PERPENDICULAR MAGNETIC RECORDING

PERPENDICULAR MAGNETIC RECORDING

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Perpendicular Magnetic Recording by

Sakhrat Khizroev Center for Nanoscale Magnetic Devices, Department of Electrical and Computer Engineering, Florida International University, Miami, Florida, U.S.A. and

Dmitri Litvinov Center for Applied Nanomagnetics, University of Houston, Houston, Texas, U.S.A.

KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW

eBook ISBN: Print ISBN:

1-4020-2723-0 1-4020-2662-5

©2005 Springer Science + Business Media, Inc. Print ©2004 Kluwer Academic Publishers Dordrecht All rights reserved No part of this eBook may be reproduced or transmitted in any form or by any means, electronic, mechanical, recording, or otherwise, without written consent from the Publisher Created in the United States of America Visit Springer's eBookstore at: and the Springer Global Website Online at:

http://ebooks.springerlink.com http://www.springeronline.com

TABLE OF CONTENTS

Page Preface Acknowledgements

ix x

CHAPTER 1 Fundamentals of Perpendicular Recording

1

1.

A Historical Perspective

1

2.

Superparamagnetic Limit

2

3. 3.1 3.2 3.3

Dodging the Superparamagnetic Limit Strong write field Perfectly aligned media Absence of the demagnetization field in bit transitions

5 5 9 10

4. 4.1 4.2 4.3 4.4

Soft underlayer as a new system component SUL as a major source of noise SUL magnetic moment SUL thickness SUL influence on the resolution

11 12 13 14 15

5.

Playback: New Signal Processing Schemes

16

6. 6.1 6.2

Challenges of New Materials Hard layer materials High anisotropy SUL materials

17 18 21

7.

How Far Will Perpendicular Recording Go

21

CHAPTER 2 Physics of Writing

23

1. 1.1

Introduction Chapter overview

23 24

2. 2.1

Different Modes of Perpendicular Recording Second perpendicular mode: a ring head and a perpendicular medium without a soft underlayer

24 25

vi 2.1.1 2.1.2 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.2.7 2.2.8 2.2.9 2.2.10 2.2.11 2.2.12 2.2.13 2.2.14

2.2.15 2.2.16 2.3 2.3.1

Gap length dependence Trailing pole thickness dependence First perpendicular mode: a single pole head and a perpendicular medium with a soft underlayer Magnetic image model Permanent magnet approximation Recording by the field in the gap (perpendicular) versus recording by the field fringing from the gap (longitudinal) Is the increase of the recording field due to the use of a SUL sufficient for adequate recording? Quadruple ratio between saturation currents in perpendicular and longitudinal recording Focused-ion-beam trimmed single pole heads Example 1: FIB trimming of a wide-track Censtor SPH into a narrow-track SPH Example 2: FIB trimming of a RH into a narrow-track SPH Single pole head: design strategy Definition of efficiency Throat height dependence Dependence on the pole trackwidth and thickness Skew angle sensitivity of single pole head Gap length dependence Field modelling SPH efficiency versus RH efficiency Skew angle versus gap length Single pole head of type III Experiments to compare different types of SPH’s Flying height limitation of single pole head design Multiple magnetic image reflection Modified first perpendicular mode: a shielded single pole head and a perpendicular medium with a soft underlayer Shielded single pole head

28 30 31 31 32 35 36 37 38 38 39 42 43 44 50 52 56 58 63 66 68 68 70 72 79 80

CHAPTER 3 Physics of Playback

85

1. 1.1

Introduction Chapter overview

85 85

2. 2.1 2.1.1 2.1.2

Playback in Perpendicular Recording Analysis Methods Direct calculation with Point-size Reader Approximation Calculation based on the Reciprocity Principle

86 86 86 98

vii 2.1.3 2.1.4 2.1.5 2.1.6 2.1.7

2.1.8 2.1.9 2.1.10

Model of the magnetic image Image “paradox” Examples of reader designs Basic reader comparison Parallels between perpendicular and longitudinal recording Influence of shields Number of shields Shield thickness Soft underlayer versus no soft underlayer Differential reader optimisation and single MR differential readers Parallels between playback in perpendicular and longitudinal magnetic recording: revisited Overview of reader designs Conformal mapping Use of a soft underlayer 3-D BEM calculation Conclusions on study of parallels between playback in perpendicular and longitudinal recording

101 102 104 106 108 111 111 113 113 114 118 118 120 123 124 126

CHAPTER 4 Perpendicular Recording Media

127

1. 1.1

Introduction Chapter overview

127 127

2. 2.1 2.2 2.3 2.4 2.5 2.6

Perpendicular Recording (“Hard”) Layer Types of Media Continuous Media Magnetic Field Calculation Demagnetisation Field in Perpendicular Recording Layer Stray Field from Perpendicular Magnetic Media Well-defined Perpendicular Easy Axis: Thicker Recording Layer?

128 128 131 131 135 137 142

3. 3.1 3.2 3.3 3.4 3.5 3.6 3.6.1 3.6.2

Soft Underlayer Saturation Moment Thickness of Soft Underlayer SUL-to-ABS Separation Anisotropy: Micromagnetics of SUL Magnetic Biasing Dynamics of Perpendicular Recording Design of a high-data-rate perpendicular system Kerr microscopy

144 145 146 149 150 153 155 156 159

References Index

162 173

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Preface This book is intended to be of use to two groups of readers. The first group includes engineers and scientists who have good familiarity with conventional magnetic recording and intend to investigate perpendicular magnetic recording in more detail. The second group comprises mostly graduate students and those who need at least a casual or, perhaps, detailed knowledge of the applications of the physics of magnetism in ultra-high density (with nanoscale bit and transducer dimensions) magnetic recording. Today, many leading companies in the multi-billion-dollar data storage industry demonstrate an unprecedented interest in perpendicular magnetic recording. It is commonly believed that perpendicular magnetic recording is the most likely alternative to the conventional magnetic technology – longitudinal magnetic recording. Longitudinal magnetic recording that has been the core technology since the inception of the magnetic data storage industry more than half a century ago is finally coming to terms with reality. Reality screams that the areal density is limited by thermal instabilities in the longitudinal magnetic media at areal densities not far beyond 100 Gbit/in2. As never before, the industry is desperate to find an alternative technology that could maintain the “usual” steady areal density growth rate to which it got used over the many years in the past. This explains why recently perpendicular magnetic recording could suddenly revitalize such a strong attention from the data storage community. Out of a number of alternatives, perpendicular magnetic recording is technically the closest technology to longitudinal recording. Therefore, with a switch to perpendicular recording it would cost the least for the industry to change its current infrastructure. Perpendicular recording promises to defer the superparamagnetic density limit in the magnetic media by at least a factor of ten compared to longitudinal recording. As companies in the ultra-competitive data storage industry promptly get in the global race to develop perpendicular magnetic recording, many engineers find themselves not adequately trained and experienced in the new technology. Although perpendicular magnetic recording is similar to the conventional technology, it still has a number of peculiarities and open issues that have never been encountered in longitudinal recording and thus remain to be understood and resolved. The authors of this book believe that to adequately address these issues, every engineer in the field should acquire sufficient knowledge of the physics of perpendicular magnetic recording and be well aware of the fundamental and sometimes barely perceptible differences between perpendicular and longitudinal recording.

ix

x The purpose of this book is to provide engineers and graduate students with the basic knowledge in perpendicular magnetic recording. The book’s emphasis is on the basic physics of perpendicular magnetic recording rather than a detailed description of one or another particular technical implementation. The authors attempt to provide the most basic guidelines to design a complete system to record and retrieve information from a perpendicular magnetic medium with areal densities up to one terabit per square inch and beyond. This defines also the structure of this book. The book is divided into four chapters. Chapter 1 provides background on the most recent development in the field and describes open questions and issues in perpendicular recording. Chapter 2 and Chapter 3 cover the recording (writing) and playback mechanisms, respectively. In these chapters, not only the authors explain the physics of data recording and playback but also propose technical solutions to the design optimization of recording and playback transducers. Chapter 4 introduces the essentials of perpendicular magnetic recording media. Throughout the book, the authors compare perpendicular and magnetic recording. Currently, the authors of this book, Khizroev and Litvinov, are with Florida International University and the University of Houston, respectively. The material presented in this book has been collected as a result of several years of research of the authors together with Florida International University, the University of Houston, Seagate Research, IBM Almaden Research Center, and Carnegie Mellon University. Based on the accomplishments of this research, Khizroev and Litvinov have co-authored over 12 patents that were granted to Seagate and IBM.

Acknowledgements The authors would like to acknowledge the numerous insightful discussions on different topics of perpendicular magnetic recording with Mark Kryder, Kent Howard, Roy Chantrell, Dieter Weller, Song Xue, Erik Svedberg, Bin Lu, Alex Shukh, and many others of Seagate Technology, David Thompson of IBM Research, Hal Rosen, Kurt Rubin, Alex Taratorin, Pat Arnett, Yoshihiro Ikeda, Margaret Best, Neil Smith, Roger Wood, Andy Moser, Wipul Jayasekara, Y. Sonobe, and many others of IBM Almaden Research and currently of Hitachi Global Storage Division, Michael Mallary, Adam Torabi, Anna Kostrov, and others of Maxtor Corporation, Stanley Charap, David Lambeth, Robert White, Jim Bain, Albert Theile, Jimmy Zhu, David Laughlin, and others of Carnegie Mellon University, Roman Chomko and Venkatesan Renugopalakrishnan of Florida International University, Neil Bertram of the University of California at San Diego, Heng Gong of Iomega Corporation, Jack Judy of the University of Minnesota, Leon Abelmann and Cock Lodder of the University of Twente, Mark Re, Francis Liu, Jing Zhang, Kroum Stoev, and others of ReadRite Corporation, Jim Miles of the University of Manchester, Gerardo Bertero and David Wachenschwanz of Komag Corporation, Yasushi Kanai of Niigata Institute of Technology, Kevin O’Grady and Jing Wu of York University, Masaaki Futamoto of Hitachi Central Research Laboratory, Hiroaki Muraoka of the University of Tohoku, Carolyn Ross of Massachusetts Institute of Technology, Ben Hu of Headway Corporation, Paul Frank of Information Storage

xi Industry Consortium, Robert Doyle and Hideo Fujiwara of the University of Alabama, and many others. The authors would like to express special gratitude to David Thompson, retired IBM Fellow, for encouraging, challenging, mentoring, and supporting this research since 1996. The authors would like to thank Shun-ichi Iwasaki who has always been an inspiring symbol and supporter of the international efforts to develop perpendicular magnetic recording. The authors would like to thank Simona Stefanescu for invaluable help with reading the manuscript and the derivation and analysis of many analytical expressions used throughout the book. Finally, the authors greatly appreciate the dedicated efforts of Helga Melcherts of Kluwer Academic Publishers during the preparation of the manuscript. S. Khizroev, Miami D. Litvinov, Houston February 2004

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Chapter 1 Fundamentals of Perpendicular Recording

Chapter 1

Fundamentals of Perpendicular Recording

1. A Historical Perspective It is believed that formally perpendicular magnetic recording [1,2,3,4] was proposed for the first time by Professor Shun-ichi Iwasaki about two decades ago. However, sporadic research efforts on the development of perpendicular recording started much earlier. It is very likely that perpendicular recoding was mentioned for the first time in a new computer design program at the University of California at Berkeley in the late 1940s [5]. This program was funded by the Office of Naval Research to pursue an intermediate sized computer based on a magnetic drum memory. Among the first companies that have demonstrated a serious interest in perpendicular magnetic recording are IBM Corporation [1] and Ampex Corporation [3]. From 1955 until 1961, perpendicular recording was the major disk-drive project at IBM. Ampex Corporation was another company that considered perpendicular recording as a solution in the magnetic tape industry in the late 1950s. However, despite such a long history, because of the strong position of the conventional technology (longitudinal magnetic recording), there have been only a few products based on perpendicular recording. Among these products are a hard disk from Censtor Corporation [6] and a floppy disk from Toshiba Corporation [7]. Today, as the conventional magnetic recording technology is finally facing its fundamental limit due to thermal instabilities in the longitudinal magnetic media [8], the strong interest in perpendicular recording as the prime alternative is coming back [9,10]. As envisioned by industry and academia leaders, perpendicular recording is the most likely candidate for the technology to be implemented in the next generations of hard disks. The most competitive virtue of this technology is the fact that while being technically the closest alternative to longitudinal recording, it is capable of deferring the (superparamagnetic) density limit beyond what is achievable with longitudinal recording.

1

Perpendicular Magnetic Recording

2

It is believed that perpendicular magnetic recording will enable to retain the current rapid technology advancement for the next several generations of magnetic storage solutions. The chapter will cover the basic principles underlying perpendicular recording as well as the challenges associated with implementing the technology [11,12,13,14].

Magnetizing Coil

MR Reader

Inductive “Ring” Writer

S

N N

S

S

Write field

N N

S S

N N

S

Recording Media

Figure 1. A schematic diagram of a conventional longitudinal recording scheme employed in today’s hard drives.

2. Superparamagnetic Limit Data on a magnetic recording medium are stored by means of recording certain spatial variations of the magnetization, where the variations represent the data. The relation between the data and the magnetization pattern is defined through the special data encoding. Figure 1 shows a simplified schematic diagram of a conventional longitudinal recording system. The recording medium is engineered so that the preferred direction of the magnetization, a so-called easy axis, lies in the plane of the recording layer. Using an inductive “ring”-type writer, the magnetization of the grains (in the medium) is aligned along the track in either positive or negative direction. The recorded data pattern is read back using a magnetoresistive element. A change or no change in the magnetization direction at the bit transitions corresponds to a 1 or to a 0, respectively. The lateral dimensions of a bit, the smallest feature realized in a particular drive design, define the areal bit density that the drive could support. A conventional magnetic medium has a granular structure such that each bit consists of several magnetic grains or magnetic clusters. The magnetic clusters/grains are usually shaped irregularly and are randomly packed, as shown in Figure 2a. Consequently, the

Chapter 1 Fundamentals of Perpendicular Recording

3

recording bits and bit transitions are usually not perfect, which is illustrated in Figure 2b. These imperfections lead to noise in the playback signal. The noise is kept below a certain acceptable level by means of including a sufficiently large number of magnetic grains into each bit. The resulting averaging reduces the level of noise. Therefore, to satisfy the Scaling Law, as the areal density increases, the bit size and the average size of the grains that constitutes each bit should be decreased. Independently, Figure 2b also illustrates that the reduction of the average grain size (which is necessary to maintain the signal-to-noise ratio with the density increase) results in the reduction of the statistically defined bit transition. Typical grains in today’s media range from 5 to 15 nm.

Magnetic grains

Bit transition (a)

(b)

Figure 2. (a) A transmission electron micrograph (TEM) of a typical granular medium. (b) A schematic diagram of a single bit transition in a granular medium.

One of the critical factors characterizing the reliability of a data storage device is the data stability. Various parameters control the stability of the data against external factors. Relative to the ambient temperature, which is manifested by thermal fluctuations in the recording medium, the magnetic anisotropy energy stored in each magnetic grain is one of the major determinants (assuming that the grains are magnetically independent). The magnetic anisotropy energy defines the approximate amount of energy necessary to reverse the direction of the magnetization of a grain. For a single grain, it is equal to KUV, where KU is the magnetic anisotropy energy per unit volume and V is the volume of the grain, as shown in a schematic diagram in Figure 3. According to the Statistical Physics, even for a relatively large value of KU (compared to the characteristic thermal energy density) there is a finite probability for the grain to reverse its magnetic moment due to thermal fluctuations, as given by the following Expression.

fr

f 0 exp ( 

'Er ), k BT

(1)

where subscripts “+” and “-“ stand for the “upward” and “downward” magnetization directions, respectively, f0 is the characteristic frequency in the range from 109 to 1012 Hz [58]. The exact value depends on the intrinsic medium properties linked to the quantum-

Perpendicular Magnetic Recording

4

mechanical interactions within the grain, 'E is the energy barrier between the two energy states. In the extreme case, 'E = KUV. (It should be reminded that the demagnetization field reduces the effective energy barrier. The demagnetization field strongly depends on the recording mode: it achieves its maximum in the bit transitions in longitudinal media, while it achieves its minima in the bit transitions in perpendicular media.) For a medium to be thermally stable, the above quantity KUV should be substantially greater (>~ 40 times) than the energy of a single quantum of thermal fluctuation, kBT, where kB is Boltzman’s constant and T is the temperature [8]. As the ratio, KUV/ kBT , approaches this “magical” number (~40), due to the above described exponential dependence, the relaxation time, W = 1/f, (which defines for how long can a particle remain in a stable state) drastically changes. For example, Charap et el showed that when for a typical longitudinal medium the ratio is reduced from 60 to 25, the relaxation time drops from more than 3.5 x 106 years to 72 seconds, respectively [8]. From the above discussions, one could derive an approximate “stability” expression to evaluate the minimum average grain size below which the longitudinal recording medium would become thermally unstable. (Conventionally, the boundary value for the ratio KUV/ kBT is assumed to be 40).

KU V k BT

a~

t 40 ŸV t 40Kk

B

U

1 ArealDensi ty

T

,

t a min # 3

(2)

40 k B T KU ,

(3)

As mentioned above, according to the Law of Scaling, higher areal densities require smaller grain sizes. It follows that to sustain thermal stability with reduction of the average grain size, the anisotropy energy density KU should be increased. There is a choice of potential medium materials (e.g., Co/Pd multilayer) that would provide the necessary increase in the anisotropy energy density. Unfortunately, as KU increases, so does the write field necessary to efficiently write onto the medium. The problem lies right here. It is believed that the saturation magnetization for “soft” magnetic materials (of which recording heads are made) is fundamentally limited [15]. The current state-of-theart recording heads already use materials with magnetization close to the predicted fundamental limit, ~ 26 kGauss. In conventional longitudinal recording, the upper limit of the write field that a recording head could generate is equal to 2SMS, where MS is the saturation magnetization of the head material. The highest value of 4SMS for the materials available today is rapidly approaching what is believed to be a fundamental limit of ~26 kGauss. This defines the upper limit of the KU values that could be employed in a longitudinal medium and, consequently, the maximum areal density achievable with conventional longitudinal recording. It has been predicted that with the materials available today, the highest areal density achievable with conventional longitudinal recording is ~100 Gbit/in2 [10,8].

Chapter 1 Fundamentals of Perpendicular Recording

5

Figure 3. A schematic diagram describing the anisotropy energy of a magnetic grain.

3. Dodging the Superparamagnetic Limit Several aspects native to perpendicular recording make it superior to longitudinal recording with respect to the superparamagnetic limit. Among the advantages are higher write-field amplitude and sharper write-field gradients, thicker recording layers, absence of the demagnetizing field in the bit transitions, higher playback amplitude, etc. The specific nature of these advantages is discussed in detail below. 3.1. STRONG RECORDING FIELD Figure 4 shows comparative schematic diagrams of conventional longitudinal and perpendicular recording modes. While in longitudinal recording, the preferred direction of the magnetization (in other words, the “easy” axis) lies in the plane of a recording medium, in perpendicular recording, the “easy” axis is perpendicular to the plane of a medium. In longitudinal recording, writing is performed by the fringing field emanating from the gap region between the write-poles of a conventional “ring”-type recording head. It is the geometry of the ring head that defines the upper limit of the write field of 2SMS, where MS is the saturation magnetization of the write-pole material. In perpendicular recording, the write field is generated between the trailing pole of a single pole head and a soft underlayer (SUL). SUL, a new component in a recording system, is a soft magnetic material located below the recording layer. In such geometry, the upper limit of the write field is equal to 4SMS, which is two times higher than the highest field

Perpendicular Magnetic Recording

6

achievable with the longitudinal ring head. Higher write efficiency of the perpendicular single-pole recording head (in combination with the SUL) will ba explained in greater detail and is illustrated in Figure 5. Yoke

Trailing edge Coil

Written moment in media

“Gap” field

Record. layer Transition

(a)

SUL

Coil

Yoke

Recording medium

Fringing fields

(b)

Transition

Written moment in media

Figure 4. Schematic diagrams showing a side cross-section of (a) a typical perpendicular system including a SPH and a double-layer medium with a SUL and (b) a longitudinal system, including a ring-head and a singlelayer recording medium.

Chapter 1 Fundamentals of Perpendicular Recording

7

Real head Coil “Gap” fields Physical Gap

Effective Gap

SUL boundary

(a) Image head

(b) Figure 5. A schematic diagram and 3-d drawing of the magnetic imaging principle in perpendicular recording using a medium with a soft underlayer.

It could be shown (the proof of this concept is beyond the scope of this book [13]) that to evaluate the magnetic field above the SUL boundary, the SUL could be thought of as a perfect magnetic mirror such that the magnetic field above the SUL boundary is equal to

Perpendicular Magnetic Recording

8

the net field generated by both the magnetic elements above the SUL boundary and their images located below the SUL boundary. This concept is illustrated in Figure 5, where the SUL is replaced with an image of the recording head. From this picture it is clear that in perpendicular recording the write process effectively occurs in the “gap” between the magnetic poles of the real head and its image. This is in contrast to longitudinal recording, where writing is performed by the field fringing from the gap, as outlined above. From simple superposition arguments, it is straighforward to show that the “ingap” field is equal to 4SMS while the highest value of the fringing field is equal to 2SMS [11]. As shown above, the maximum write field achievable in perpendicular recording is almost twice as large as the maximum write field achievable in longitudinal recording. The direct consequence is the ability to write onto a higher anisotropy medium (higher KU). The use of higher anisotropy media materials allows higher areal densities without compromising thermal stability of the recording data. As illustrated in Figure 6, the spatial profile of the write field in perpendicular recording is also more beneficial for achieving higher areal density (compared to longitudinal recording). The side gradients, i.e. the rate at which the field rolls off at the side edges of a recording head, are usually substantially sharper than what one observes in longitudinal recording. This property leads to better-defined tracks with a relatively narrow erase band. Along with better magnetic alignment of the media (see below), extremely narrow tracks are possible to achieve. H x (Oe) 19397

H x (Oe) 6734

9698

3373

0.5 um Along the track

(a)

(b)

Figure 6. Longitudinal head field contours and perpendicular head field contours from (a) a longitudinal head with a 150 nm gap and (b) a perpendicular pole head with a pole thickness of 700 nm. The trackwidth is 50 nm in both cases.

The single pole perpendicular write heads used to acquire the experimental data presented in this chapter were fabricated via focused ion-beam (FIB) modification of conventional longitudinal writers [16]. It should be emphasized that the main difference in the design of conventional perpendicular and longitudinal writers is the length of the gap between the magnetic write-poles. In terms of the write process, while in longitudinal recording the writing occurs near the gap region, in perpendicular recording, the writing occurs near the trailing edge of the trailing pole [17]. Figure 7 shows a state-of-the-art perpendicular recording head manufactured by FIB trimming of a conventional longitudinal write head

Chapter 1 Fundamentals of Perpendicular Recording

9

by increasing the gap length and trimming the trailing pole and the reader to specified dimensions. Both the trailing pole and the reader are designed for a 60 nm track width.

FIBed Writer

FIBed Reader Figure 7. A FIB image single pole perpendicular write head made by focused ion-beam etching of a conventional longitudinal ring head. The trailing pole width is 60 nm.

3.2. PERFECTLY ALIGNED MEDIA In conventional longitudinal recording, the easy axes of individual grains are randomly oriented in the plane of a medium. (It should be recalled that the easy axis is the energetically favorable axis/direction along which the magnetization of a grain is aligned in the absence of external magnetic fields.) Thus, in longitudinal recording, a large fraction of the grains forming a bit has their easy axes severely misaligned with the bit magnetization direction. Writing well-defined bit transitions on such randomly oriented media imposes stringent requirements onto the spatial profile of a write-field. If one neglects the imperfections of a bit transition due to the granular nature of a medium, the quality of the bit transition is defined mainly by the write-field profile. This is drastically different from perpendicular recording, in which the easy axis of each magnetic grain is relatively well aligned in the direction perpendicular to the plain of the medium. Thus, in a perpendicular recording, the magnetization direction of a recorded bit always coincides with the orientation of the easy axes of individual grains that form the bit. Well-defined easy axis orientation relaxes the stringent requirements for the trailing and side writefield gradients necessary to achieve sharp transitions, thus enabling the use of thicker media [14]. The intrinsically better alignment of perpendicular media helps to record narrow tracks with well-defined transitions even into a relatively thick recording layer. A MFM image of two adjacent tracks with a 65 nm trackpitch written into a 50 nm thick CoCr recording layer using a 60 nm wide single pole head is shown in Figure 8 [11]. This is equivalent to a track density of ~400ktpi. It should be stressed that the state-of-the-art in longitudinal

Perpendicular Magnetic Recording

10

recording for the track density is ~100 ktpi. The possibility of using thicker recording layers further assists with improving thermal stability.

Figure 8. A MFM image of two tracks with a 65 nm trackpitch.

With respect to using well-aligned media, it should be remembered that previously it was shown that, although a well-aligned perpendicular medium might have a relatively small average angle between the magnetization and the recording field, the torque created is still sufficiently large to relatively rapidly switch the magnetization [18, 19].

perpendicular longitudinal S

N N

N

S

S

N

S

More stable magnet configuration Figure 9. A schematics of the influence of demagnetizing field in longitudinal and perpendicular media.

3.3. ABSENCE OF THE DEMAGNETIZATION FIELD IN BIT TRANSITIONS One of the major destabilizing factors in longitudinal recording medium is the strong demagnetizing field in the bit transitions. The destabilizing influence of the demagnetizing field in the bit transitions is easy to see if one observes that two adjacent bits with opposing magnetization directions repel in a similar way as two bar magnets

Chapter 1 Fundamentals of Perpendicular Recording

11

with the poles of the same polarity, such as north-north or south-south, facing each other. The magnets would try to flip so that poles of opposite polarities are next to each other. This is illustrated in Figure 9. The calculated demagnetizing field for the cases of longitudinal and perpendicular media for a single bit-transition is shown in Figure 10. In the longitudinal recording mode, high demagnetizing field in the bit-transitions destabilize individual grains leading to a finite transition width. This is opposite to perpendicular recording, in which the demagnetizing field reaches its minima in the bit-transitions, thus promoting ultra-narrow transitions and, consequently, high-density recording. It could also be noted that, unlike in longitudinal recording, the demagnetization field in perpendicular recording decreases as the thickness increases, thus promoting a thicker recording layer, which in turn is beneficial for the thermal stability. With this respect, it is common to note that although perpendicular recording promotes high densities, the stronger influence of the demagnetization field at lower densities is a disadvantage of perpendicular recording. 2000

T = 10 nm T = 20 nm

T = 10 nm T = 20 nm

1000 Hx (Oe)

Hz (Oe)

1000

2000

0

0

-1000

-1000

-2000

-2000 -0.04 -0.02 0.00 0.02 0.04 Distance down the track (um)

-0.04 -0.02 0.00 0.02 0.04 Distance along the track (um)

(a)

(b)

Figure 10. The demagnetization field versus the distance down the track along the central planes of 10 nm and 20 nm thick recording layers for (a) perpendicular and (b) longitudinal recording media.

4. A Soft Underlayer as a New System Component

One of the key aspects of perpendicular recording that makes it superior to the longitudinal recording with respect to superparamagnetic effects is utilization of media with a SUL. A single-pole head and a medium with a SUL perpendicular recording system enables write fields in excess of 80% of 4SMS of the pole head/SUL material. This doubles the fields available in longitudinal recording, thus opening the possibility to write on substantially higher anisotropy media and leading to better thermal stability. Acting as a magnetic mirror, SUL effectively doubles the recording layer thickness, facilitating substantially stronger readout signals. Also, the effective thickness increase due to the mirroring effects by a SUL leads to the reduction of the demagnetizing fields with a potential to further improve thermal stability.

Perpendicular Magnetic Recording

12

Fields from Wall (Source of Noise)

& M

& M

Domain wall (source of “magnetic charges”) Figure 11. A schematic of the stray fields generated by a SUL

While the utilization of perpendicular media with a SUL should make it possible to postpone the superparamagnetic limit, the SUL introduces a number of technical challenges. Some of the issues related to the presence of the SUL are discussed below. 4.1. SUL AS A MAJOR SOURCE OF NOISE Among the technical challenges introduced by the presence of a SUL is the fact that a not properly optimized SUL material can introduce a significant amount of noise into the playback signal. The noise results from the stray field generated by the effective charges resulting from domain walls in the SUL as illustrated in Figure 11.

head

Hard layer Soft underlayer

----

 ----Magnets

Figure 12. A schematic of experimental setup to magnetically bias SUL film.

Chapter 1 Fundamentals of Perpendicular Recording

13

Magnetic biasing of the SUL, i.e. forcing the SUL into a single magnetic domain state, allows to minimize the SUL noise. The biasing can be achieved either by application of an external magnetic field or by engineering a SUL material with a built-in biasing field. Figure 12 shows a schematic of the experimental setup to study the effect of magnetic biasing of the SUL on the noise. The magnetic biasing was achieved using two NdFeB permanent magnets placed in the vicinity of the media. The placement of the magnets was such that it allowed achieving complete saturation of the SUL underneath the reader. Special care was necessary to arrange the magnets sufficiently far from the recording head ~2cm away in order not to affect the properties of the read element. Figure 13 shows the playback signals from the two media with as deposited non-biased (a) and magnetically biased (b) SUL’s. A substantial level of noise attributed to presence of a large number of domain walls (confirmed by magnetic force microscopy) in the SUL can be seen in Figure 13a. A drastic reduction of the noise (by at least 10dB) is clearly observed in Figure 13b where the SUL is magnetically biased.

(a)

(b)

Figure 13. Playback signal from two media with different SUL’s. (a) SUL with a large number of stripe domains. The presence of stripe domains was confirmed using magnetic force microscopy. (b) Biased SUL with domain walls swept out from the SUL material.

The magnetic biasing saturates SUL film forcing it into a pseudo-single domain state effectively sweeping the domain walls out of the SUL material. This results in elimination of the SUL noise. 4.2. SUL MAGNETIC MOMENT To properly design a perpendicular recording system that utilizes a medium with a SUL, it is critical to choose an appropriate SUL material. As illustrated in Figure 14, if the

Perpendicular Magnetic Recording

14

magnetic moment of a SUL material is lower than the magnetic moment of the recording pole tip, saturation of the SUL underneath the pole tip can occur.

SUL 4SMS < Head 4SMS

(saturated region under the pole tip deteriorates gradients)

SUL 4SMS > Head 4SMS (not saturated under the pole tip)

Pole tip

Pole tip

H

H

Soft underlayer

Soft underlayer

Saturated region Figure 14. A schematic illustrating the saturation effect in the SUL is the magnetic moment of a SUL is lower than the magnetic moment of the write pole tip.

The results of boundary element modeling for two different head/SUL combinations are presented in Figure 15. It can be noticed that it is possible to generate strong recording fields with the magnitude approaching 4SMS of the pole tip even if the SUL has a lower magnetic moment than the pole tip. However, saturation of the SUL will lead to a substantial deterioration of the trailing field gradients. The trailing gradients in the case of the Permalloy based SUL are substantially worse than the trailing gradients in the case when a FeAlN based SUL is used. It follows that if high moment materials are used for write heads, e.g. CoFeB, FeAlN, etc., the moment of the SUL material should match or exceed the moment of the pole tip material. 4.3. SUL THICKNESS Another important issue related to the optimized design of a SUL is the SUL thickness. Using simple considerations of magnetic flux conservation, the minimum thickness required for the SUL to function properly is given by Expression 4.

tsoft underlayer t

1 M S pole tip wpole tip , 2 M S soft underlayer

(4)

where the wpole tip is the width of the write pole tip, i.e. the dimension of the write pole tip defining the track width. The evaluation of the above equation for the case of 100

Chapter 1 Fundamentals of Perpendicular Recording

15

Gbit/in2 areal density and 4:1 bit aspect ratio, i.e. a 160 nm wide pole tip, and the same pole tip and SUL materials, gives the lower boundary on the SUL thickness of 80 nm. It should be stressed that this thickness is substantially smaller than the “required minimum” (as often quoted in the literature, of “hundreds of nanometers to a micron). Permalloy Isat=100mA FeAlN Isat=75mA

Hz (kOe)

15

10

5

0

-0.5

-0.4

-0.3

Distance down the track (Pm) Figure 15. Trailing fields from a single pole perpendicular write head made out of FeAlN (4SMS =20kG) for FeAlN and Permalloy (4SMS =10kG) SUL’s.

This important observation needs to be strongly emphasized. Due to material properties, the above mentioned problem of the SUL noise becomes increasingly aggravated with the increasing thickness of the SUL. 4.4. SUL INFLUENCE ON THE RESOLUTION An additional challenge that the presence of a SUL imposes is the potential deterioration of the system resolution. During reading from a medium with a SUL, due to the magnetic imaging of the SUL, the resolution could get distorted if the separation between the ABS and the SUL (sum of the recording layer thickness and the flying height) were comparable to the reader thickness. This phenomenon is clearly illustrated in the calculated [20] PW50 and the playback signal versus the SUL-to-ABS distance, shown in Figure 16. PW50 is the physical width of a single transition, the measure of the spatial resolution of a recording system. In these calculations, a fixed recording layer thickness of 10 nm was assumed, and the separation between the bottom side of the recording layer and the SUL was varied from zero to a finite value. For comparison, the dotted straight lines indicate the values for the case when there is no SUL used. It could be clearly observed that the resolution of the modeled recording system substantially deteriorates at certain values of the ABS-to-SUL separation. This suggests that a special care (of this separation) has to be taken to properly optimize the system’s resolution.

Perpendicular Magnetic Recording

16

PW50 (with SUL) PW50 (without SUL) Signal (with SUL) Signal (without SUL)

PW50 (nm)

50

1.0 0.9

45

0.8 0.7

40

0.6 10

15 20 25 30 ABS to underlayer distance (nm)

Normalized Signal (arb.units)

Although, in a properly designed system this resolution distortion could be almost completely eliminated, it causes the resolution of a typical read head in a system with an underlayer to be at most as good as the resolution of an equivalent head in a system without a SUL. It should be noted, however, the SUL definitely increases the playback signal, which is desirable at high areal densities.

Figure 16. PW50 and the normalized playback vs. the ABS to underlayer spacing. 30 nm GMR element and a 70 nm shield-to-shield spacing are assumed.

5. Playback: New Signal Processing Schemes

Hstray + +

(a)

+ +

M

charges in the transition

Hstray (b)

++++++++ -------------------

+++++++

Figure 17. Diagrams showing the sources of stray fields in the case of (a) longitudinal recording, and (b) perpendicular recording.

One of the drastic differences between perpendicular and longitudinal recording is the difference in the playback signal. To help understand the basic difference in the playback process between longitudinal and perpendicular recording, schematic diagrams of the stray field emanating from a longitudinal medium and perpendicular media without and with a SUL are shown in Figures 17a and b, respectively. As could be noticed, in the

Chapter 1 Fundamentals of Perpendicular Recording

17

longitudinal case, the stray field emanates only from the transitions, with the fields near the transitions oriented perpendicular to the disk plane. On the contrary, in the perpendicular cases, the stray field emanates from the effective magnetic “charge” at the top and effective (due to the SUL) bottom surfaces of the recording layer, with the field near the transitions oriented parallel to the disk plane. As a result of the different magnetic “charge” distributions, the playback waveforms differ drastically between longitudinal and perpendicular recording schemes. This difference is illustrated in Figure 18, where typical low-density playback waveforms are shown for both perpendicular and longitudinal recording. The above shown waveforms for perpendicular and longitudinal recording modes outline a major difference between perpendicular and longitudinal recording. While in longitudinal recording the signal is present only near the bit transitions, in perpendicular recording the signal is read not only near the bit transition but rather across the entire bit area. It is possible to differentiate the perpendicular playback signal to make it similar to the playback signal in longitudinal recording. However, it should be remembered that the differentiated perpendicular playback waveform is similar but not identical to the longitudinal playback waveform. The difference arises in the absence of a transition when there is no longitudinal playback signal while the differentiated perpendicular playback, although is relatively small in amplitude, is still finite (non-zero). Longitudinal Playback

Playback Signal

Playback Signal

Perpendicular Playback

Time

Time

Figure 18. Typical playback waveforms for perpendicular and longitudinal recording schemes.

It should be stressed that while not entirely suited to be processed by conventional longitudinal channels, perpendicular playback waveforms clearly contain more information than typical longitudinal waveforms in which the signal is concentrated mostly near the transitions. This property could be used to advantage in future channel designs. 6. Challenges of New Materials

18

Perpendicular Magnetic Recording

While the requirements for the head materials used in perpendicular recording are similar to those for the head materials used in longitudinal recording, the major differences exist with respect to the media materials. A typical perpendicular medium consists of two magnetically active layers, a hard layer and a SUL, and a number of non-magnetic layers, as shown in Figure 19. The hard layer has rather different magnetic properties compared to the hard layer utilized in conventional longitudinal recording. It should also be noted that SUL has no analogy in longitudinal recording. The requirements for these two layers are outlined below.

Figure 19. A schematics of a typical perpendicular medium.

6.1. HARD LAYER MATERIALS The primary approach to the design of a perpendicular recording layer is in many ways similar to the design of a conventional longitudinal recording layer. All the media in use today has granular structure, i.e. made of polycrystalline materials. Major goals inherent to both longitudinal and perpendicular recording layer development are small grain size, small grain size distribution, texture control, optimization of the inter-granular exchange de-coupling, etc. The large variety of today's perpendicular magnetic recording layer types can be clearly divided into the two major categories: 1) Alloy based media, such as CoCr-alloys [21, 22], and 2) media based on magnetic multilayers, such as Co/Pt, Co/Pd or other materials [23, 24]. Figure 20 contrasts the major difference between alloy and multilayer media. In alloy media, the magnetic anisotropy is controlled by magnetic crystalline anisotropy. The alloy media are usually highly textured to insure well-defined magnetic easy axis [25]. In magnetic multilayers, the magnetic anisotropy is controlled

Chapter 1 Fundamentals of Perpendicular Recording

19

through interfacial effects between a magnetic layer, such as Co, and a highly polarizable spacer layer, such as Palladium or Platinum. In contrast to the alloy media, this set of materials as used in perpendicular media usually possesses a very weak texture.

Multilayer

Alloy

Co

n

n n n

Pd

n n Bi-layer

Single crystal grains, arrows represent the easy axes orientations

(a)

(b)

Figure 20. A schematic representation of major microstructural differences between (a) an alloy medium and (b) a multilayer medium.

Material-wise, perpendicular CoCr-based alloy recording layers are similar to conventional longitudinal CoCr-based media, with the major difference being the orientation of the magnetic easy axis. Therefore, a significant amount of information accumulated in the course of the longitudinal media development can be used to control the critical parameters such as the grain size and the inter-granular exchange coupling. At the same time, CoCr-based perpendicular media have some open issues. For example, it is not clear yet if it is possible to make a CoCr-based medium with sufficiently high anisotropy to avoid superparamagnetic instabilities at ultra-high areal densities. It also has proven to be difficult to make CoCr-alloy based perpendicular recording layers with a remanent squareness of 1. The remanent squareness is defined as a ratio between the remanent magnetization, the value of magnetization on a M-H loop at H=0, and the saturation magnetization, the maximum value of magnetization. It is believed that a remanent squareness of 1 is necessary for low-density bit pattern stability. Also, a remanent squareness of less than 1 can lead to substantial amounts of DC noise. Various magnetic alloys such as L10 phases of FePt, CoPt, etc. are being studied as higher anisotropy alternatives for the recording layer. The magnetic multilayer based recording layers typically have significantly larger anisotropy energies (Coercive fields of above 15 kOe have been reported.) and are thus promising to be extendable to significantly higher recording densities. Another advantage of the magnetic multilayers is the fact that typically these materials have a remanent squareness of 1. To compare basic magnetic properties of CoCr-alloy and mutlilayer based recording layers, typical M-H loops by a Kerr magnetometer for a 50 nm thick perpendicular CoCr thin-film and a 52 nm thick Co/Pd structure (a stack of 40 sets of adjacent 3 and 10

Perpendicular Magnetic Recording

20

4

Kerr Signal (a.u.)

Kerr Signal (a.u.)

Angstrom thick layers of Co and Pd, respectively) are shown in Figures 21a and b, respectively. It can be noticed that in addition to the remanent squareness of 1, the Co/Pd structure exhibits nucleation fields in excess of 3kOe, a useful characteristic to avoid data self-erasure due to stray fields. Meanwhile, the CoCr material shown in Figure 21a has a squareness of 0.75. The CoCr and Co/Pd recording layers have coercive fields and magnetizations of approximately 3 kOe and 9 kOe and 300 emu/cc and 200 emu/cc, respectively.

2 0 -2 -4 -6

-4

-2

0

2

4

6

5 0 -5

-10

-5

0

5

Field (kOe)

Field (kOe)

(a)

(b)

10

Figure 21. An M-H loop of a 50nm thick (a) CoCr-alloy layer and (b) Co/Pd multilayer.

The direct consequence of remanent squareness less than 1 is shown in Figure 22, which compares the spectral SNR distributions for the two media types. The CoCr medium exhibits a significant amount of noise at lower linear densities. This is mainly due to the fact that the dominant contribution to the noise at low linear density in the CoCr-based medium comes from the DC noise which results from the relatively low value of remanent squareness, as described below in more detail.

Figure 22. SNR versus the linear density for a CoCr-alloy (hollow circles) and a Co/Pd multilayer (hollow squares).

Chapter 1 Fundamentals of Perpendicular Recording

21

6.2. HIGH ANISOTROPY SUL MATERIALS Several design guidelines for SUL’s were discussed above including thickness requirement and magnetic moment requirement. An additional parameter, which is critical to achieve optimized performance of a SUL in a perpendicular recording system, is magnetic anisotropy of the SUL material. The dynamic properties [26, 27] and influence of a SUL on system’s resolution [28] are affected by the value of the anisotropy field. The latter is illustrated in Figure 23, where the playback versus the linear density (roll-off) curves are shown for identical perpendicular recording systems with different SUL materials. The explanation of the quantum-mechanical nature of this effect is beyond the scope of this book. However, it should be mentioned that the deterioration of the system’s resolution arises from the inability of lower anisotropy SUL materials to perfectly respond to rapid spatial variations of magnetization in the recording layer.

Playback (dBm)

0

FeAlN (Hk ~ 15 Oe) Ni45Fe55 (Hk ~ 50 Oe) Permalloy (Hk ~ 5 Oe)

-10 -20 -30 -40 -50 -60 0

200

400

600

800 1000

Linear Density (kfci) Figure 23. Playback roll-off curves for perpendicular recording media with identical recording layer but different SUL’s.

The extent of the roll-off curves to higher linear densities for higher anisotropy SUL indicates the advantage of using high anisotropy SUL materials.

7. How Far Will Perpendicular Recording Go?

It should be emphasized that perpendicular recording does not eliminate but rather defers the superparamagnetic limit of longitudinal recording to higher areal densities. A number of factors, including the availability of higher recording fields, the possibility of using thicker and well-aligned media, and the absence of strong demagnetizing fields in the bit transitions, contribute into deferring the superparamagnetic limit to substantially higher areal densities. It has been shown that with all the factors taken into account, the maximum areal density achievable with perpendicular recording configuration in the development today is 500-1000 Gbit/in2 [10,29,30]. Once perpendicular magnetic

22

Perpendicular Magnetic Recording

recording reaches its superparamagnetic limit, a new wave of technological innovations will have to take place. As mentioned in the beginning of this chapter, the foremost fundamental reason for the existence of the superparamagnetic limit is the head materials constraint imposing the limitation on the available head field that limits the utilization of higher anisotropy media. Among the potential successors of perpendicular recording is heat-assisted magnetic recording (HAMR) [31]. In HAMR, the anisotropy of a recording medium is substantially reduced via local heating of the medium during the writing instance. To accomplish the heating mission, a source of heat (envisioned as an ultra-small light source) should be added in a recording system to locally increase the temperature of the recording medium. The local increase of the medium temperature leads to the local decrease of the medium coercivity enabling recording with relatively small magnetic fields. Additionally, patterned media can be utilized to further extend the limits of magnetic recording [31]. In a patterned medium, the location and the size of the magnetic features are pre-determined by the medium manufacturing process. Elimination of the element of randomness characteristic to today’s polycrystalline recording media is a clear advantage of the patterned medium approach. However, for such a medium to become a serious contender to replace conventional alloy or multilayer media, an economically viable manufacturing process will have to be developed [32,33]. It should be emphasized that due to the advantageous nature of perpendicular recording in promoting extremely high areal bit densities (high write field amplitude, well aligned medium, sharp field gradients, absence of demagnetizing field at transitions, etc.), the future technologies such as mentioned above HAMR and recording on a patterned medium, are likely to be developed as extensions of perpendicular magnetic recording schemes [31] rather than to be based on conventional longitudinal recording.

Chapter 2 Physics of Writing

Chapter 2

Physics of Writing

1. Introduction

After fierce struggles to extend the life of longitudinal magnetic recording as the main technology for another couple of years, the data storage industry is finally coming to terms with reality. Reality tells that the areal density in cutting-edge laboratory demonstration systems is limited by thermal instabilities in the longitudinal magnetic media [34]. Recent high areal density demonstrations of perpendicular recording clearly demonstrate the strong interest of the data storage industry in this alternative technology today [35,36,37,38]. Compared to the conventional longitudinal recording mode, it is believed that perpendicular recording is capable of deferring the superparamagnetic limit to a substantially higher areal density due to the thicker recording layer and/or the use of a soft underlayer (SUL) [39]. Although perpendicular recording is certainly the closest alternative to the conventional technology, its novelty also brings up new issues, not ever encountered in longitudinal recording. These issues have to be well understood before the technology can be fully and most efficiently implemented. Major questions related to perpendicular media and perpendicular playback and writing heads have been previously considered. However, relatively little attention has been given to the writing process at areal densities beyond 100 Gbit/in2. For example, the role of soft magnetic shields in the writing process is still an unresolved question: although the use of soft shields around the main pole of the writing head certainly increases the field gradient, its influence on the magnitude of the recording field is still controversial. Another fundamental question is the role of the soft underlayer in the writing process. These and many other questions associated with the writing process need to be considered altogether for the most efficient design of the write head. Therefore, the intention of this Chapter is to analyze the writing process in perpendicular recording from the global perspective of maximizing the achievable areal density.

23

24

Perpendicular Magnetic Recording

1.1. CHAPTER OVERVIEW In this Chapter, a detailed overview of the methodology to design a write transducer for recording onto perpendicular media at areal densities beyond 1Tbit/in2 is presented. The two basic modes of perpendicular recording, single-layer recording media in combination with a ring type head and double-layer recording media with a soft underlayer in combination with a single pole head, are compared with each other theoretically and experimentally. In addition, perpendicular recording is compared to longitudinal recording from the perspective of the writing process. The system efficiency is redefined for perpendicular recording to take into account the critical role of the soft underlayer. The effects of using “soft” magnetic shields around the trailing pole are analyzed. It is shown that at least a factor of two increase in the field can be obtained at areal densities beyond 500 Gbit/in if shields are used. Such an open issue as the skew angle sensitivity in perpendicular recording is analyzed. It is shown that using “soft” magnetic shields around the trailing pole substantially improves the skew angle sensitivity. Moreover, using shields substantially improves the system efficiency and to some degree fulfils the role of the soft underlayer in perpendicular recording.

2. Different Modes of Perpendicular Recording

There are two basic modes of perpendicular recording [40]. The 1st mode utilizes a single pole head (SPH) for recording onto a double-layer perpendicular medium consisting of a recording layer and a SUL, as shown in a diagram in Figure 1a [41]. As described below, the use of the SUL is one of the most critical factors contributing to one of the bestknown advantages of perpendicular recording, which is the ability to generate a recording field of the order of 4SMs, where Ms is the saturation magnetization for the recording head material [42-43]. For comparison, in conventional recording, the maximum longitudinal recording field generated by a ring head (RH) is approximately 2SMs [44]. The ability to generate a stronger field makes it feasible to record on a medium with higher coercivity, which in turn further defers the superparamagnetic limit to a higher areal density [45]. The 2nd mode utilizes a regular RH for recording onto a single-layer perpendicular medium, as shown in a diagram in Figure 1b. Although, the 1st mode is more widely exploited due to the advantages of the SUL, it is still reasonable to start with the description of the 2nd mode, because the latter is fairly similar to the conventional longitudinal mode and, therefore, is going to be a good transitional step towards development of a structured theory of perpendicular recording. Both the longitudinal recording mode and the 2nd perpendicular recording mode rely on the utilization of a ring head along with a medium without a soft underlayer. Through the comparison of these two recording modes, some of the critical features of perpendicular recording can be made fairly apparent. Besides the two basic modes, in some cases, some kind of an intermediate mode, e.g., a RH and a medium with a SUL, or a SPH or a RH and a medium with a tilted magnetization with or without a SUL can also be preferred, as discussed below. Moreover, it is shown that substantial modifications to basic head structures are required

Chapter 2 Physics of Writing

25

for the ability to record at densities beyond 1Tbit/in2. In the following Chapters, advantages and issues associated with different recording modes are discussed in detail. Yoke

Trailing edge Coil

Written moment in media

“Gap” field

Record. layer Transition

(a)

SUL

Coil

Yoke

Trailing pole Recording medium

Fringing fields

(b)

Transition

Written moment in media

Figure 1. Diagram showing a side cross-section of a perpendicular system of (a) the 1st mode, including a SPH and a double-layer medium with a SUL, and (b) the 2nd perpendicular mode, including a RH and a single-layer recording medium.

2.1. SECOND PERPENDICULAR MODE: A RING HEAD PERPENDICULAR MEDIUM WITHOUT A SOFT UNDERLAYER

AND

A

As mentioned above, the second mode of perpendicular recording, which uses a conventional longitudinal ring head and a medium without a SUL, still remains an arena of exploration because of its resemblance to the conventional longitudinal mode and the lack of the "not-yet-understood peculiarities” of the SUL in the first mode [42]. A

Perpendicular Magnetic Recording

26

diagram showing a conventional longitudinal system is shown in Figure 2a. The only structural difference between the second perpendicular mode and the conventional longitudinal mode is in the medium magnetization orientation: the magnetization is in the plane and perpendicular to the disk plane for the longitudinal and perpendicular modes, respectively. Also, in the perpendicular mode, the medium's "easy" axis is ideally aligned in one direction (in the direction perpendicular to the disk plane), while in the longitudinal mode the "easy axes" are randomly oriented in the disk plane [46]. Coil Yoke

Recording medium

Fringing fields

(a)

Transition

Written moment in media

Drive coil

TH

P2

P1

W2 (b)

T1

G

T2

Figure 2. (a) Diagram showing a side cross-section of a typical longitudinal system, including a RH and a recording medium. (b) 3D schematic diagram of a RH.

Chapter 2 Physics of Writing

27

Because RH is a critical part of longitudinal recording, a more detailed diagram of the conventional RH is shown in Figure 2b. Although, in most practical cases, the leading pole, P1, is typically substantially wider than the trailing pole, P2, in this Section, the assumption that both poles, P1 and P2, have the same thickness, T1=T2, is used for simplicity of explanation of the key issues. Because the actual recording takes place near the trailing edge of the gap length, the effective trackwidth is dominantly determined by the width of the trailing pole, W2, and does not strongly depend on the width of the leading pole [47]. Moreover, in the past, some recording head manufacturing companies, for example, ReadRite Corporation, indeed, utilized a ring head with identical leading and trailing poles of the type shown in Figure 2b [48]. Using a specially developed magnetic force microscopy (MFM) technique to separately measure individual components generated by such a RH, the perpendicular and longitudinal field profiles at the ABS of such a RH were directly measured, as shown in Figure 3a [49]. The crosssections of these field profiles along the central line in the track direction are shown in Figure 3b.

P2 GAP

P1

(a)

Hx and Hz (au)

1.0 0.5

Ti/CoCrPt

0.0

-0.5

Cr/CoCrPt

-1.0 -10

-5 0 5 Axis along the track, X (Pm)

10

(b) Figure 3. (a) MFM images of the perpendicular and longitudinal field profiles taken at the ABS of a RH with a 200 nm gap length. (b) The cross-sections of these field profiles taken along the central line in the track direction.

Perpendicular Magnetic Recording

28

In general, the RH structure has been widely studied in its association to longitudinal recording, and there is plenty of literature, which contains more detailed information about the RH design. In this work, the authors only discuss the aspects of the RH design, which are of interest for perpendicular recording. Before going into details of the head design analysis, it is worth reminding that traditionally, the Karlqvist’s two-dimensional (2D) model has been utilized for describing the magnetic properties [50]. However, today, as the areal density reaches the point, at which the trackwidth becomes fairly small, 2D calculations cannot give sufficient accuracy. Therefore in this Chapter, results of 3D calculations made with boundary element model (BEM)–based commercial field solver Amperes are shown [51].

1.0 G = 30 nm

Hz

0.5 Hx 0.0 Gap region

-0.5

W2 = 200 nm PT = 500 nm

-1.0 -1000 -500 0 500 1000 Down the track (nm)

(a)

Hx and Hz (1/2SMs)

Hx and Hz (1/2SMs)

2.1.1. Gap Length Dependence The 3D calculated along-the-track (X-) and perpendicular (Z-) field components for a RH without a SUL at saturation for a set of 4 values of the gap length, 30, 70, 150, and 500 nm, are shown in Figures 4a-d, respectively. In these calculations, value for the flying height was 5 nm, and the trackwidth and the pole thickness were modeled to be 200 and 500 nm, respectively. Nevertheless, the efficiency depends on the gap length exactly as in longitudinal recording [44,52]. The dependence of the system efficiency on the gap length is reflected in the saturation current dependence on the gap length, as shown in Figure 5. The normalization factor, NF, necessary for determining the exact drive current value depends on specific head parameters, including its dimensions and the location of the drive coil with respect to the ABS [52]. The saturation current is determined as the current at which the recording field under the gap center at a 5 nm flying height starts to saturate. Going back to the description of Figures 4a-d, with a gap length of 70 nm, the chosen parameters approximately correspond to areal density of 50 Gbit/in2.

1.0 0.5 0.0 -0.5

G = 70 nm Hz

Hx

Gap region W2 = 200 nm PT = 500 nm

-1.0 -200 -100 0 100 200 Down the track (1/fly height)

(b)

1.0 0.5 0.0 -0.5

G = 150 nm

Hz

Hx Gap region W2 = 200 nm PT = 500 nm

-1.0 -1000 -500 0 500 1000 Down the track (nm)

(c)

Hx and Hz (1/2SMs)

Hx and Hz (1/2SMs)

Chapter 2 Physics of Writing

1.0

29

G = 500 nm Hz

0.5 Hx 0.0 -0.5

Gap region W2 = 200 nm PT = 500 nm

-1.0 -1000 -500 0 500 1000 Down the track (nm)

(d)

Figure 4. Longitudinal and perpendicular field components versus the distance down the track for a RH with a trackwidth of 200 nm, a pole thickness of 500 nm at four values of the gap length, (a) 30 nm, (b) 70 nm, (c) 150 nm, and (d) 500 nm.

Although, in practice, both field components, in-plane and perpendicular, simultaneously influence each recording event, ideally, the perpendicular and longitudinal field components reflect the perpendicular and longitudinal recording modes, respectively. From the plots, it can be seen that the longitudinal field component is fairly well localized in the gap region. In this case, the field near the trailing edge of the gap produces recording. As a result, by having the gap length sufficiently small, a fairly sharp field profile and fairly large areal densities can be produced. However, there is a limit to reducing the gap length. As the efficiency increases with the gap reduction, the less flux leaks out through the gap region, thus resulting in the weaker recording field. Eventually, the recording field becomes too small for overcoming the medium coercivity field. For example, in this particular case, this trade-off value of the gap, below which the longitudinal field component starts to drop, is in the vicinity of the 70 nm value, as seen from Figures 4a-d. The tradeoff value is mostly determined by the flying height and the trackwidth. The scenario is different for the perpendicular field component, for which a fairly large value can be noted far beyond the gap region. As a result, in this case, recording is produced not by the field in the immediate vicinity of the gap region, but rather by the field near the trailing edge of the trailing pole, as long as the field near the trailing edge is larger than the coercivity field [53]. Also, it can be noted that the perpendicular field component at saturation even increases as the gap length is increased in the considered range. This is caused by the reduction of the longitudinal field contribution into the net flux as the gap increases and thus making the net field dominantly perpendicular. It can be noted that unlike in longitudinal recording, the maximum field and the trailing field gradient are defined not only by the physical gap length but also to substantial degree by the trailing pole tip geometry. However, for the both systems, the efficiency fairly strongly depends on the gap length because of the use of a RH. For any recording mode, for which a RH is utilized with a medium without a SUL, transitions are produced by the fields that fringe out from the gap of the "closed" magnetic loop of the RH [54]. In other words, the gap region becomes a

Perpendicular Magnetic Recording

30

part of the magnetic flux loop, and therefore the efficiency of the loop strongly depends on the gap region. The dependence of the efficiency on the gap length is proportional to the dependence of the saturation current on the gap length. Assuming that the saturation current is defined as the current at which the longitudinal field at the center of the gap reaches 2SMs, the calculated saturated current versus the gap length is shown in Figure 5.

Imax (au)

200 150 100 50

0

200 400 Gap (nm)

Figure 5. The maximum field current versus the gap length.

1.0

PT = 100 nm

0.5 0.0

Hz

Hx

Gap region G = 70 nm W = 100 nm

-0.5

-1.0 -1000 -500 0 500 1000 Down the track (nm) (a)

Hx and Hz (1/2SMs)

Hx and Hz (1/2SMs)

2.1.2. Trailing Pole Thickness Dependence The calculated field components for a gap length of 70 nm and a trackwidth of 200 nm are shown for a set of three values of the pole thickness, 100, 200, and 500 nm, in Figures 6a-c, respectively. It can be noted that although the longitudinal field component does not vary substantially as the pole thickness is increased from 100 to 500 nm, the perpendicular component increases more than by a factor of three. Moreover, while the perpendicular component is noticeably smaller than the longitudinal component at the smallest value of the pole thickness, i.e. 100 nm, it becomes comparable to the longitudinal component as the pole thickness in increased to 500 nm. 1.0

PT = 200 nm Trailing edge

0.5 0.0

Hx

Hz Gap region G = 70 nm W = 100 nm

-0.5

-1.0 -1000 -500 0 500 1000 Down the track (nm)

(b)

Hx and Hz (1/2SMs)

Chapter 2 Physics of Writing

1.0 PT=500 nm

Trailing edge

0.5 0.0

31

Hz

Hx Gap region

-0.5

G = 70 nm W = 100 nm

-1.0 -1000 -500 0 500 1000 Down the track (nm) (c) Figure 6. Longitudinal and perpendicular field components versus the distance down the track for a RH with a trackwidth of 100 nm, a gap length of 70 nm at three values of the pole thickness, (a) 100 nm, (b) 200 nm, and (c) 100 nm.

In general, it could be noted that with respect to the recording field, the 2nd perpendicular mode is quantitatively similar to the longitudinal mode. In both cases, the maximum field never exceeds 2SMs of the head material. Previously, the implementation of the RH writer in combination with perpendicular media with a SUL has also been published and furthermore, the related material is going to be presented in this Chapter [55]. In Section The First Perpendicular Mode: A SPH and a Perpendicular Medium With a SUL, it is shown that the perpendicular recording field can be increased by at least a factor of two, i.e. can reach 4SMs, if a medium with a SUL is utilized. 2.2. FIRST PERPENDICULAR MODE: A SINGLE POLE HEAD AND A PERPENDICULAR MEDIUM WITH A SOFT UNDERLAYER As shown in Figure 1a, besides the presence of a SUL, the first mode is different from the second mode also in the type of the recording head: it is a SPH instead of a RH. Unlike the RH, the SPH, utilized in combination with the SUL, has a physical gap that is substantially larger than the flying height. The purpose of the large gap is to force the magnetic flux to flow through the SUL rather than through the gap region, thus to enhance the perpendicular component of the magnetic field. Therefore, the SUL is an indispensable part of the recording head, as well as it is of the recording medium. 2.2.1. Magnetic Image Model It is convenient to use the so-called "magnetic image" model for more transparent description of recording processes in a system with a SUL [53]. According to this model, the SUL is replaced with a half-space, which contains a mirror image of the recording head, as shown in a schematic diagram in Figure 7. Such replacement is theoretically justified provided the SUL can be approximated to be ideal [56]. According to a theorem of differential equations, the Laplace’s Equation (a consequence of the Maxwell’s Equations, convenient to use for the calculation of the magnetic field) has an

Perpendicular Magnetic Recording

32

unambiguous solution if adequate boundary conditions are satisfied [57]. It appears that in the above case with an ideal SUL the boundary conditions at the SUL top surface are the same as in the case with a mirror half-space provided that the magnetic "charge" reverses its polarity when reflected into the mirror half-space. Together with the image head, there are effectively two heads involved in each recording event, thus the net recording field becomes fairly large, as compared to the equivalent longitudinal case, as discussed below. 2.2.2. Permanent Magnet Approximation The fastest way to estimate a magnetic field generated by a SPH at saturation is probably to use the permanent magnet approximation. In this approximation, the SPH is presented as an infinitely long vertical magnetic bar with finite cross-section dimensions, W (trackwidth) and T (thickness), with its magnetization aligned (saturated) along the vertical axis. In such scenario, the magnetic field components can be directly calculated using, for example, the equivalent “charge” model [58]. Thus derived formulas for a saturated SPH without the presence of a soft underlayer are shown by Expressions 1a to c. Because of the problem symmetry, it is sufficient to calculate the field in one coordinate quadrant, x > 0 and y > 0.

Real head Coil “Gap” fields Physical Gap

Effective Gap

SUL boundary

(a) Image head

Chapter 2 Physics of Writing

33

(b) Figure 7. (a) A cross-section diagram and (b) a 3-d drawing showing a mirror image representation of a perpendicular system with an ideal soft underlayer.

H

H

x

­ ª b ° « M s ° « y 2  ®ln « b 2 ° « ° «¬ y  2  ¯

2 2 §¨ x  a ·¸  §¨ y  b ·¸  z 2 2¹ © 2¹ © 2 2 §¨ x  a ·¸  §¨ y  b ·¸  z 2 2¹ © 2¹ ©

ª º » « y b  2 « » »  ln « « y b  » »¼ «¬ 2

2 2 §¨ x  a ·¸  §¨ y  b ·¸  z 2 2¹ © 2¹ © 2 2 §¨ x  a ·¸  §¨ y  b ·¸  z 2 2¹ © 2¹ ©

º½ » ° (1a) »° »¾ »° »¼ ° ¿

y

­ ª ° « a M s °° « x  2  ®ln « 2 ° « a ° « x 2  °¯ ¬

2 2 §¨ x  a ·¸  §¨ y  b ·¸  z 2 2¹ © 2¹ © 2 2 §¨ x  a ·¸  §¨ y  b ·¸  z 2 © 2¹ © 2¹

º ª » « x a  » « 2 »  ln « » « x a  » 2 «¬ ¼

2 2 §¨ x  a ·¸  §¨ y  b ·¸  z 2 2¹ © 2¹ © 2 2 §¨ x  a ·¸  §¨ y  b ·¸  z 2 2¹ © 2¹ ©

º½ » ° (1b) » °° »¾ »° »¼ °° ¿

Perpendicular Magnetic Recording

34

H

M

s 4

z



M

­ ª ° « ° « ° «  1 ° « ® tan « ° « ° «z ° « ° ¬ ¯

s 4

­ ª ° « ° « ° « 1  ° « ® tan « ° « ° «z ° « ° ¬« ¯

º » » x a y b » 2 2 »  » 2 2 » § a· § b· 2 »     x y z ¨ ¸ ¨ ¸ » © 2¹ © 2¹ ¼

 

ª « « «  1 tan «« « «z « ¬

º » » §¨ x  a ·¸§¨ y  b ·¸ » © 2 ¹© 2 ¹ »  » 2 2 » a b · · § § 2 » ¨ x  ¸ ¨ y  ¸  z » © 2¹ © 2¹ ¼»

º » » x a y b » 2 2 » » 2 2 » § a· § b· 2 »     x y z ¨ ¸ ¨ ¸ » © 2¹ © 2¹ ¼

ª « « « 1  tan «« « «z « ¬«

 

½ ° ° ° ° ¾  ° ° ° ° ¿

º » » §¨ x  a ·¸§¨ y  b ·¸ » © 2 ¹© 2 ¹ » » 2 2 » § a· § b· 2 » ¨ x  ¸ ¨ y  ¸  z » © 2¹ © 2¹ ¼»

½ ° ° ° ° ¾ ° ° ° ° ¿

.

(1c)

The origin of the coordinate system is at the center of the pole tip air-bearing surface (ABS) with the vertical axis, Z, directed downward, as shown in Figure 8. Moreover, the presence of the SUL can be simply taken into account through using the described above "magnetic image model." In other words, the same expression can be utilized to calculate the extra recording field due to the image head located at the other side of the recording layer. The spacing difference between the real and image heads is equal to the recording layer thickness plus the separation between the recording layer and the SUL. The sum of the two fields gives the total recording field.

Y

Main Pole Tip

X W Recording layer

G

Z

Figure 8. A diagram showing the location of the origin of the coordinate system utilized in the calculations.

Chapter 2 Physics of Writing

35

2.2.3. Recording By The Field In The Gap (Perpendicular) Versus Recording By The Field Fringing From The Gap (Longitudinal) When using the “magnetic mirror" image model, besides the physical gap length, also, the effective (magnetic) gap length can be introduced [59]. The effective (magnetic) gap, defined as the spacing between the air bearing surfaces (ABS’s) of the real and image heads, i.e. the two-fold separation between the ABS and the SUL, can be meaningfully compared to the physical gap of the RH [53]. It can be noticed that the SPH considered together with its image resembles the RH rotated 90 degrees around the axis along the cross-track direction, with the difference that the recording is produced in the “gap” itself [60]. In contrast, in the longitudinal case as well as in the case of the 2nd perpendicular mode, the field fringing from the gap region produces recording, as shown in Figure 9. Any system exploiting a RH without a SUL is intrinsically built so that for the system to be efficient, the gap length should be fairly small. It should be reminded, that the more efficient a system is, the smaller amount of the magnetic flux leaks out on its way from the source (drive coil) to its destination (ABS). Consequently, substantial amount of the magnetic flux just circulates in the magnetic ring yoke without being exploited for the recording purpose itself, and, as noticed above, only the fringing field produces the actual recording. Typically, the maximum fringing field, which can be achieved in a recording system of this type, is less than 2SMs, where Ms is the saturation magnetization of the head material [54]. This limits the coercivity of a longitudinal medium, on which the recording head can record [61]. On the contrary, it is due to the recording by the field in the “gap” region why the use of the SUL in the 1st perpendicular mode provides such a drastic increase in the recording field at saturation. The calculated perpendicular and longitudinal field components for a SPH with a gap length, G, of 1000 nm, a pole thickness, PT, of 500 nm, and a trackwidth, W, of 100 nm at saturation are shown in Figure 10. It can be noticed that in this case the maximum perpendicular field is of the order of 4SMs. This allows writing on a medium with a higher anisotropy field. The anisotropy field defines the field, which needs to be applied for switching the magnetization in the recording layer. In turn, the higher anisotropy medium means the higher density, to which the superparamagnetic limit can be deferred. Real SPH

RH

Fields in the Gap Medium

(a)

Image SPH

Fields fringing from

(b) the Gap

Figure 9. Schematic diagrams showing (a) recording by the field in the "gap" in perpendicular recording and (b) recording by the fringing field in longitudinal recording.

Perpendicular Magnetic Recording

36

Hx and Hz (1/2SMs)

At this point, the SUL is assumed to be ideal. Also, the default modeling settings included a physical gap length of 1000 nm, a trackwidth of 100 nm, and a throat height of 500 nm, with a 20-nm separation between the ABS and the SUL. It can be noticed that for the 1st perpendicular mode, the field profiles are qualitatively similar to the field profiles for the longitudinal mode, as shown in Figure 4, provided that the field components are exchanged with each other according to the transformation Hx o Hy and Hy o -Hx [60]. However, as previously mentioned, from the quantitative perspective, due to the use of the SUL the maximum perpendicular field in perpendicular recording is approximately by a factor of two larger than the maximum longitudinal field in longitudinal recording.

2.0 G = 1000 nm 1.5 1.0 0.5

Trailing edge

Hz

Hx

0.0 -0.5 -1.0

Pole region

0

W = 100 nm PT = 500 nm

750 1500 2250 Down the track (nm)

Figure 10. Longitudinal and perpendicular field components versus the distance down the track (along the central line) for a SPH with a gap length, G, of 1000 nm, a pole thickness, PT, of 500 nm, and a trackwidth, W, of 100 nm.

2.2.4. Is the Increase of the Recording Field due to the Use of a SUL Sufficient for Adequate Recording? Indeed, the use of a soft underlayer provides a two-fold increase of the recording field component, as compared to the conventional longitudinal recording mode [42]. However, this comparison of the two recording modes is not equivalent. Above, it was shown that in the longitudinal mode, in the gap region, besides the longitudinal field component, there is also a substantial perpendicular component. For example, for a typical gap length of approximately 150 nm, as shown for the case in Figure 4c, both longitudinal and perpendicular components reach approximately the same value, i.e. 2SMs of the head material. On the contrary, in the perpendicular mode, the maximum longitudinal field component is substantially smaller than the perpendicular field component. For example,

Chapter 2 Physics of Writing

37

for the case shown in Figure 10, the perpendicular component almost reaches 4SMs, while the longitudinal component is substantially less than 1SMs, i.e. the different is almost by a factor of four. As a result, the actual recording field is directed at angle values of approximately 45 and 15 degree with respect to the medium magnetization for the longitudinal and perpendicular modes, respectively. From the idealistic StonerWohlfarth model, the switching field differs from the anisotropy field depending on the angle between the recording field and the "easy" axis [62]. Moreover, the switching is expected to be substantially easier for the 45-degree case, as compared to the 15-degree case. Another difference between these two recording modes results from the different nature of the recording medium. For the perpendicular case, the magnetization is aligned in one direction, i.e. the direction perpendicular to the disk plane, while, for the longitudinal case, the magnetization is directed randomly in the disk plane. Therefore, although the realistic recording media might be substantially different from the idealistic Stoner-Wohlfarth model, for the fair comparison of the two recording modes, all the described factors should be taken into account in more precise calculations. It is not done in this Chapter, because the purpose is to describe the main concepts that help distinguish perpendicular recording. However, it could be noted that the second perpendicular mode, i.e. the mode without a soft underlayer, is based on the use of the ring type head, similar to the longitudinal mode, with all the advantages resulting from the larger torque angle between the recording field and the magnetization. This similarity to the longitudinal mode makes the implementation of the second mode more straightforward, as compared to the implementation of the first mode. Therefore, the second mode should not be totally ignored. 2.2.5. Quadruple Ratio between Saturation Currents in Perpendicular and Longitudinal Recording Another advantage of perpendicular recording that can be noted from the mirror image model is the fact that due to the SUL the effective number of the current sources is effectively doubled (see Figure 8). As a result, the perpendicular system needs approximately only half as much current to generate the same magnetic field in the effective gap, as compared to an equivalent longitudinal system. Below (in the following Section), it is shown that in the perpendicular case recording is produced in the effective gap region. This is unlike the longitudinal mode, for which recording is produced by the field fringing from the gap. Because the field fringing from the gap is about only a half of the field in the gap and the effective number of the drive current sources in the perpendicular system is twice as many as in the longitudinal system, for the perpendicular system it takes approximately four times less drive current to generate the same recording field as in the longitudinal system, with the other conditions equivalent. Although such a fairly rough estimate does not take into account any non-linear effects that can take place in a recording system, it provides a good sense for the saturation currents in the two systems. As an example, the maximum recording fields generated by RH and SPH versus the drive current are shown in Figure 11.

Perpendicular Magnetic Recording

38

Hx and Hz (1/2SMs)

2.0

SPH: Hz

1.5 1.0

RH: Hx

0.5 0.0 -10 0

10 20 30 40 50 60 70 Drive Current (au)

Figure 11. The maximum recording fields for a RH and a SPH, each with a trackwidth of 500 nm, and a RH with a throat height of 500 nm, a gap length of 70 nm, and a SPH with a gap of 1000 nm, an ABS to SUL separation of 35 nm, a throat height of 250 nm.

In this calculation, each of the two heads was assumed to have the same trackwidth of 500 nm. The RH was modeled with a gap length of 70 nm and a throat height of 500 nm, while the SPH was modeled with a pole thickness of 500 nm, a gap length of 1000 nm, an ABS to SUL separation of 35 nm, a throat height of 250 nm. It can be noted that the linear region slope for the SPH is almost four times as large as the linear region slope for the RH. If recorded on media with the same coercivity field, the saturation current for a perpendicular system should be expected to be four times as less as it is for an equivalent longitudinal system. 2.2.6. Focused-ion-beam Trimmed Single Pole Heads Using FIB trimming of regular relatively large RH’s or/and SPH’s, it is possible to fairly economically fabricate a set of individual recording SPH's, with a required set of parameters, including the trackwidth, the pole thickness, the gap length, the throat height, the shape of the leading (trailing) edge, and others [63]. Study of the FIB-fabricated devices could give a good insight into the operation of realistic magnetic devices. 2.2.7. Example 1:FIB Trimming of a Wide-track Censtor SPH into a Narrow-track SPH By courtesy of Censtor Corporation, relatively wide SPH's (approximately, with a 1-Pm trackwidth) were available for further modification via FIB trimming [64,65]. The modification included the reduction of the trackwidth down to approximately 100 nm. Scanning electron microscope (SEM) image of a FIB-fabricated 120-nm wide SPH is shown in Figure 12.

Chapter 2 Physics of Writing

39

Track direction, X

Probe ABS

Two side FIB-made trenches Figure 12. SEM image of a FIB trimmed Censtor head (ABS view at a 20-degree tilt).

MFM images of the perpendicular and longitudinal components of the field generated at the ABS of this head with a drive current in the over-saturated regime (above 1000 mA˜turn) are shown in Figures 13a and b, respectively. The central cross-sections of these field component profiles are shown in Figure 13c. Although, there is no SUL in this case, the symmetry of the measured field profiles look similar to the symmetry of the modeled profiles with a SUL, as shown in Figure 10. As mentioned above, the SUL has mostly a quantitative effect and thus does not substantially change the shape of the field profile. 2.2.8. Example 2:FIB Trimming of a RH into a Narrow-track SPH By courtesy of IBM Corporation, relatively wide track RH's (approximately, with a 1-Pm trackwidth) were available for further modification via FIB trimming. The modification included not only the reduction of the trackwidth down to approximately 60 nm, but also the increase of the gap length from its original value of 150 nm to the required value of approximately 1 Pm. Scanning electron microscopy (SEM) image of thus FIB-fabricated 60 nm wide SPH with a 1-Pm gap length is shown in Figure 14.

Perpendicular Magnetic Recording

40

0 nm

400 nm 0 nm (a)

400 nm (b)

MFM signal (au)

0.8 0.6 0.4 0.2

Perpendicular

0.0 -0.2 -0.4

Longitudinal

-0.6 -200

-100 0 100 200 Distance down the track (nm) (c)

Figure 13. MFM images of the (a) perpendicular and (b) longitudinal field components generated by a FIB trimmed Censtor head with a trackwidth of 120 nm and a pole thickness of 200 nm. (c) Cross-section profiles of the perpendicular and longitudinal components.

Chapter 2 Physics of Writing

W

=

m n 0 6

41

Main pole tip

G = 1000

Figure 14. SEM image of a SPH FIB-made from a RH (ABS view at a 40-degree tilt).

A MFM image of two adjacent 65 nm wide tracks with periodic sets of transitions recorded onto a CoCr-based perpendicular medium with a SUL, as shown in Figure 15, clearly indicates the functionality of thus fabricated SPH despite its nanoscale size trackwidth. It should be reminded that there is a general concern that as the SPH pole tip dimensions are reduced to sizes substantially less than the characteristic domain wall width in the soft material of which the pole tip is made of, the magnetization not only might become fairly "hard" to switch but also might display substantially non-zero remanence [66]. A more detailed analysis of this issue is presented below.

130 nm

Figure 15. MFM image of two adjacent 65 nm wide tracks recorded onto a CoCr-based perpendicular medium with a SUL.

Perpendicular Magnetic Recording

42

2.2.9. Single Pole Head: Design Strategy In this Section, a more detailed description of the SPH structure is presented with the purpose to explain the approach chosen to design the SPH geometry, as shown in Figure 16, and thus clarify the limitations of this head design and motivate an approach for future modifications. The limitations are fundamentally caused by the inability to infinitely maintain the linear scaling of the system dimensions (for increasing the areal density) below the value, at which the flying height reaches its smallest value that is physically feasible. It is believed that it is unlikely to be able to maintain a steady flying height below approximately 5 nm because of the proximity to the size of the air molecules critically participating in the recording head flying process. Therefore, a deviation from the straightforward scaling law is necessary for further increasing the areal density. This deviation can be accomplished through the modification of the SPH design. Hence, the understanding of the principles utilized to design SPH geometry shall make SPH modifications most efficient for satisfying the demand for the areal density increase.

Drive coil

I Leading pole

TH TL

G

M

Main pole

W T ABS

Figure 16. A schematic diagram of a SPH.

Before going into details, it is worth reminding the major requirements towards a write head in perpendicular recording: 1)the ability to generate a sufficiently strong field for recording onto a medium with adequate coercivity, 2)the ability to generate sufficiently large trailing and side field gradients for recording sufficiently sharp transitions and narrow tracks, respectively, 3)the ability to localize the recording field in a fairly limited region along the track so that the skew angle sensitivity is minimized (see Section Skew Angle Sensitivity), 4)the ability to maintain reasonable efficiency of a recording system.

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43

Below, the analysis of the parameters, which influence the above-listed requirements, is presented. Before going into a description of the design methodology, it is worth reminding that already today the flying height in every state-of-the-art recording system is of the order of 5 nm, which is already close to the size of the air molecule. Therefore, it is hard to see how the flying height can be further reduced, because the air flow process is a critical link in the ultimate operation of a magnetic hard-drive. This means that the established over decades trend of increasing the recording density in a magnetic recording system only through the direct application of the scaling law should be adjusted for creating next generation magnetic technologies. In other words, for obtaining the maximum benefit and achieving the highest possible areal density, special attention should be given to each component of the magnetic recording system. 2.2.10. Definition of Efficiency Before going into details of the design, such basic quality indicator as the efficiency of a recording system should be redefined for perpendicular recording [44]. In longitudinal recording, the efficiency is the ratio of the magnetic flux generated in the deep gap of the RH and the flux in the drive coil 52. As mentioned above, for perpendicular recording it is not the physical gap but rather the effective (magnetic) gap, defined as the separation between the SPH and its image, is equivalent to the physical gap of the RH. Therefore, it makes sense to define the efficiency, K, of a magnetic system of the 1st perpendicular mode as the ratio of the magnetic flux in the magnetic gap (the flux under the pole tip ABS) and the flux in the drive coil, as shown in Figure 17, K = Bgap˜ Agap / N˜I,

(2)

where Agap is the deep gap cross-section area.

Longitudinal “Circuit”

Perpendicular “Circuit”

I Bdrive Bdrive

SUL boundary

I

SPH Bgap

Bgap

Image

RH

Figure 17. Diagrams showing magnetic “circuits” in a longitudinal system with a RH and a perpendicular system with a SPH and a SUL.

Perpendicular Magnetic Recording

44

2.2.11. Throat Height Dependence The throat, being the narrowest part of the magnetic flux loop (circuit), typically, is also the highest reluctance link of the magnetic loop [67]. Thus, by reducing the throat height, the relative contribution of the throat region into the net reluctance of the magnetic circuit is also reduced and therefore, the overall efficiency of the system is increased. Also, as described below, by reducing the throat height, the recording field at saturation is increased. There are two competing factors contributing into the increase of the recording field as a result of the throat height reduction. First, the field is increased because as a result of the throat height reduction the point inside the pole tip at which the saturation starts to occur is shifted closer to the ABS. Calculated magnetization contours along the central cross-track planes inside the main pole tip in two extreme cases, with fairly tall and sufficiently short throats, are shown in Figures 18a and b, respectively. The magnetization profiles at saturation along the central vertical line inside the pole tip for these two cases are shown in Figures 18c. It can be observed that for the tall throat, the saturation occurs near the top region of the throat, thus only a relatively small part of the initial magnetic flux generated by the drive coil reaches the ABS. As the current is increased beyond the saturation value, the most of the flux is going to leak out from the magnetic loop on its way from the drive coil to the ABS. In contrast, for the short throat, the saturation starts to take place at the ABS, thus the maximum possible flux reaches the ABS and therefore the maximum possible field (for a flat surface, of the order of 4SMs) can be generated. In other words, in the latter case, there is effectively more magnetic "charge" generated at the ABS. The "charge" at the ABS is the required source of the recording field.

Saturated region

2 kOe

10 kOe

19.6 kOe

Saturated region

19.8 kOe

TH

TH

20 kOe

+++

“charge” at the ABS (a)

20 kOe

+++++++

“charge” at the ABS

(b)

Mz/Ms

Chapter 2 Physics of Writing

1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3

45

500 nm

100 nm

ABS

0 200 400 600 Along the central line inside the Pole Tip (nm)

(c) Figure 18. Calculated magnetization contours along the central cross-track planes inside the main pole for two extreme cases at saturation: (a) a fairly tall throat and (b) a sufficiently short throat. (c) Magnetization profiles along the central vertical line for the two cases at saturation.

Because the charge is located at the ABS, the ABS dimensions of the pole tip determine the recorded bit sizes. Therefore, being local by its origin, this is a favorable effect of the throat height reduction. Unfortunately, the throat height reduction results in another effect, which deteriorates the field gradients. This effect is due to the "charges" created on the tilted walls above the throat height of the main pole, as shown in Figure 19. These "charges" generate an extra field, which is not localized and therefore results in the deterioration of the field gradients, as shown below. As the throat height is reduced, the "charge" at tilted walls is effectively moved closer to the ABS, and thus, the effective contribution of this unfavorable field increases. It should be remembered that although a perpendicular medium is ideally symmetric with respect to any of the two in-plane directions, i.e. along and across the track, a typical SPH, as shown in Figure 19, is not [68]. Because of fabrication process limitations, typically, the throat top boundaries (the line at which walls starts to deviate from being vertical) are defined only at the two cross-track side walls of the main pole, and not at any of the two along-track side walls, as shown in Figure 19. It should be reminded that the magnetic "charge" is proportional to the change of the magnetization component normal to the boundary surface [69]. Therefore, in the particular case, the magnetic "charge" is concentrated on the cross-track sides rather than on the leading and trailing sides of the main pole. As a result, because of the different amount of the "charge" in these two cases, the throat height dependencies might be quantitatively different for the field profiles along and across the track, respectively, as shown below.

Perpendicular Magnetic Recording

46

Trailing side wall M ++

LS

++

M

+

Cross-track side wall

++ + ++ + Magnetic + “charges” on side walls TH

PT

W

Figure 19. A diagram of a SPH pole tip showing location of side wall “charge."

1.5 1.0

TH = 100 nm

TH = 100 nm

Trailing edge 500 au 200 100 50

1.0

Hz / Hz 0

Hz (10 x kOe)

2.0

500 au 0.5

0.5 0.0

0.0

0.0 0.2 0.4 0.6 0.8 1.0 Distance down the track (Pm)

(a)

Trailing edge

200 100 50

0.0 0.2 0.4 0.6 0.8 1.0 Distance down the track (Pm)

Chapter 2 Physics of Writing

TH = 500 nm 1.0

Trailing edge

1.5

500 au 200 100 50

1.0 0.5 0.0

TH = 500 nm

Hz / Hz 0

Hz (10 x kOe)

2.0

47

Trailing edge 500 au 200 100 50

0.5

0.0

0.0 0.2 0.4 0.6 0.8 1.0 Distance down the track (Pm)

0.0 0.2 0.4 0.6 0.8 1.0 Distance down the track (Pm)

(b) Figure 20. Along-track profiles of the perpendicular field component and its normalized value for two values of the throat height, (a) 100 and (b) 500 nm.

The along-track profiles of the perpendicular field component and its normalized value at a 5 nm flying height and a 20 nm separation between the ABS and the SUL at different values of the drive current (in the arbitrary units) for two values of the throat height, 100 and 500 nm, are shown in Figures 20a-b, respectively. In this case, the side-wall tilt angle, M, as shown in Figure 19, was modeled to be 45 degrees. The perpendicular fields and their normalized values for the same set of parameters across the track are shown in Figures 21a-b, respectively. As expected, it is observed that although it is easier to drive more recording field in the case of the shorter throat, the undesirable off-track side field also increases due to the increased contribution from the "charge" at the tilted sidewalls. To explicitly illustrate this effect, two cross-track perpendicular field profiles corresponding to the two throat height values at saturation are put together in Figure 22a. The same profiles normalized to the corresponding values at the center of the track are shown in Figure 22b. The normalized profiles directly illustrate the fact that the shape of the field profile is substantially wider in the shorter throat height case.

Hz (1/2SMs)

1.5

TH=100nm

100

50

TH=100nm 500 au 200

500 au 200

1.0

1.0

Hz / Hz 0

2.0

100 0.5

50

0.5 0.0 0.0

0.0 0.0

0.1 0.2 0.3 0.4 Distance across the track (Pm)

(a)

0.1 0.2 0.3 0.4 Distance across the track (Pm)

Perpendicular Magnetic Recording

48

2.0

1.0

1.0

1000 au 500 200

Hz / Hz 0

Hz (1/2SMs)

1.5

TH=500nm

TH=500nm

100

0.5

1000 au 500 200 100

0.5 0.0 0.0

0.0 0.0

0.1 0.2 0.3 0.4 Distance across the track (Pm)

0.1 0.2 0.3 0.4 Distance across the track (Pm)

(b) Figure 21. Cross-track profiles of the perpendicular field component and its normalized value for two values of the throat height, (a) 100 and (b) 500 nm.

1.6 Hz (10 x kOe)

1.4 1.2 1.0 0.8

TH = 100 nm

0.6 0.4 0.2 TH = 500 nm 0.0 0.1 0.2 0.3 Distance across the track (Pm) (a)

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49

Normalized Hz (Hz/Hz o)

1.2 1.0 0.8 0.6 TH = 100 nm 0.4 0.2 TH = 500 nm 0.0 0.1 0.2 0.3 Distance across the track (Pm) (b) Figure 22. (a) The cross-track profiles at saturation for two values of the throat height, 100 and 500 nm. (b) The normalized profiles at saturation.

The field 5 nm below the center of the main pole versus the drive current at three values of the throat height, 100, 200, and 500 nm, is shown in Figure 23a. The drive current is given in arbitrary units because the exact value of the current depends on a number of specific to each head design parameters, such as the exact location of the drive coil with respect to the ABS, the yoke geometry, etc. The saturation current can be defined at the value, at which the first discontinuity (change of the slope) in the field dependence on the current takes place. Thus derived saturation current (reflecting the system efficiency) versus the throat height is shown in Figure 23b. In summary, it can be noticed that reduction of the throat height has two favorable effects, the increase of the recording field and the reduction of the saturation current. However, the throat height can not be reduced entirely to zero, because the smaller the throat height is, the worse the side and trailing field gradients are, as noted above (see Figure 22). Here, it should be mentioned that for an ideally saturated state, the field due to the side charge could be easily calculated according to the Coulomb Law for the “magnetic” charge on the sidewalls. It can be shown that at zero throat height and at a tilt angle of 45 degree, the extra field due to the side "charge" could substantially overcome 4SMs (the maximum field assuming the pole tip with no side wall charge) provided the side wall is sufficiently tall. Considering the side nature of the source of this field, the field due to the side “charge” not only increases the field under the pole tip (on the track) but also creates an unfavorable field at the sides (off the track) and thus deteriorates the field gradient. Therefore, for minimizing the contribution due to the side “charges”, it is preferable to

Perpendicular Magnetic Recording

50

keep a sufficiently tall throat. In other words, there is a trade-off between the field magnitude and the field gradient, and this trade-off can be controlled by the throat height. TH=100 nm 200 nm

1.5

500 nm

1.0 0.5 0.0 0

200 400 600 800 1000 Drive Current (au)

(a)

Saturation Current (au)

Hz 0 (1/2SMs)

2.0

90 85 80 75 70 100 200 300 400 500 Throat Height (nm)

(b)

Figure 23. (a) The perpendicular field versus the drive current at three values of the throat heigth, 100, 200, and 500 nm. (b) The saturation current versus the throat height.

2.2.12. Dependence on the Pole Trackwidth and Thickness Another way to increase the recording field is to make each ABS cross-section dimension of the SPH pole tip (pole thickness and trackwidth) as large as possible [70]. The characteristic dimension, at which the field starts to substantially change, is determined by the doubled (due the “image” by the SUL) distance between the ABS and the SUL. The trackwidth, W, of the SPH determines how narrow a track can be recorded. Therefore, the trackwidth value is set by a required areal density value. For example, at an areal density beyond 100 Gbit/in2, the trackpitch (the trackwidth plus the guard band) should be smaller than approximately 160 nm, assuming a 4:1 bit aspect ratio (BAR). Assuming that the guard band occupies approximately a fifth part (20 percent) of the trackpitch, the SPH should have a trackwidth of approximately 120 nm for recording an approximately 130 nm wide track. As to the pole thickness, as previously mentioned, in perpendicular recording, ideally, the actual recording takes place only near the trailing edge of the pole, therefore, one can have the pole thickness as large as necessary for the maximum increase of the recording field. The maximum recording at saturation versus the pole thickness for a given trackwidth of 120 nm is shown in Figure 24a. However, in practice, as explained in Section below, the pole thickness cannot be made infinitely long because in a realistic hard-drive, the skew angle is not always zero. The non-zero skew angle results in effectively recording a substantially wider track, as compared to the trackwidth of the pole tip. As shown below, the pole thickness value of approximately 200 nm should reduce the skew angle sensitivity to few percent of the trackwidth value, assuming approximately a 10 degree maximum skew angle and areal densities below approximately 400 Gbit/ in2. At 400 Gbit/ in2 areal density, assuming a 4:1 bit aspect ratio, the recorded trackpitch should

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51

be 80 nm wide. Therefore, the trackwidth of the pole tip should be smaller than 80 nm. The maximum saturation field versus the pole trackwidth at a fixed value of the pole thickness of 200 nm is shown in Figure 24b. At this point, it is worth reminding that the image head is located effectively further away from the center of the recording layer as compared to the real head, as shown in Figure 9, with the spacing difference being equal to the recording layer thickness. Ideally, the net recording field of 4SMs can be produced as a result of the contributions of the fields generated by both, the real and image heads, respectively, with a 2SMs field per each head. Assuming a 30-nm separation between the ABS and the SUL, at such high areal densities, the trackwidth (~< 80 nm) is of the same order of magnitude as the doubled separation between the ABS and the SUL. Therefore, it is not unnatural that the net recording field starts to substantially drop as the trackwidth is further reduced.

Hz / 2SMs

1.5 1.0

W = 120 nm

0.5 0.0

0

(a)

100 200 300 Pole Thickness (nm)

Hz / 2SMs

2.0 1.5 1.0

T = 200 nm

0.5 0.0 (b)

0

100 200 300 Trackwidth (nm)

Figure 24. The maximum field at saturation (a) versus the pole thickness for a SPH with a trackwidth of 200 nm, and (b) versus the pole trackwidth at a fixed value of the thickness, 200 nm.

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52

In summary, ideally, assuming a zero skew angle, the pole thickness can be made infinitely large because the recording is produced only near the trailing edge. Nevertheless, the increase of the thickness results only in approximately 30 percent increase if the trackwidth is kept as small as 120 nm. Moreover, in real conditions with a non-zero skew, the non-zero length of the pole thickness, T, results in substantial side recording, as explained below in Section Skew Angle Sensitivity. 2.2.13. Skew Angle Sensitivity of Single Pole Head One of the most serious issues during the future implementation of perpendicular recording is believed to be the excessive sensitivity of a typical perpendicular recording system to the skew angle [41,71]. As mentioned above, unlike in longitudinal recording, for which the recording is produced by the fringing field in the physical gap region of a RH, as shown in Figure 2, in perpendicular recording, the recording is produced in the effective gap near the trailing edge of the main pole of a SPH, as shown in Figure 1. As a result, one of the principal differences is the order of magnitude difference between typical sizes of the gap region of the RH and the trailing pole thickness of the SPH.

(a)

Trailing Pole

Recorded Track

Trailing edge T2

W

W2 T

Skew angle

Side Band (b)

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53

(c) Figure 25. (a) A schematic diagram illustrating the definition of the skew angle. (b) A diagram showing the ABS of the trailing pole with a skew with respect to the track direction and transitions recorded with the skewed trailing pole, thus creating a recorded track. (c) Guide diagram and MFM images of tracks recorded by a SPH onto a CoCr-based perpendicular medium at zero skew and a 15 degree skew angle, respectively.

For the state-of-the-art recording RH and SPH suitable for areal densities of the order of 100 Gbit/in2, for example, the gap thickness and the trailing pole thickness are of the order of 50 nm and 1000 nm, respectively. Such a substantially larger thickness of the SPH pole results in its extreme sensitivity to the skew angle. To help understand this issue, the top view of the head assembly over the surface of a disk is shown in Figure 25a. This Figure also illustrates the definition of the skew angle, which is the angle between the direction of the track and the head axis of symmetry. A diagram of a track recorded by a SPH at a non-zero skew angle is shown in Figure 25b. It can be noted that at the condition of non-zero skew angle the recording is produced not only by the trailing edge but also by one of the sides of the trailing pole. For comparison, MFM images of two real tracks recorded with a SPH with a 500 nm thick pole at zero and a 15-degree skew angle onto a CoCr -based perpendicular medium are shown in Figure 25c. Consequently, it is clear that the thicker the trailing pole is, the more sensitive the system is to the skew angle. To a sufficient degree of approximation, the side written region is proportional to the product T2 x sinT, where T2 and T are the pole thickness and the skew angle, respectively, as shown in Figure 26. Assuming typical values for T2 and T of approximately 1000 nm and 10 degrees, respectively, the side written region can be of the order of 150 nm, which is unacceptable at areal densities beyond 100 Gbit/in2. It should be reminded that the entire trackwidth is expected to be less than 150 nm at such high densities assuming a 4:1 bit aspect ratio (BAR). Trailing edge P2

Track direction

T

Side written region

Figure 26. A diagram showing how the side recording is generated due to a non-zero skew angle.

54

Perpendicular Magnetic Recording

One of the “perks” about the skew angle sensitivity in perpendicular recording is its dependence on the linear density. MFM images of recording tracks written with a SPH at a 15-degree skew angle at five values of the linear density, 20, 40, 60, 80, and 100 kfci, are shown in Figures 27a to e, respectively. These images clearly illustrate the disappearance of the undesirable side written region with the increase of the linear density. The following describes another experiment illustrating the density dependence of the skew angle sensitivity. Two sets of three tracks were recorded by a SPH at zero and a 15-degree skew angle, respectively. The central track was recorded at a relatively low linear density of 25 kfci, while the two side tracks were recorded at a relatively high linear density of 250 kfci. Then, a relatively narrow (80-nm wide) read head was used to scan the tracks in the cross-track direction. Thus obtained track profiles are shown in Figure 28. Clearly, broadening of the effective trackwidth could be noticed for the central track (which was recorded at the lower linear density value). Previously, the described linear density dependence was explained by the insufficient magnetic field gradient in the side region [71].

Figure 27. MFM images of recording tracks written with a SPH at a 15 degree skew angle with linear densities of a) 20 kfci, b) 40 kfci, c) 60 kfci, d) 80 kfci and e) 100 kfci.

Chapter 2 Physics of Writing

Playback Signal (mV)

25kfci

55

Skew = 0 degrees Skew = 15 degrees

2.5 250kfci

250kfci

2.0

1.5 -50

-25 0 25 50 Offset across the track (Pin)

Figure 28. Track profiles for a set of three tracks (with the central and side tracks recorded at 25 and 250 kfci linear densities, respectively) at two values of the skew angle, 0 and 15 degree, respectively.

The most straightforward "solution" for eliminating the skew sensitivity is the reduction of the pole thickness. However, this solution is not adequate for ultra-high density recording, because in this case the recording field, as shown earlier (See Section 2.2.12. Dependence on the Pole Trackwidth and Thickness), drastically drops and thus recording on a sufficiently high coercivity medium becomes problematic [61]. For example, the calculated perpendicular field at saturation versus the distance down the track near the trailing pole edge with a 120-nm trackwidth and with a 20-nm separation between the ABS and the SUL at two different values of the pole thickness, 100 and 500 nm, is shown in Figure 29. In this calculation, the material of which SPH and SUL were made was modeled as a relatively high moment material with a 4SMs of 2 T [72]. It can be seen that the reduction of the pole thickness to 100 nm reduces the field almost by a factor of two. Another approach should be found to solve the fundamental issue of the skew angle sensitivity. For example, it was shown that the use of a trapezoidal write pole could partially reduce the skew sensitivity [73,74]. Good understanding of the mechanisms determining the recording field shall help find a more drastic solution.

Perpendicular Magnetic Recording

56

PT = 0.5um

Hz (Oe)

16000 12000 8000 4000 PT = 0.1um 0

0.9 1.0 1.1 1.2 Distance along the track (um)

Figure 29. Modeled vertical fields near the trailing edge for two values of the pole thickness, PT, of 0.5 Pm and 0.1 Pm, with the same trackwidth of 0.1 Pm.

2.2.14. Gap Length Dependence Above, in Section Second Perpendicular Mode: a Ring Head and a Perpendicular Medium Without a Soft Underlayer, it was shown that as a direct consequence of the recording by the field fringing from the gap, properties of a system utilizing a RH without a SUL, regardless of whether it is perpendicular or longitudinal recording, fairly strongly depend on the physical gap length, as discussed above. This is in contrast with the case of the 1st perpendicular mode, for which no significant dependence on the physical gap length can be expected as long as the gap length is substantially larger than the separation between the ABS and the SUL. Source of magnetic flux MMF (coils)

P2

P1 P1

tgap

P2

wP2

tgap tP2

hP2

tP2-to-SUL Recording Layer

SUL

(a)

(b)

Figure 30. A schematic diagram of: (a) the air bearing surface (ABS) view and (b) the side view of a single pole head (SPH).

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For the 2nd perpendicular mode, the physical gap is a part of the main path for the magnetic flux in a recording system. As a result, in case of the 2nd mode, a stronger dependence on the gap length is expected. On the contrary, for the 1st perpendicular mode, the main path for the magnetic flux does not go through the physical gap region, it rather goes through the SUL, which explains the substantially weaker dependence on the physical gap length as long as the gap length is substantially larger than the separation between the ABS and the SUL. However, if the gap length is reduced to the values comparable with the separation between the head and the SUL, the dependence on the gap becomes essential for the 1st perpendicular mode as well. Because of the potential practical benefits that could be envisioned based on the understanding of the dependence on the gap length, a detailed analysis of the physical gap influence on the recording characteristic of a system with a SUL is given below [55].

8 6 4 2 0

-2 0.0 0.5 1.0 1.5 Distance down the track (Pm) Ring Head (writing by “gap field”)

P1

12 8

P2

Trailing edge of the write field

P2

Vertical Field (kOe)

Vertical Field (kOe)

P1

Gap Trailing edge of the write field

Gap

4 0

-0.5 0.0 0.5 Distance down the track (Pm) Ring Head + SUL = Single Pole Head (writing by the trailing edge of P2)

Figure 31. Vertical field profiles for a RH (medium without a SUL) and a SPH (medium with a SUL).

As mentioned above, commonly, it is believed that the gap, tgap, between the leading and trailing poles in a SPH (See Figure 30) has to be sufficiently large (compared to the separation between the air-bearing-surface (ABS) of the head and the SUL, tP2_to_SUL), i.e. tgap >> tP2_to_SUL, to achieve satisfactory recording performance. In this Chapter, such SPHs will be referred to as type I. On the other hand, SPHs, for which the gap length is comparable or smaller than the ABS-to-SUL separation, will be referred to as type II. In addition, there exists an additional qualititavely different type of a SPH, in which the gap length is substantially (orders of magnitude) larger than any other magnetic dimension of the recording head. Such a head design will be referred to as type III SPH. As shown below, the type III SPH is equivalent to a SPH in which the leading pole is not present at all. The recording performance of the type III SPH design is also discussed below.

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58

Leading Edge

0nm 30nm 70nm 150nm 300nm 700nm

Hz (kOe)

1.5 1.0 0.5

Trailing Edge

0.0 0.0 0.2 0.4 0.6 0.8 1.0 Distance down the track (Pm) Figure 32. Modeled perpendicular write field for type I and type II SPH’s under a pole tip. The variation parameter is the gap length.

One can note that physically the type II SPH design looks exactly as the RH design. However, in the following analysis, it is chosen to differentiate between these two head designs. For clarity, a SPH and a RH imply the presence and the absence of a SUL, respectively. It should be reminded that recording with a RH onto a medium is fundamentally different from recording with a SPH due to the different nature of the write fields generated by these types of recording heads [53]. As mentioned earlier, during recording with a SPH, the writing is accomplished by the trailing edge of the trailing pole. This is in contrast to the recording with a RH, during which the writing is accomplished near the trailing edge of the gap. If a conventional RH is used to write onto a medium with a SUL, it acts as a SPH of type II. This is emphasized in Figure 31, where the two cases of along-the-track vertical field profiles for the same writer are shown: when the writer is used with a medium without a SUL (the writer acts as a RH) and when the writer is used with a medium with a SUL (the writer acts as a SPH). Field Modeling. The magnetic properties of SPH’s are investigated using the above mentioned 3D-boundary element modeling software Amperes. In the calculations presented below, an ideal SUL (with ideal magnetic imaging properties) was assumed. For explanation clarity and without sacrificing the physical substance, the finite thickness and permeability and the micromagnetic effects are ignored [12]. In these calculations, the following values are chosen: the ABS-to-SUL separation = 30 nm; CoFe alloy with a 4SMS of 22 kG is chosen as the SUL and yoke material; the separation between the ABS and the point at which the field values are measured is 10 nm; unless specified otherwise, a 200 nm wide trailing pole is assumed.

Chapter 2 Physics of Writing

Type II SPH 20

Hz (kOe)

15 10

Trailing edge

59 1 mA turn 2 mA turn 5 mA turn 10 mA turn 20 mA turn 40 mA turn 60 mA turn 100 mA turn 160 mA turn 200 mA turn

5 0 0.0 0.2 0.4 0.6 0.8 1.0 Distance down the track (Pm)

Figure 33. Modeled perpendicular write field profile for a type II SPH (70nm gap) under the trailing pole tip at different values of the magnetizing current.

The perpendicular field profiles for SPH’s with several different values of the gap thickness are shown in Figure 32. According to the definition above, the 30-nm and 70nm gap SPH’s represent typical type II SPH’s (or RH’s if there is no SUL) while the 700nm gap SPH represents a type I SPH. A gapless SPH, i.e. a SPH with no gap at all (gap thickness is equal to zero), is also a type II SPH. It could be observed that regardless of the gap thickness, the magnetic properties at the trailing edge are essentially identical including a maximum write field of ~18 kOe and a maximum trailing gradient of ~ 350 Oe/nm, as shown in Figure 32. The maximum write field here and later in the Chapter is defined as the value of the write field at a certain maximum value of the magnetizing current, IMAX, above which the trailing gradient starts to deteriorate. It should be stressed that a higher field is possible to achieve at the expense of a deteriorated trailing gradient. As the gap becomes thinner, the field profile under P2 changes dramatically so that a substantial decrease of the magnitude of the vertical field towards the leading edge of P2 is observed in the type II SPH.

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60

Type I SPH

Trailing Edge 1 mA turn 2 mA turn 5 mA turn 10 mA turn 20 mA turn 40 mA turn 60 mA turn 100 mA turn 160 mA turn

20

HZ (kOe)

15 10 5 0

0.2 0.4 0.6 0.8 1.0 1.2 Distance down the track (Pm) Figure 34. Modeled perpendicular write field profile for a type I SPH (700nm gap) under the pole tip at different values of the magnetizing current.

The longitudinal component of the write field under the P2 is negligible. It becomes essential in the gap region and its maximum value, located near the trailing edge of the P1, varies from ~3.5kOe to ~4.5kOe as the gap thickness is decreased from 700 nm to 30 nm. In this region, the vertical field is negligible and the net field amplitude is far below the threshold value of the write field necessary to write onto media designed to be written onto with a 18 kOe write field at the trailing edge of the P2. The write field profiles for a type II SPH for several values of the magnetizing current are shown in Figure 33. It can be noted that the above mentioned decrease of the vertical component of the write field towards the leading edge of P2 in type II SPH’s is observed only at larger values of the magnetizing current. For comparison, the perpendicular field profiles for a type I SPH for several values of the magnetizing current are shown in Figure 34. It is instructive to plot the maximum value of the write field at the trailing edge and the maximum value of the trailing gradient versus the current value in the magnetizing coils. These dependencies are shown in Figures 35 and 36, respectively. Two distinct regimes of the P2 magnetization process can be clearly identified: an initial steep increase of the field and gradient values in a narrow range of the magnetizing current values is followed by a weaker dependence of the field and gradient on the current. The second regime starts to manifest itself as the top portion of the pole tip (throat) starts to saturate.

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61

HZ (kOe)

20 15 30 nm gap 70 nm gap 150 nm gap 300 nm gap 700 nm gap

10 5 0 0

40

80 120 160 200 I (mA x turn)

Figure 35. The amplitude of the write field at the trailing edge versus the current in the magnetizing coils for different values of the gap thickness. The dotted line represents an 18 kOe level.

Trailing Gradient (Oe/nm)

A marginal decrease of the trailing field gradient observed as the gap thickness is decreased (see Figure 36), is only ~5% of the total trailing gradient magnitude.

300 250 200 150 100 50

30 nm gap 70 nm gap 150 nm gap 300 nm gap 700 nm gap 0 50 100 150 200 Magnetizing Current (mA turn)

Figure 36. The maximum trailing field gradient versus the current in the magnetizing coils for different values of the gap thickness.

The effect of the decrease of the write field towards the leading edge of P2 is the result of the specific magnetic configuration of the type II SPH. The difference between the magnetic configurations of the type I and type II SPH’s is illustrated in Figure 37, where the magnetic paths of the flux generated at P2 are compared for type I and type II SPH’s. While in the type I SPH most of the magnetic flux flows directly into the SUL, in the

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62

type II SPH the magnetic flux generated at P2 is distributed between the SUL and P1. Consequently, in the type II SPH more flux reaches the trailing edge (rather than the leading edge) of the ABS of P2. This can be clearly observed from Figure 38 where contour plots of the vertical component of the magnetization in the middle plane of P2 for both type I and type II SPH’s are shown.

P1

P2

tP2

P2

P1

tP2

SUL

SUL

(a)

(b)

Figure 37. A schematic diagram illustrating the difference between the magnetic flux paths in (a) a type I SPH (a) and (b) a type II SPH.

In the past, it has been shown that the trailing pole thickness is one of the limiting factors controlling the maximum write field that a SPH can generate [4]. To maximize the write field, the trailing pole has to be thicker than a certain critical thickness defined by the overall magnetic system configuration. The SUL-to-ABS separation and the width of P2, the parameter that defines the track width, are among the most critical parameters that strongly affect the critical thickness of P2. However, it should be reminded that, for a perpendicular system, the thicker the trailing pole is, the more sensitive the recording system is to the skew angle [9]. The maximum write field at the trailing edge versus the trailing pole thickness for type I and type II SPH’s with gap thickness values of 700 and 70 nm, respectively, is shown in Figure 39. It could be observed that a thinner trailing pole could be utilized in a type II SPH to achieve higher write fields. The above phenomenon could be explained if one observes that in the type II SPH configuration, a SUL could be thought of as being effectively wrapped around P2, as illustrated in Figure 40. This effectively increases the total thickness of P2, thus enabling a larger write field at a smaller value of P2 geometrical thickness.

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63

Type II SPH

Trailing edge

Leading edge

ABS

Type I SPH

ABS Figure 38. Vertical component of the magnetization in the middle plane of P2 for type I and type II SPH’s. Darker shading corresponds to a higher value of MZ.

SPH Efficiency versus RH Efficiency. One of the concerns in using a type II SPH is the efficiency of this type of head, i.e. the amount of current through the magnetizing coil necessary to bring the head to the operating regime. It is commonly believed that the relatively wide gap in a SPH is a prerequisite for the head to be efficient. However, as shown below, the efficiency loss in the narrow-trackwidth type II SPH’s is only marginal. 1.8

HMAX (kOe)

1.6 1.4 Type I SPH Type II SPH

1.2 1.0 0.2

0.4 0.6 P2 Thickness (Pm)

Figure 39. The maximum perpendicular write field for type I (700nm gap) and type II (70nm gap) SPH’s under a pole tip.

The equivalent electrical circuit for the system utilizing an SPH is shown in Figure 41 [75]. In this approximation, it is assumed that the magnetic flux path through P2 is the same for the magnetic flux flowing into the ABS of the P2 and into the gap region. If the magnetizing coil is located near the ABS, the major contribution to the overall reluctance

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64

of the magnetic circuit comes from the magnetic reluctance, RP2, of the throat region of P2, the magnetic reluctance, RGAP, of the free space between P1 and P2, i.e. the gap region, and the magnetic reluctance, RP2-to-SUL, of the free space between the P2 and SUL. Then, neglecting RP1-to-SUL, RSUL, and RYOKE, for a given magneto-motive force, MMF, generated by the magnetizing coil, the net magnetic flux, ), is given by

ĭ|

MMF , where R AIR R P2  R AIR

P1

R GAP ˜ R P2toSUL . R GAP  R P2toSUL

P2

|

(3)

P2

P1

tP2

tP2

SUL

SUL

Figure 40. A schematic drawing illustrating the effective increase of the P2 thickness in type II SPH.

It is straightforward to show that flux between the ABS of the P2 and the SUL, )P2-to-SUL, is given by

ĭ P2- to-SUL

MMF . § R P2- to-SUL · ¸  R P2- to-SUL R P2 ¨¨1  R GAP ¸¹ ©

(4)

This equation can be rewritten in terms of the physical dimensions of the SPH. Then the vertical magnetic field, B, under P2 is given by

B

P 0 ˜ MMF h P2 § t P2- to-SUL ˜ h P2 · ¨1  ¸  t P2- to-SUL P ¨© t GAP ˜ t P2 ¸¹

,

(5)

where P0 is permeability of air (free space), P is the relative permeability of P2, hP2 is the throat height, tP2 is the P2 thickness, wP2 is the P2 width (track width), tGAP is the gap thickness, and tP2-to-SUL is the SUL-to-ABS separation, as illustrated in Figure 40.

Chapter 2 Physics of Writing

65

MMF (coil)

)

~

RYOKE

RP2 RGAP RP2-to-SUL

RP1-to-SUL

)P2-to-SUL

)

RSUL

Figure 41. A schematic diagram showing an equivalent magnetic circuit for a narrow-gap SPH based recording system.

To compare the efficiency of a typical type II SPH to the efficiency of a typical type I SPH, the above equation is evaluated for the following two cases: type II SPH with tP2-toP1=tP2-to-SUL and hP2=tP2 and type I SPH with tGAP=infinity. This gives

B type I SPH

P 0 ˜ MMF h P2

P

 t P2- to-SUL

and B type II SPH

P 0 ˜ MMF 2h P2

P

.

(6)

 t P2- to-SUL

Because P is usually a large number in excess of 100, the last two equations show that the efficiency values for the two SPH types are similar. It should be remembered that the above arguments are only an approximation based on a number of assumptions such as zero flux leakage (fringing fields are neglected), the absence of non-linearity in the yoke structure, uniform flux distribution, etc. However, as shown below, based on the direct magnetic field modeling results, the above conclusion that there should be no drastic difference between the efficiency values for the two SPH types remains valid. The dependence of the saturation current, ISAT, on the gap thickness is shown in Figure 42. ISAT is defined as the value of the current in the magnetizing coils required to reach a certain operating write field. In this example, the operating write field is chosen to be 18 kOe. It can been observed that although the saturation current increases with the decrease of the gap length, a change of only ~30% is observed as the gap length is decreased from 700nm to 70nm and a factor of two increase of the operating current is observed for a 0-nm gap. It should be stressed that this is in contrary to a common belief that the decrease of the gap thickness would lead to a dramatic, by orders of magnitude, reduction of the SPH efficiency. It should also be stressed that the efficiency of a SPH is

Perpendicular Magnetic Recording

66

Operating Current (mA turn)

strongly dependent on the head design and could be adjusted to match particular recording system requirements. 250 225 200

Type II SPH

175 150 Type I SPH

125 0

200 400 600 Gap Thickness (nm)

800

Figure 42. The dependence of the saturation current, ISAT, on the gap thickness.

Both of the above mentioned properties of type II SPH’s, i.e. the decrease of the write field towards the gap and the ability to generate a stronger write field at a smaller value of the trailing pole thickness, can be used to advantage to minimize the skew angle sensitivity. Skew Angle Versus Gap Length. It is expected that a type I SPH with a 500nm thick P2 at a 15-degree skew will write a 130 nm wide detrimental side band (illustrated in Figure 43) leading to necessity to reduce the track pitch by approximately 25%.

Side band due to non-zero skew Recorded track Figure 43. Illustration of non-zero skew angle sensitivity.

This results in a substantial net loss in the areal bit density. If a type II SPH is used, e.g. with a 30nm gap, the effective length of the P2 is substantially reduced. If a recording medium with a 10% of switching field distribution is used, the effective P2 thickness of a 30-nm gap SPH reduces to ~150nm. Consequently, the detrimental side band width is reduced to 40 nm. In this case, the track pitch needs to be reduced by only 8%, which is clearly advantageous for maximizing the areal bit density. The extreme case of the type II SPH design is a gapless SPH. According to the analysis above, it is expected that the gapless SPH would have the best skew angle performance. To confirm this expectation, the following experiment was performed. A gapless SPH

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67

was manufactured according to the thin-film fabrication process. An SEM image of the air bearing surface view of one of the manufactured gapless SPH is shown in Figure 44.

P2

P1 Figure 44. SEM image of the ABS view of a gapless SPH. (There is no gap between the trailing pole, P2, and the leading pole, P1.)

To compare the skew sensitivity of a gapless SPH with an equivalent non-zero gap type II SPH, two sets of tracks were recorded with a 1-Pm wide gapless SPH and a 1-Pm wide type II SPH with a 1-Pm thick gap, respectively, at a skew angle varying from –15 to 15 degrees. At every skew angle, the effective trackwidth was measured by a relatively narrow (120-nm wide) GMR read head. The measured skew angle dependences for the two heads, respectively, are shown in Figure 45. The comparison clearly indicates a substantially weaker dependence on the skew angle for the gapless head.

Track width (Pm)

1.8

1Pm Gap Gapless

1.6 1.4 1.2 -15 -10 -5 0 5 10 15 Skew angle (degrees)

Figure 45. The effective readback trackwidth versus the skew angle for two SPH’s, with zero and a 1-Pm gap, respectively.

Perpendicular Magnetic Recording

68

Single Pole Head of Type III. As mentioned above, a type III SPH represents a distinct type of SPH. In the type III SPH design, the role of P1 is built-in to be negligible, thus a generic representative of this type of SPH is a SPH which does not have P1 pole at all. The absence of P1 leads to the deteriorated efficiency of a type III SPH, as compared to an equivalent type I SPH. However, for the dimensions considered in this book (to satisfy areal densities beyond 100 Gbit/in2) at all other equivalent conditions, the saturation current for the type III SPH is only ~30 % higher than the saturation current for the type I SPH (See Figure 46).

20

HZ (kOe)

15 10 5 0

Type I SPH (700nm gap) Type II SPH (70nm gap) Type III SPH 0 50 100 150 200 Magnetizing current (mA x turn)

Figure 46. The amplitude of the trailing field versus magnetizing current for the three types of SPH’s.

It should be noted that although a type III SPH is not the most efficient type of SPH, the absence of P1 helps to reduce the sensitivity of the SPH to a stray field [4]. Experiments to Compare Different Types of SPH’s. A type I SPH used in this experiment was fabricated from one of two identical RH’s via focused ion-beam (FIB) trimming to increase the gap thickness. The second RH was used as a type II SPH (see discussion above). Additionally, both SPH’s were FIB trimmed to allow for a 300nm trackwidth, as shown in Figure 47. The recording performances of the resultant type I SPH with a gap of 1 Pm and of the type II SPH with a gap of 80 nm were compared. A 39 nm thick Co/Pd supperlattice based recording layer and a Ni45Fe55 (or FeAlN when indicated) SUL at a 12 nm flying height were utilized.

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69

Gap

Type II SPH

Type I SPH

Figure 47. Electron micrographs of a type II and type I SPH’s prepared from two identical RH’s via focus ionbeam trimming. Both heads were trimmed to allow for a 300nm trackwidth. A white line on the type II SPH micrograph is a guide for an eye to outline the location of the gap.

Saturation current (mA turn)

The spin-stand-measured saturation current (Isat) and PW50 versus the soft underlayer thickness for the type I SPH and the type II SPH are shown in Figures 48 and 49, respectively. Although for a sufficiently thick soft underlayer the Isat is larger for the type II SPH, PW50 for the type II SPH is approximately as small as PW50 for the type I SPH. When the SUL becomes too thin, the type I SPH, which is designed to operate with the SUL cannot properly function, because the SUL saturates, while the type II SPH gradually transforms into a RH when the SUL saturates and can still write on the media with relatively low coercivity utilized in these experiments (Hc~2,500 Oe). Due to a higher value of 4SMS of FeAlN, the thickness at which the FeAlN SUL begins to saturate is smaller than the thickness at which the Permalloy SUL begins to saturate.

350 300 250

Type I and Permalloy Type II and Permalloy Type I FeAlN Type II and FeAlN

200 150 100 50 0.0

0.2 0.4 0.6 Underlayer thickness (um)

0.8

Figure 48. Saturation current and (a 70 nm thick reader with a 100 nm shield to shield separation) versus the SUL thickness.

Perpendicular Magnetic Recording

70

From Figures 48 and 49 it can be observed that the thickness at which the SUL begins to saturate is smaller for the type II SPH than for the type I SPH. The origin of this effect becomes clear if one recalls that the minimum thickness of a SUL is defined by the ability of the SUL to carry the magnetic flux emanating from the P2 (See Ref. [12]). In a type II SPH a certain fraction of the flux gets channeled into the leading pole, P1, thus, relaxing the minimum thickness requirement for a SUL. The possibility of using a thinner SUL is an additional advantage of utilizing a type II SPH in a perpendicular recording system.

180 PW50 (nm)

170 160

Type I SPH Type II SPH Permalloy SUL

150 140 130 0.0

0.2 0.4 0.6 0.8 Underlayer thickness (um)

Figure 49. PW50 (a 70 nm thick reader with a 100 nm shield to shield separation) versus the SUL thickness.

The above shown experimental results are in agreement with the theoretical prediction of only a marginal deterioration of the head efficiency in the type II SPH design versus the type I SPH design. Although, many factors contribute to the measured value of PW50, the invariance of the PW50 with respect to the type I or type II SPH design is indicative that the trailing gradient is not much affected by the gap thickness. 2.2.15. Flying Height Limitation of Single Pole Head Design As mentioned above, the fundamental density limitation of the regular SPH design is due to the inability to scale the flying height as the areal density increase demands for the reduction of the flying height to values below physically impossible [61]. For example, it is believed that the smallest achievable flying height is approximately 5 nm. It is hard to see how one can make the flying height smaller considering that 5 nm is already of the order of the size of the air molecule. Therefore, assuming a constant flying height of approximately 5 nm, as the trackwidth is reduced to satisfy the areal density increase, the field generated at the location of the recording layer also decreases. Unfortunately, the field magnitude cannot be endlessly maintained via the reduction of the throat height. As shown above, as the throat height becomes too small, the contribution to the recording

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field from the magnetic “charges" on the tilted sidewalls increases. As a result, the crosstrack and trailing field gradients deteriorate [54]. Assuming the sidewall tilt angle is approximately 45 degree, the smallest value of the throat height at which the gradient deterioration is less than 50 percent is approximately 100 nm. The recording field generated at saturation under the center of a 300 nm thick trailing pole with a 100 nm throat height at a 5 nm flying height versus the distance across the track at 3 values of the trackwidth, 25, 50, and 100 nm, is shown in Figure 50a. For example, at a 1Tbit/in2 density, the trackwidth is approximately 50 nm assuming a 4:1 BAR. The signal halfwidth defined as the distance along the track, at which the signal drops twice from its maximum value, versus the trackwidth is shown in Figure 50b. It can be noted that as the trackwidth becomes narrower than approximately 50 nm, the half-width ceases to strongly depend on the trackwidth. This is explained by the fact that as the trackwidth is reduced below this critical value, the half-width is dominantly determined by the doubled separation between the ABS and the SUL along with the flying height, which, in this case, are 20 and 5 nm, respectively. Also, as the trackwidth is reduced below approximately 50 nm, the field magnitude drastically decreases for two reasons: 1) the field generated by an individual SPH drops as the trackwidth is reduced because the net magnetic charge is reduced, and 2) the contribution of the field generated by the image SPH drops faster with the trackwidth reduction because it is effectively further away from the center of the recording layer, as compared to the real SPH.

T = 300 nm

1.5 Hz / 2SMs

W = 100 nm 1.0 50 nm 25

0.5

10 HW/2

0.0 0

20 40 60 80 100 Distance across the track (nm) (a)

Perpendicular Magnetic Recording

Vertical Field Halfwidth (nm)

72

100 80 60 40 20 0 0

20

40

60

80

100

Trackwidth (nm) (b) Figure 50. (a) The vertical field versus the distance across the track at saturation for a SPH with a 300 nm thickness at 3 values of the trackwidth, 25, 50, and 100 nm. (b) The field half-width versus the trackwidth.

In summary, the main question regarding the write head at areal densities of the order of 1Tbit/in2 can be formulated: "How to maintain the recording field magnitude with the reduction of the bit dimensions without deteriorating the field gradients?" 2.2.16. Multiple Magnetic Image Reflection In a perpendicular system of the 2nd type, recording is produced by the trailing edge of the trailing pole (TP) of a single pole head (SPH), as shown in Figure 51. The recording field is controlled by the electrical current in a coil wrapped around the TP.

However, additional field sources contribute to the net recording field under the TP. Previously, it was shown that the additional sources are due to the magnetic “charge” in a recording medium, which, unlike in longitudinal recording, is concentrated not in the transitions, but rather more uniformly distributed at the top and effective (due to the presence of the SUL) bottom sides of the recording layer. Among these sources is the field generated by tracks adjacent to the main track under the TP. From the field superposition principle, the maximum field in this case is less than 2SMs, where Ms is the saturation moment of the recording layer. This effect exists in both longitudinal and perpendicular recording leading to non-linear transition shift (NLTS).

Chapter 2 Physics of Writing

Leading Pole (LP)

73

Trailing edge

I Write coil

TP H

Recording Layer

SUL

Figure 51. A schematic diagram showing how due to the multiple reflection a relatively small field under the leading pole can be magnified into a relatively strong field under the trailing pole.

There is an additional effect inherent only to perpendicular recording with a SUL, which contributes to the net field under the TP. The magnetic flux due to a bit-pattern in the recording layer can be transferred from the leading pole to the trailing pole, as shown in Figure 52. Although indirect, this effect is capable of generating a relatively large additional field under the TP, as shown below. As described below, the process underlying this effect can be explained in terms of the multiple (magnetic) image reflection (MIR) of the surface magnetic charges in the recording layer, sandwiched between two magnetic “mirrors”, the soft underlayer and the leading pole.

Non-zero net magnetization

TP LP Recording Layer

SUL Figure 52. A schematic diagram showing the origin of the MIR effect.

74

Perpendicular Magnetic Recording

The intention of this Section is to utilize 3-D boundary element modeling (BEM) supported by spin-stand and magnetic force microscopy (MFM) experiments to investigate in detail the dependence of various parameters on the MIR effect. In the absence of two magnetic mirrors above and below the recording layer, the stray field emanating from a DC magnetized recording layer is negligibly small. The net stray field is a sum of the oppositely directed fields generated by the top and bottom surface “charge” of the recording layer. In other words, the magnetic field is “trapped” inside the recording layer, as shown in Figure 53. Next, the effect of the presence of the soft underlayer and the leading pole can be analyzed. As earlier described, the soft underlayer acts as a mirror that creates an image of the surface charge in the recording and thus increases the effective separation between the effective bottom and top charge in the recording layer. The leading pole is the second magnetic mirror added to the opposite side of the recording layer. This second mirror creates another set of surface charge images and further increases the effective separation between the effective bottom and top charge. The following analogy could illustrate the rest in this process. Imagine yourself standing between two facing each other mirrors. Ideally, due to the multiplicative reflection you should be able to see an infinite number of images of yourself. Similarly, this multiplicative process leads to the effective substantial separation of the surface charges from each other and thus, releases non-zero magnetic flux (See Figure 53). Ideally, assuming the head/medium system to be a 100% efficient magnetic flux guide, i.e., with no flux leakage, according to the magnetic flux conservation, assuming a DCmagnetized medium under the leading pole, the magnitude of the additional field, Haddition, generated under the trailing pole is expected to be directly proportional to the net magnetic moment of the recording layer, Ms, Haddition ~ 4SMs ALP/ ATP ,

(7)

where ALP and ATP are the ABS areas of the leading and trailing poles, respectively. The linear dependence on the Ms and the ratio ALP/ ATP is valid as long as no saturation occurs in the system. The linear dependence on the ratio ALP/ ATP becomes a crude approximation when the trackwidth and, consequently, the area, ATP, is reduced down to a size, at which the efficiency of the system starts to drop. Previous calculations indicate that for a given perpendicular system configuration the efficiency drops to values less than 60 percent as the trackwidth becomes narrower than approximately 300 nm. At such narrow trackwidths, significant amount of the magnetic flux generated in the

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75

region under the leading pole due to the MIR effect leaks out on its way to the trailing pole. Magnetic “charges” moved to f due to multiple magnetic reflections

------------

DC Magnetized Recording Layer Soft Magnetic Material No magnetic flux gets out

LP

Magnetic Flux

-----------

+

=

++++++++ Soft Magnetic Material

Magnetic Flux

SUL Magnetic “charges” in the recording layer

++++++++ f Magnetic “charges” moved to due to multiple magnetic reflections

Figure 53. A schematic diagram helping understand the difference between two cases: a) magnetic flux trapped inside the recording layer and b) the magnetic flux extracted from the recording layer region located between SUL and LP due to the MIR effect.

Calculations indicate that in the presence of a SUL, a focused ion beam (FIB) trimmed single pole head with a 300 nm trackwidth and a leading pole’s ABS cross-section of 2Pm x 5Pm is capable of generating an extra field under the trailing pole of up to approximately 2000 Oe due to a DC magnetized recording medium with a Ms of 400 emu/cc located between the SUL and the LP. If a recording medium possesses relatively low coercivity, i.e. less than ~ 2000 Oe, such a large additional field itself is sufficient to entirely erase a previously recorded track. The following experiment was performed for observing the effect. A double-layer CoCr based perpendicular medium with coercivity of ~1800 Oe with a remanent squareness of 0.7 was preliminarily DC-erased in the presence of an external vibrating sample magnetometer’s (VSM) field of approximately 3 T. The SPH was run across the medium at the condition of zero current in the write coil. A MFM image of a track recorded as a result of the described experiment is shown in Figure 54a. Because the medium was preliminarily DC-erased and the write current was zero, the only contribution to the net recording field could be due to the above described effect of the extraction of the magnetic flux due to the MIR effect in the region under the

Perpendicular Magnetic Recording

76

leading pole. For comparison, a MFM track recorded using the same head, now by applying an alternating write current of 200 mA˜turn, is shown in Figure 54b.

(a)

(b) Figure 54. MFM images of tracks recorded onto a CoCr based perpendicular medium with coercivity of approximately 1800 Oe with a recording field (a) due to the effect of MIR to relocate the magnetic flux from the region under the leading pole into region under the trailing pole and (b) by an alternating write current.

To study effects of MIR on recording, the following parameters were chosen: TP trackwidth, W2, = 300 nm, TP thickness, T2, = 500 nm, throat height, TH, = 500 nm, LP width, W1, = 2 P, LP thickness, T1, = 2 Pm, gap length = 1 Pm, head magnetic moment, Bs, = 2 T, recording layer thickness, t, = 20 nm, recording layer moment, Ms, = 200 emu/cc, and ABS to SUL separation = 30 nm. Considering a DC-erased medium under the LP with a 2 x 2 P cross-section, the perpendicular component of the field generated under the TP at zero write current at 3 different values of the TP trackwidth, W2 = 100 nm, 300 nm and 500 nm, versus the distance along the central line down the track at a 5 nm flying height is shown in Figure 55a. The reason why the field does not strongly depend on the trackwidth in this range is the existence of the two essentially competing effects. On one hand, the field should increase with the trackwidth reduction according to the Flux Conservation Law, as described above. On the other hand, less flux reaches the TP as the trackwidth is

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77

reduced, because the efficiency of the system drops with the trackwidth reduction in this particular range. In any case, it can be seen that recording layer magnetization of only 200 emu/cc is sufficient to generate an extra field of more than 1000 Oe just due to the MIR effect. 1.75

1.4 Hz (kOe)

1.2 1.0 0.8

W = 100 nm W = 300 nm W = 500 nm

1.50 1.25 1.00

0.6

0.75

0.4

0.50

0.2

0.25

0.0

0.00 0.0 0.2 0.4 0.6 0.8 Distance down the track (um)

(a)

1000 x 500 1000 x 1000 1400 x 1400 2000 x 2000 6000 x 2000

0.0 0.2 0.4 0.6 0.8 Distance down the track (um)

(b)

Figure 55. Recording field under the TP due to the effect of MIR of the DC-erased recording layer between the LP and SUL at different values of (a) TP widths, 100, 300 and 500 nm, and (b) at different cross-sections of the LP.

The perpendicular recording field under the TP at a 5 nm flying height versus the distance down the track is shown for a set of different values of the LP width and thickness (W1xT1 nm2) in Figure 55b. It can be noted that the field depends on the net ABS area of the LP, A = W1 x T1, rather than independently on the LP width and thickness as long as the non-zero magnetization regions under the LP are relatively not far from the location of the main track defined by the TP. Moreover, the dependence on the LP area is rather significant in comparison with the dependence on the TP area. This is explained by the relative proximity of the LP to the main source of the field, the region at which the MIR effect occurs, along with relatively large cross-section dimensions of the LP, as compared to the ABS-to-SUL separation. It can be noted that for fairly realistic LP’s cross-section dimensions, e.g. such as 6 x 2 P2, a recording field of as high as 1400 Oe could be generated under the TP, as shown in Figure 55b. The important question to answer is: “What is more important: a particular bit pattern or the net (average) magnetic moment under the leading pole?” At an initial condition of zero net magnetic moment (ac-demagnetized medium) under the LP, a 500 nm wide DCerased track was modeled to be recorded along the central line under the LP along with two 250 nm wide tracks located 250 nm away from the main track at each side with the magnetization directed opposite to the direction of the magnetization in the main track, as

78

Perpendicular Magnetic Recording

shown in Figure 56. As a result of this track pattern, the net magnetic moment under the leading pole is still zero.

LP 500 AC-demagnetized background

SUL

250

250

DC-magnetized tracks

Figure 56. A front view cross-section diagram showing 3 DC-tracks with respect to the LP.

The magnetic field generated under the TP as a result of the described track pattern is shown in Figure 57a. For comparison, the field generated at a condition of an entirely DC-erased medium is also shown on the same plot. It could be observed that in the case of zero net magnetic moment the generated field is negligibly small. Indirectly, this result indicates that the dependence on the track location is not significant at least for the considered off-center region of the order of 1Pm, otherwise, the opposite polarity magnetic flux currents under the LP would not have been able to cancel each other, and therefore, the net field under the TP could not have been as small as it is. In another modeled scenario, the location of a 500 nm wide track on the background of an ACerased medium was varied. The perpendicular field under the leading pole generated when the track under the leading pole is located along the central line and 500 nm away from the central line is shown in Figure 57b. It could be observed that the field for the both cases is very similar. For the both cases, the track is entirely covered by the leading pole. It should be noted that when the track was located outside the area covered by the leading pole, the field under the leading pole was found to become negligibly small. For reference, the dependence of the perpendicular field component versus the net magnetization in the recording layer is shown in Figure 58.

Chapter 2 Physics of Writing 12 10

DC-erased Track Compensated Bit Pattern

HZ (kOe)

8 6 4 2 0

79

0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00

Centered Track Off side Track

0.00 0.25 0.50 0.75 Distance down the track (um)

0.0 0.2 0.4 0.6 0.8 Distance down the track (um)

(a)

(b)

1.5 1.0 0.5

Hz (kOe)

Figure 57. Stray field versus areal density at a 200 ktpi at the cetner and at 5 percent off the edte of a bit at a 5 nm flying height.

0.0 0.0

Mave (4SMs) 0.5

1.0

Figure 58. Vertical field under the TP due to the MIR effect for a 2 Pm x 6Pm LP cross-section versus normalized net magnetization in the recording layer.

In conclusion, the phenomenon of the MIR along with the physics of the magnetic flux propagation explains why by making the leading pole sufficiently large it is possible to generate an unacceptably large magnetic field under the trailing pole even at zero current in the write coil. 2.3. MODIFIED FIRST PERPENDICULAR MODE: A SHIELDED SINGLE POLE HEAD AND A PERPENDICULAR MEDIUM WITH A SOFT UNDERLAYER

80

Perpendicular Magnetic Recording

2.3.1. Shielded Single Pole Head One of the previously proposed solutions is to build "soft" magnetic shields around the main pole, as shown in Figure 59 [76, 77].

Shield

SS

X

G (Gap) Main Pole

Y

Figure 59. A diagram of the ABS view of a shielded single pole head (SSPH) pole tip configuration.

It can be noted that the shields are wrapped only around the trailing side and the two cross-track sides of the main pole. Only these three sides are critical for recording, because the two cross-track sides define the trackwidth and the trailing side defines the quality of each linear transition. No recording is supposed to take place at the leading side, therefore, this side does not necessarily have to be covered with a shield. The direct effect of the shielding is screening the unfavorable side field away from the recording medium, as shown in a cross track cross-section diagram in Figure 60. Consequently, the constraints on the head structure, which were put on the regular SPH (without shielding) for the purpose of reducing the effect of the side field, are substantially relaxed if shields are utilized. It should be reminded that for the regular SPH, the pole tip geometry is chosen with a fairly large throat height with the purpose to reduce the side field. The cost of the fairly tall throat is the substantial reduction of the field magnitude and the system efficiency, as shown above. On the contrary, for the case with shields, the throat height can be substantially reduced for maintaining the fairly large field magnitude without loosing the field gradients. In other words, if shields are used, a substantially more efficient pole structure can be implemented without loosing the field gradient. As an example, the calculations were made to compare the recording field generated by a regular SPH with a throat height, TH, of 100 nm with the recording field generated by a shielded SPH (SSPH) with a 50 nm throat height with a cross-track shield to shield separation of 90 nm and a downtrack gap, G, between the write pole and the trailing

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81

shield of 20 nm. The shield throat height, STH, was modeled to be 10 nm. In both cases, the pole tip was modeled to be 50 nm wide and 300 nm thick. The central cross-track profiles for these two cases at saturation are shown in Figure 61. In practice, however, there might be limitations due to the processing difficulties. For example, the shortest possible today throat height is going to be dictated by the lapping process accuracy. As a direct consequence of the ability to exploit a more efficient pole tip configuration, the improved skew angle performance of SSPH can be mentioned. Due to the higher efficiency, as compared to SPH, a much thinner pole tip can be utilized to generate the same recording field. Therefore, SSPH has substantially improved skew angle performance, as compared to that of SPH, as discussed below.

(a)

(b)

TH

STH

Hside Hrec SUL

Shield Recording layer

Shield Hrec SUL

Figure 60. Diagrams showing the magnetic field propagation for the two cases of interest: (a) without and (b) with shields.

For the SSPH design, the side cross-track and trailing field gradients are dominantly determined by the spacing between the main pole and the shields. This is in contrast with the regular not-shielded SPH design, for which the gradients are determined not only by the flying height and the separation between the ABS and the SUL but also to significant degree by the throat height. Evidently, the deadly limitation of a nonzero throat height in the case of the regular SPH design is automatically removed in the case of the SSPH design. In the latter case, even for a substantially shorter throat height, the undesired side cross-track and trailing fields are reduced due to the existence of a relatively lowreluctance and well-defined return flux path via the shields. As noted above, the reduction of the throat height to zero drastically increases the system efficiency and allows a substantially larger amount of the magnetic flux generated by the drive coil to reach the ABS. This automatically results in an improved skew angle performance of the SSPH design, as compared to the conventional SPH design, because in this case a head with a substantially thinner pole tip can be utilized to generate a field as strong as the field generated by an equivalent conventional SPH with a much thicker

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82

main pole tip. As discussed above, the skew angle sensitivity is proportional to the pole tip thickness.

Hz (10 x kOe)

1.5

1.0 Regular

0.5

Shielded

0.0

W = 100 nm 0.0 0.1 0.2 0.3 Distance across the track (Pm)

Figure 61. Cross-track profiles for the two cases, a regular SPH and a shielded SPH (SSPH), at saturation.

The maximum trailing field at saturation versus the main pole thickness for the two cases, a regular SPH with a 500 nm throat height and SSPH with a zero throat height, both with a 120 nm trackwidth, is shown in Figure 62. To clearly illustrate the point, the field is shown normalized to its saturation value. It can be noted that indeed the field starts to drop at a smaller value of the thickness for the case with shields.

Hz max/Hz max sat

1.0 0.8

SSPH

0.6 Regular SPH

0.4 0.2 0.0 0

200 400 600 Thickness (nm)

Figure 62. The maximum trailing field at saturation (normalized to its saturation value) versus the pole tip thickness for the two cases, a regular SPH with a 500 nm throat height and a shielded SPH with a zero throat height. The trackwidth is 120 nm.

Another observation that can be made is the fact that if shields around the main pole are utilized, as described above, there is absolutely no need for the return pole separated with

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83

a fairly large gap from the leading edge of the main pole, as shown in Figure 63a. The shields wrapped around the main pole not only act as the gradient shapers but also perform the role of a return pole. As a result, a system with shields around the main pole and without a return pole remains approximately as efficient as a regular system with a return pole. As a consequence of the shields acting also as a flux return pole, the requirements on the use of a SUL are much less tight as compared to the regular SPH case. It can be also observed that the shielded structure resembles the typical ring head structure. The purpose of the separation between shields and the main pole is to avoid the side field and thus to distinctly define the recording transitions. Similarly, the purpose of the gap between the two poles of the ring head structure is to define the recording transitions. Moreover, similar to a system with a ring head, a system utilizing a shielded writer can be utilized without any SUL at all. As shown in Figure 63b, the fairly small separation between the main pole and the shield provides sufficient efficiency. In most implementations, shields are coupled to the main pole through the back of the pole, as shown in Figure 63c. In this case, the trailing and cross-track side field gradients are determined by the flying height and the separation between the main pole and the shields rather than by the separation between the ABS and the SUL.

Return pole

Main Shield Main pole pole

Shield

B

B

(b)

(a)

SUL

Main pole Shield B

(c) SUL

Figure 63. Diagrams showing the flux return paths in cases with a regular (a) SPH (along-track cross-section view) and (b) a shielded SPH (SSPH) (front view cross-section), respectively. (c) Full-scale front view crosssection of SSPH.

Perpendicular Magnetic Recording

84

In fact, for the considered dimensions, the only noticeable difference between the two modified systems, with and without a SUL, is the fact that the system without a SUL needs approximately 20 percent more current to saturate, as shown in the calculated current dependencies in Figure 64. To clearly illustrate the main dependence, the field is shown normalized to its saturation value. In summary, it can be concluded that the utilization of soft magnetic shields around the main pole results in the following advantages:

Hz max/ Hz max sat

1)The recording field can be maintained to be fairly large (compared to the case without shields) with no substantial field gradient degradation at higher areal densities. 2)The field gradient can be controlled via varying the separation between the main pole and each of the shields as well as by the pole tip and shield geometry. 3) If shields are used, the system efficiency could be increased through the reduction of the throat height. As a consequence, the skew angle sensitivity could be also substantially reduced.

with SUL

1.0

with NO SUL

0.5 0.0 0

50 100 150 200 Drive current (au)

Figure 64. The maximum field versus the drive current for two configurations: SSPH with and without a SUL. The field is shown normalized to its saturation value.

Chapter 3 Physics of Playback

Chapter 3

Physics of Playback

1. Introduction

Recent high areal density demonstrations of perpendicular recording clearly indicate an increasing interest in this technology [78,79,80,81]. It is believed that, as compared to conventional longitudinal recording, perpendicular recording is capable of deferring the superparamagnetic limit to a significantly higher areal density due to a thicker recording layer and/or the use of a soft underlayer (SUL) [82]. Although perpendicular recording is certainly the closest alternative to the conventional technology, its novelty also brings up new issues, not ever encountered in longitudinal recording. These issues have to be well understood before the technology can be fully and most efficiently implemented [83,84,85]. Major issues related to perpendicular media and perpendicular write heads have been previously considered [86,87,88,89]. However, relatively little attention has been given to the playback process. For example, the role of the SUL in the playback process is still an open question: although the SUL certainly increases the magnitude of the playback signal, its influence on the signal resolution is still controversial. Another fundamental source of the difference between the playback processes in longitudinal and perpendicular recording is the difference in the magnetic “charge” configuration in longitudinal and perpendicular media, respectively. Therefore, the intention of this Chapter is to investigate the physics of the playback process in perpendicular recording. 1.1. CHAPTER OVERVIEW In this Chapter, the physics of the playback process in perpendicular recording is explored. It is shown that due to the existence of the two layers of the “magnetic charge”, at the top and effective bottom surfaces of the recording layer, the stray field sensed by a reader rolls off with the areal density essentially differently than it does in longitudinal recording. Unlike in longitudinal recording, in perpendicular recording, the recording layer thickness is an extremely sensitive parameter, which provides extra flexibility in controlling the density roll-off. It is illustrated that for areal densities beyond

85

86

Perpendicular Magnetic Recording

approximately 200 Gbit/in2, the slowest roll-off for both longitudinal and perpendicular recording occurs at a bit aspect ratio of 1:1. A fundamental role of the soft underlayer in the playback process is investigated. It is illustrated that although at relatively low linear and track densities the use of a soft underlayer increases the playback signal, the signal does not depend on the use of a soft underlayer at high densities. It is shown that for both perpendicular modes, although at sufficiently low track densities (below ~ 50 ktpi), the signal disappears at relatively low linear densities, there is a significant non-zero signal even at zero linear density if the track density is sufficiently high (above ~ 300 ktpi). A magnetic image model is introduced to illustrate that the use of a soft underlayer could not improve the resolution of a recording system. Moreover, it is shown that there is range of the air-bearing-surface-to-soft-underlayer separation in which the playback resolution deteriorates. The guidelines are given on how to design a playback head to avoid the operation in the region of the deteriorated resolution. Besides the conventional playback head design including a single read element surrounded by two soft shields along the track, a number of other optimized for perpendicular recording designs are explored. For example, it is illustrated that compared to conventional single-read-element shielded configurations, differential reader configurations display superior playback properties in terms of both the playback amplitude and the spatial resolution.

2. Playback in Perpendicular Recording

2.1. ANALYSIS METHODS In this Chapter, the playback process is analyzed as not just a detailed study of another read head design but rather an integral process, including a reader and a medium. Such an integral consideration is especially critical for the perpendicular mode with a medium with a soft underlayer (SUL). In this mode, the SUL is often viewed as an indispensable part of the recording head [85]. For broader and more insightful comprehension of the playback process in perpendicular recording, two analytic approaches, direct and reciprocity, are considered. The “direct calculation” method addresses exclusively the fundamental contribution of a recording medium into the playback process. Thus, the fundamental issues related to the different “charge” configuration in a perpendicular medium could be more explicitly studied. As to the “reciprocity calculation,” it reflects the magnetic properties of the playback head. 2.1.1. Direct Calculation with Point-size Reader Approximation To calculate the magnetic field emanating from a perpendicular recording medium with a periodically written bit pattern, an analytical 3D expression could be derived, as shown in Equation 1 [90]. This expression takes into account the recording layer effective thickness, G (twice the physical thickness if a SUL is used), the hard layer saturation magnetization, Ms, and the bit length and width, a and b, respectively. The origin of the reference coordinate system is chosen to be located at a corner of a bit at the top surface of the recording layer, as shown in Figure 1.

Chapter 3 Physics of Playback

87

X

Z

Y (0,0,0) Periodic bits

G

a

b

Figure 1. A diagram showing the location of the reference system with respect to a “effective” recording layer with a periodicall written bit pattern.

To study the dependence on the bit aspect ratio (BAR) and the areal density (AD), the transformation equations representing the bit length and width, a and b, through BAR and AD, could be used, as given by Equations 2 and 3, respectively.

H z stray

f f 32 M s §S n · §Sk · sin ¨ y¸ x ¸ sin ¨ ¦ ¦ © a ¹ © b ¹ n 1 k 1 S nk odd odd (1)

u"

S

2 2 §n· §k· ¨ ¸ ¨ ¸ ©b¹ ©a¹

zª S «1  " « ¬«

a

BAR , AD

b

1 BAR ˜ AD

2 2 §n· §k· ¨ ¸ ¨ ¸ ©b¹ ©a¹

Gº », z ! 0 » ¼» (2)

,

(3)

In the “direct calculation” approximation, no head finite size effects are taken into account. Therefore, the “direct calculation” reflects only the contribution of a medium to the playback process. Also, considering that the effect of the use of an “ideal” SUL on the

Perpendicular Magnetic Recording

88

stray field is equivalent to a two-fold increase of the recording layer thickness, the same expressions could be used also to model the case with a SUL just by replacing G with 2G. However, it should be remembered, that the use of the SUL is not necessarily equivalent to the two-fold increase of the recording layer thickness in the sense of the energy, and, therefore, this approximation could not be applied to predict phenomena associated with the bit energy, e.g. the thermal instability effect [87].

Hstray + +

(a)

+ +

M

charges in the transition

Hstray (b)

(c) Medium image

++++++++ -------------------

+++++++

++++++++ ----------

----------

+++++++

Underlayer boundary

Figure 2. Diagrams showing the sources of stray fields in the case of (a) longitudinal recording, and perpendicular recording (b) without and (c) with a SUL.

To help understand the basic difference in the playback process between longitudinal and perpendicular recording, schematic diagrams of the stray fields emanating from a single transition in a longitudinal medium and a perpendicular media without and with a SUL are shown in Figures 2a-c, respectively [91]. As can be noticed, in the longitudinal case, the stray fields emanate only from the transitions, with the fields near the transitions oriented perpendicular to the disk plane. On the other hand, in the perpendicular cases, the stray field emanates from the effective “magnetic charge” at the top and effective (due to the SUL) bottom surfaces of the recording layer, with the field right above the transitions oriented parallel to the disk plane. The calculated stray fields for two values of the recording layer thickness, 10 and 20 nm, with a Ms of 200 emu/cc, above an isolated magnetization transition at a 5 nm flying height are shown for the three cases in Figures 3a-c, respectively. In these calculations, the transition is assumed to be ideal. Also, in this example, for the description simplicity, infinitely wide tracks are assumed. Below in this

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Chapter it will be shown that consideration of a finite trackwidth is critical to predict realistic waveforms. To calculate the field in a longitudinal recording system, threedimensional (3-D)-boundary element modeling (BEM) was exploited [92]. It should be noted that, in general, the stray field in the perpendicular cases looks similar to the stray field in the longitudinal case, provided that the perpendicular and in-plane components are interchanged. It is obvious that the effective distance away from the transitions, at which most of the drop in the stray field occurs, is determined by the effective recording layer thickness. Longitudinal

Perpendicular 1000 No Su

Hz and 20nm Hz and 10nm

500 0 Hx and 10nm -500

Hx and Hz (Oe)

Hx and Hz (Oe)

1000

Hx and 10nm

500 0 -500 H and 10nm z

Hx and 20nm

Hx and 20nm

-0.10 -0.05 0.00 0.05 0.10 Distance along the track (um)

Hz and 20nm

-0.10 -0.05 0.00 0.05 0.10 Distance along the track (um)

(a)

(b)

Hx and Hz (Oe)

1500 Perpendicular With SU 1000

Hx and 20nm Hx and 10nm

500 0 Hz and 10nm

-500

Hz and 20nm -0.10 -0.05 0.00 0.05 0.10 Distance along the track (um)

(c) Figure 3. The along-the-track and perpendicular stray field components, Hx and Hz, versus the distance along the track over a single transition in (a) a longitudinal medium and perpendicular media (b) without and (c) with a SUL, with 10 and 20nm recording layer thickness values with a Ms of 200 emu/cc at a 5 nm flying height.

For example, because of the effective factor-of-two increase in the recording layer thickness when the SUL is used, in the perpendicular case without a SUL the field drops more rapidly than it does in the case with a SUL. Below it is shown that the amount, by which the field drops, is determined mostly by the trackwidth.

90

Perpendicular Magnetic Recording

Before going into a more detailed study of the perpendicular stray field dependence on different parameters, such as the recording layer thickness, BAR, and others, it is helpful to clearly understand the basic physics behind the origin of the stray field in perpendicular recording. As previously mentioned (see Figures 2b and c), the net stray field in perpendicular recording consists of the oppositely directed fields generated by the top and effective bottom “charges” of the recording layer. Therefore, because the net stray field is the difference between the two fields, it is relatively strongly sensitive to the bit dimensions and the effective recording layer thickness. As an example, the field profiles emanating from the top and bottom “charge,” as well as the net stray field at a 5 nm flying height for a 10 nm thick recording layer with a periodically written bit pattern with a 500 x 500 nm2 bit cell cross-section is shown in Figure 4a. The magnitude of the stray field is normalized to 4SMs, where Ms is the recording layer magnetization. It is obvious that for a relatively wide and long bit (as compared to the flying height) and if the recording layer thickness is significantly smaller than each of the bit cell sizes, the magnitudes of the two fields over the center of a bit should be approximately equal to the field from a uniformly “charged” plane (with a “charge” density of 4SMs), i.e. to 2SMs. Thus, in this case, being equal in magnitude and oppositely directed, the two fields substantially cancel each other, so the net stray field should be relatively small at the center of a bit. On the contrary, for sufficiently small bits, as compared to the flying height, although each of the fields, from the top and bottom “charges,” is less than 2SMs, the fields are also more different from each other in their strengths, thus the net stray field might not be negligible, as shown for a 50 x 50 nm2 bit cell cross-section in Figure 4b. By changing the recording layer thickness from a relatively small to a sufficiently large, the contribution of the field from the effective bottom “charge” is changed from equal to negligible compared to the contribution from the top “charge”. As a result, the net stray field at the center of a bit changes from zero to the field due to only the top “charge,” as shown in Figure 4c for four different values of the square bit side, 25, 50, 200 and 500nm. It can be noted that the characteristic length, at which the field reaches its saturation value, is more sensitive to the bit length as the bit length becomes comparable to the recording layer thickness. So far, the calculation has involved only the stray field at the center of a bit. The field cancellation effect (FCE) due to the top and bottom “charge” is quantitatively different if the field is considered closer to one of the edges of a bit. It is worth mentioning that, in general, the FCE is the cause of a typically observed maximum value in perpendicular roll-off curves [84-86]. At sufficiently low densities, the net signal is small because of the FCE, while at sufficiently high densities, the net signal is naturally small because the bit area of the top side of the recording layer (containing the field generating “charge”) becomes fairly small. To illustrate how the FCE depends on the position with respect to the bit cell, the stray field versus the linear density at a 200 ktpi track density at a 5 nm flying height at the center and five percent (relative to the bit length) away from one of the edges of a bit is shown in Figure 5.

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0.6

91

Top surface

Hz/4SMs

0.4 0.2

Net field

0.0

-0.2 Bottom surface

-0.4 -0.6

0 100 200 300 400 500 Central line along the track (nm)

(a)

Hz/4SMs

0.4

Top surface

0.2

Net field

0.0 Bottom surface

-0.2 -0.4

0

10 20 30 40 50

Central line along the track (nm)

(b)

500 x 500

Hz/4SMs

0.5 0.3

200 x 200 50 x 50

0.2

25 x 25

0.4

0.1 0.0 0 (c)

200

400

600

Recording layer thickness (nm)

Figure 4. The field profiles at a 5 nm flying height due the magnetic “charges” at the top and bottom surfaces and the sum of the two fields from a 10 nm thick recording layer with a periodically written bit pattern with (a) a 500x500nm2 and (b) 50x50nm2 bit cell cross-section. (c) The net stray field (at the bit center) at a 5 nm flying height versus the thickness of the recording layer at different bit-cell cross-sections.

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92

It can be noticed that the perpendicular field above the center peaks up at a significantly higher density (~600kfci) than it does above the edge of a bit (~100 kfci). In the following Reciprocity Calculation Section, the existence of the maximum in a perpendicular roll-off curve is explained with the help of a different model.

0.3 Normalized Hz

At the Center

0.2 At the Edge

0.1

0.0

0

250 500 750 Linear Density (kfci)

1000

Figure 5. The stray field versus the areal density at a 200 ktpi at the cetner and at 5 percent off the edge of a bit at a 5 nm flying height.

To explain the dependence of the signal on the trackwidth, the calculated normalized isolated transition response at a 5 nm flying height for a 20nm thick recording layer without a SUL (equivalent to a 10nm thick layer with a SUL) at three values of the recorded trackwidths, 80, 200, and 1000nm, provided the adjacent tracks are acdemagnetized (in other words, average magnetization is zero for the adjacent tracks), is shown in Fig. 6. Assuming an ideal transition, the perpendicular stray field could be calculated via straightforward integration of the field produced by the point “charge” uniformly distributed within an individual track with a trackwidth of W and a recording layer thickness of G with the transition at X=0. It can be observed that the narrower the track is the smaller the amount of the field, which is lost away from the transition, is. This is in agreement with the above-described field cancellation effect, according to which the fields from the top and bottom “charges” essentially cancel each other only for a sufficiently wide track sufficiently far away from the transition. Going back to the calculation of the field due to a periodic bit pattern (see Eqs. 1-3), the stray field emanating from the center of a bit in a periodically written bit pattern at a 5 nm flying height, versus the linear density for media with three values of the recording layer

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thickness, 10, 20 and 40nm, is shown for two values of the track density, 50 and 316ktpi, in Figs. 7a and b, respectively. Considering that track densities of 316 and 50ktpi correspond to track pitches of approximately 80 and 500nm, respectively, it can be noticed that for the narrower trackpitch, the stray field does not disappear at a low linear density. In fact, agreeing with the symmetry of perpendicular recording with respect to the along and across the track directions, 316ktpi at a linear density of 50kfci corresponds to 50ktpi at a linear density of 316kfci.

0.15

Hz/4SMs

0.10 0.05

80nm 200nm 1000nm

0.00

-0.05 -0.10 -0.15 -60

-40 -20 0 20 40 Distance down the track (nm)

60

Figure 6. The isolated transition response for a 20nm thick recording layer without a SUL (or 10nm thick recording layer with a SUL) at 3 values of the trackwidth, 80, 200, and 1000nm at the condition of acdemagnetized adjacent tracks.

To stress the significance of this result, it is worth remembering that in perpendicular recording, typically (at relatively large trackpitch values), it is expected that the stray field drastically would drop at low densities [93]. The current calculations indicate that this non-desirable effect of the field reduction at low linear densities does not exist at a sufficiently narrow trackpitch. Another consequence of the FCE, which can be observed from the graphs, is the fact that, although the net stray field, as expected, increases as the recording layer thickness increases, at sufficiently high linear densities the stray field does not strongly (compared to the low density case) depend on the thickness.

316kfci

Perpendicular Magnetic Recording

94

Hz/4SMs

0.4 0.3

10 (5nm with SUL) 20 (10nm with SUL) 40 (20nm with SUL)

50ktpi

0.2 0.1 0.0

0

0.4 Hz/4SMs

0.3

50kfci

(a)

316

750 1500 2250 3000 Linear density (kfci) 10 (5nm with SUL) 20 (10nm with SUL) 40 (20nm with SUL)

316ktpi

0.2 0.1 0.0

(b)

050 750 1500 2250 3000 Linear density (kfci)

Figure 7. The stray field above the center of a bit in 10, 20 and 40 nm thick recording layers at a 5 nm flying height versus the linear density for two values of the track density, (a) 50 ktpi and (b) 316 ktpi.

Also, it can be noticed that, considering that the use of a SUL is equivalent to a two-fold increase of the recording layer thickness, the use of a SUL can noticeably increase the net signal only at relatively low linear and track densities and has a much weaker effect at relatively high densities. For example, considering that a 20nm thick recording layer

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95

without a SUL is equivalent to a 10nm thick recording layers with a SUL, it could be noted that at linear densities below approximately 50 kfci at a 50 ktpi track density, the use of a SUL increases the net stray field almost by a factor of 2. The effect is weaker at a 316 ktpi track density: in this case, the use of a SUL increases the net stray field at low densities by approximately a factor of 1.5.

0.4 1:1 2:1 4:1 8:1

Hz/4SMs

0.3 0.2 0.1 0.0

0

50 100 150 200

Recording layer thickness (nm) Figure 8. The stray field over the center of a bit at a 5 nm flying height versus the recording layer thickness at a 200 Gbit/in2 at 4 values of BAR, 1:1, 2:1, 4:1 and 8:1.

It is obvious that the BAR value is also going to influence the degree of the field cancellation effect. For example, aiming at the next year projected areal density of 200 Gbit/in2, the net stray field 5 nm away from the center of a bit in periodic 200 Gbit/in2 patterns for a set of values of BAR, 1:1 (57x57nm2), 2:1 (80x40nm2), 4:1 (112x28nm2), and 8:1 (160x20nm2), versus the recording layer thickness is shown in Fig. 8. It gives an additional insight into understanding the difference between perpendicular and longitudinal recording in general, if one compares the playback signal for different recording modes in the point-size sensor approximation [94]. Although, from the above noticed 90 degree symmetry of the stray fields between perpendicular and longitudinal recording, it follows that a conventional longitudinal reader is not optimal in perpendicular recording, today perpendicular recording still relies on the use of the longitudinal reader configuration. Therefore, because the conventional reader configuration is designed to be most sensitive to the perpendicular stray field component, it makes sense to compare the perpendicular stray field components for the three recording modes [95].

Perpendicular Magnetic Recording

Stray Field, Hz (Oe)

96

No SUL

600

400 BAR = 1:1 BAR = 2:1 BAR = 4:1 BAR = 8:1

200

0 0

500

1000

1500

2000 2

Areal density (Gbit/in ) 2 Areal Density (Gbit/in )

Stray Field, Hz (Oe)

(a)

with SUL

800 600 400

BAR = 1:1 BAR = 2:1 BAR = 4:1 BAR = 8:1

200 0 0

500

1000

1500

2000 2

Areal density (Gbit/in ) 2 Areal Density (Gbit/in )

(b)

BAR = 1:4 BAR = 1:2 BAR = 1:1 BAR = 2:1 BAR = 4:1 BAR = 8:1

Hz (Oe)

800 600 400 200 0 (c)

500 1000 1500 2 Areal Density (Gbit/in )

2000

Figure 9. The perpendicular stray field component versus the areal density for the equivalent perpendicular systems with a 10 nm thick recording layer (a) without and (b) with a SUL at 4 values of BAR, 1:1, 2:1, 4:1 and 8:1, and (c) a 10 nm thick longitudinal recording medium at 6 values of BAR, 1:4, 1:2, 1:1, 2:1, 4:1 and 8:1.

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The bit averaged (averaged over the bit area) zero-to-peak perpendicular stray field at a 5 nm flying height over a periodic bit pattern recorded onto a 10 nm thick recording layer versus the areal density at different values of BAR are shown for the two perpendicular modes, without and with an ideal SUL, in Figs 9a and b, respectively. The track averaged (averaged over the track) zero-to-peak perpendicular stray field at a 5 nm flying height over the ideal transition in a periodic bit pattern recorded onto a 10 nm thick longitudinal recording medium versus the areal density at different values of BAR is shown in Fig. 9c. It should be remembered that perpendicular recording in the sense of the stray field is symmetric with respect to the directions along and across the track because the magnetization in the recording medium is directed perpendicular to the plane of the disk. Therefore, the results for BAR values of 2:1 and 4:1 are identical to the results for BAR values of 1:2 and 1:4, respectively. On the contrary, longitudinal recording has a preferred orientation in the disk plane because the magnetization is recorded along the track. Therefore, in the longitudinal case, it makes sense to consider BAR values corresponding to the complete range of values, i.e. 1:4 is not the same as 4:1. From these graphs, it definitely can be seen that in the perpendicular and longitudinal cases, the stray fields change very differently with the areal density. In the perpendicular cases, a density cross-over region can be identified, at which the stray field increase with the BAR increase changes to the stray field decrease with the BAR increase. This cross-over density region is around 500 and 200 Gbit/in2 for the cases without and with a SUL. In other words, the larger the BAR value is, the larger the signal is if the areal density is less than approximately 500 and 200 Gbit/in2 for the two cases, respectively. On the contrary, if the areal density is larger than approximately 500 and 200 Gbit/in2 for the two cases, the smaller the BAR value is, the larger the signal is, i.e., in this case, the maximum signal is reached at the BAR value of 1:1. No cross-over region can be noticed in the longitudinal case. Moreover, in the longitudinal case, regardless of the density, the signal reaches the maximum when the BAR value is approximately 1:1. Although, it is not trivial to equivalently compare the playback capability in longitudinal and perpendicular recording, to get a feel for how fast the signal drops with the areal density, the normalized graphs corresponding to the best longitudinal case with a BAR value of 1:1 and the perpendicular cases with the same BAR values are shown in Fig. 10. From the above conclusions it is clear that the perpendicular modes are relatively more sensitive to the recording layer thickness, and, for the current assumptions of the calculations it can be concluded that for both perpendicular modes, the signal drops slower with the areal density than it does in the best equivalent longitudinal case. Between the two perpendicular cases, in the case without a SUL the signal rolls off slower with the areal density than it does in the case with a SUL. The only relevant difference between these two cases is in the effective thickness of the recording layer, which is twice as large for the case with a SUL. In summary, the slower density roll-off in perpendicular recording could be explained by a relatively strong dependence of the stray field on the ratio between the BAR value and the recording layer thickness due to the field cancellation effect described above. In longitudinal recording, the density roll-off is mostly determined by the reduction of the stray field with the reduction of the amount of the effective “charge,” which generates the perpendicular stray field at the location separated from the transition by a flying height.

Perpendicular Magnetic Recording

Hz normalized

98

1.0 0.8 0.6 0.4

Longitudinal Perpendicular with no SUL Perpendicular with SUL

0.2 0

500

1000 1500 2000 2

Areal density (Gbit/in ) Figure 10. The normalized perpendicular stray field component for the best longitudinal case (with a BAR value of 1:1) and the two perpendicular cases without and with a SUL with a 10 nm thick recording layer at a 5 nm flying height versus the areal density.

The difference between the two perpendicular cases, with and without a SUL, is described below with the help of the Reciprocity principle [96]-[22]. As previously described, the Reciprocity Principle is a convenient theoretical technique, which allows one to take into account the finite playback head dimensions [85].

2.1.2. Calculation Based on the Reciprocity Principle Previously, it was shown that a linear field response of the head magnetic material is a sufficiently good approximation for the playback process even at trackwidths as small as 60 nm [97, 85]. Therefore, the Reciprocity Principle can be used for calculating the playback signal [98,99,100]. According to the Reciprocity Principle, the playback signal is proportional to the convolution of the magnetization distribution in the recording layer and the so-called sensitivity field of the head. The sensitivity field is the magnetic field generated by the imaginary unit currents in imaginary coils wrapped around the read element [85]. Because the sensitivity field depends on the head configuration, the Reciprocity Principle, as mentioned above, takes into account the contribution of the head on the playback signal. Because the sensitivity field also depends on the presence of a SUL, it is clear that the SUL should be treated as a part of the playback head. The Reciprocity Principle for the playback signal is applied via Equation 4,

Chapter 3 Physics of Playback

S~

1 I

³

& & & M ˜H wrˆ

,

99 (4)

where M is the magnetization in the recording layer and H is the sensitivity field generated by the imaginary current, I. Considering that the magnetization in the recording layer is in the perpendicular orientation with a negligibly small transition parameter, from Equation 4 it follows that for understanding the basic playback phenomena in perpendicular recording, it is sufficient to calculate only the perpendicular component of the sensitivity field [85].

SSS

Read element Shield 1 ABS to SUL

Shield 2 t

Fly height

interlayer Recording layer Interlayer SUL Figure 11. A schematic diagram showing a conventional longitudinal reader design incorporated in perpendicular recording.

At this stage of the analysis, non-zero dimensions of the playback head can be taken into account. For simplicity, a regular longitudinal read head with a shielded read element was assumed in this analysis. A schematic diagram of such a head is shown in Fig. 11. The sensitivity field was calculated as the field due to the imaginary coils around the read element with actual head/media parameters, thus, the interaction between the read head and the SUL was taken into account [85]. To model an ideal SUL, the boundary conditions were defined so that the top surface of the SUL had a constant scalar magnetic potential, M, of zero.

Perpendicular Magnetic Recording

Playback (a.u.)

100

ideal SUL No SUL 10 ABS-to-SUL = 30nm Flying Height = 5 nm 5

0 200

400

600

800

1000

Linear Density (kfci) Figure 12. Roll-off curves for the perpendicular modes without and with a SUL, with a ABS-to-SUL separation of 30 nm.

It should be mentioned that although the playback characteristics in general are sensitive to the playback head dimensions, such as the trackwidth, the shield-to-shield separation (SSS) and others, some of the fundamental conclusions could be made through a study of one specific head configuration. In the calculation described below, a 100-nm wide and 30-nm thick read element with a linear MH-response in the direction of the dominant magnetic flux propagation and with a 100-nm SSS separation was assumed. The calculated linear density roll-off curves at a 5-nm flying height with a 30-nm ABS-to-SUL separation without and with a SUL are shown in Fig. 12. It can be noticed that at the low-density limit the signal is smaller than at the intermediate densities for the both cases. This is in agreement with the above-described field cancellation effect. Another observation, which can be made from these graphs is the fact that although, at intermediate densities, in the case with a SUL the signal is significantly (by approximately 40 percent) larger than the signal in the case without a SUL, the signals for the two cases converge at linear densities above approximately 450 kfci. This is in agreement with the previous conclusions stating that at sufficiently high linear densities the use of a SUL does not contribute to the net signal. Also, it can be noticed that the signal maxima occur at approximately 380 and 290 kfci for the cases without and with a SUL, respectively. The smaller linear density, at which the maximum occurs, for the case with a SUL is explained by the fact that the use of a SUL effectively doubles the recording layer thickness, thus, the characteristic bit size, at which the influence of the effective bottom “charged” layer can be neglected, is larger for the case with a SUL. The reason why these characteristic densities do not also differ by a factor of two is explained by the signal dependence on the BAR, which is another variable in these two cases.

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Although the convergence of the two curves at high linear densities indicates that the use of a SUL does not necessarily improve the resolution of a recording system, the final conclusions about the influence of the SUL on the resolution are still not trivial to make from these simple calculations. Therefore, another model, which allows to make more direct conclusions, is described below.

Real head Center of the recording layer

Real head to the center of the recording layer Recording layer Buffer layers etc.

Image head to the center of the recording layer

The underlayer boundary line

Image head

Figure 13. A diagram showing the image representation of a system with a SUL.

For better understanding of the following material, first, the so-called magnetic image model, which is used to explain the dependence of the sensitivity field on the use of a SUL, is described [94]. 2.1.3. Model of the Magnetic Image To sufficiently good approximation, during the playback process, the SUL can be assumed to be ideally soft, i.e. it can be represented as a semi-infinitely thick layer with infinite magnetic permeability and an infinite moment. Therefore, the magnetic mirror image model can be applied [14]. In this model, the ideally SUL is replaced by a halfspace, which contains a mirror image of the recording head, as shown in a schematic diagram in Fig. 13. According to a theorem of differential equations [101], Laplace’s Equation (a consequence of the Maxwell’s Equations, convenient to use for the calculation of the magnetic fields) has an unambiguous solution provided sufficient boundary conditions are satisfied. Using an ideal SUL automatically provides the same boundary conditions as in the case with a half-space with a mirror image, providing the following rule is applied to the currents determining the magnetic structure of the mirror image head: the vertical components of the magnetic fields generated by the real and image heads add up, while the in-plane components subtract from each other. Below, the magnetic model is used to describe a “paradoxical” phenomenon caused by the use of a SUL.

Perpendicular Magnetic Recording

102

With SUL No SUL

PW50

50

45

40

35 10 (a)

15 20 25 30 ABS to SUL distance (nm)

Normalized Signal

1.0 0.9 0.8 0.7 0.6 0.5 10 (b)

With SUL No SUL

15 20 25 30 ABS to SUL distance (nm)

Figure 14. (a) PW50 and (b) the normalized playback signal versus the ABS-to-SUL separation. An equivalent dependence for the case without a SUL is shown by the dotted curve. The oval surrounds the “bad” points, for which the PW50 values are substantially larger than the PW50 value at the infinite separation between the ABS and the SUL (equivalent to no presense of the SUL at all).

Image “Paradox.” A diagram showing the location of the playback head and its image due to the SUL with respect to the recording layer is shown in Fig. 13. It can be noticed that due to the presence of the SUL, the finite thickness of the recording layer adds asymmetry into this system. The center of the recording layer is closer to the real head than to the image head by the thickness of the recording layer plus all the bottom interlayer (or buffer layers). According to the Reciprocity Principle, the resolution of the

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103

final system is determined by the sensitivity field in the region of the recording layer. Therefore, the final resolution of the system with a SUL is determined by the sensitivity field, which, in this case, is the sum of the fields generated by the real head and the image head. It can be noticed that the resolution by the image head is worse than the resolution of the real head because of the effective spacing loss due to the finite recording layer thickness plus the interlayer thicknesses. Therefore, the resolution of a system with a SUL intrinsically can not be better than the resolution of a system without a SUL. However, the signal does go up due to a SUL, because, according to the image model, there is an extra contribution to the net signal due to the image head.

50

PW50 (nm)

45 40

t = 30nm

35

SSS = 50nm

30 25

t = 10nm

20 15 (a)

t = 5nm 10 15 20 25 30 ABS to SUL distance (nm)

50

PW50 (nm)

45

35

SSS = 100nm

30 25

t = 10nm

20

t = 5nm

15 (b)

t = 30nm

40

10

15 20 25 30 ABS to Underlayer (nm)

Figure 15. PW50 versus the ABS to underlayer separation at three different values of the read element thickness, 5, 10, and 30 nm, for a shield-to-shield separation (SSS) of (a) 50 nm and (b) 100 nm.

104

Perpendicular Magnetic Recording

The calculated PW50 and the normalized amplitude versus the distance between the air bearing surface (ABS) and the SUL are shown in Figs. 14a and b, respectively. PW50 was defined as the half-width of the peak-to-peak signal [17]. In these calculations, the variable parameter, contributing to the net change in the ABS-to-SUL separation, was the net thickness of all the interlayers used, providing the flying height and the recording layer thickness remained constant, 5 nm and 10 nm, respectively. In other words, a variation in the ABS-to-SUL separation is produced via the change of the bottom net interlayer thickness. Naturally, at this condition no variation either in PW50 or in the amplitude is observed, if no SUL is used, as shown by the horizontal dotted lines in Figs. 14a and b. As a reader, a conventional design with a 30 nm thick read element and a 100 nm shield-to-shield separation (SSS) was modeled. The feature to notice is the fact (which is expected from the image model argument above) that there is a range of the ABS-to-SUL separation, in which PW50 for the case with a SUL peaks up by approximately 30 percent relative to the PW50 value for the case without a SUL. This characteristic region (in this particular case, approximately between 10 to 25 nm) of the deteriorated resolution is determined by the read head dimensions and to some degree by the flying height and the recording layer thickness. Also, it can be noticed that the signal with the SUL is always larger than the signal without the SUL, which agrees with the magnetic image model argument above. The PW50 versus the ABS-to-SUL separation for 3 different values of the read element thickness, 5, 10, and 30 nm, is shown for two different values of the SSS, 50 and 100 nm, in Figs. 15a and b, respectively. It can be seen that with the reader thickness reduction from 30 to 10 nm, the maximum PW50 change decreases as well (it should be remembered that with the reader thickness reduction the playback amplitude decreases as well). Another feature to observe is the fact that, although the amount of the maximum PW50 change does not strongly depend on the SSS, the range over which it changes is significantly smaller for the smaller value of the SSS. For example, for the both values of the SSS, 50 and 100 nm, the maximum PW50 change of above 30 % is observed for a 30 nm thick reader. At the same time, the range of the PW50 change is approximately 5 to 10 nm and 5 to 30 nm for SSS values of 50 and 100 nm, respectively. 2.1.4. Examples of Reader Designs In this Section, the Reciprocity Principle is applied to analyze and compare four different reader designs (See Figure 16): a) unshielded reader [102]; b) shielded [103,104]; c) differential reader [105,106,107,108]; d) shielded differential reader[109]. Variations of shielded, differential, and shielded differential readers with the emphasis on various aspect of recording performance and manufacturability will be considered as well. Playback from a perpendicular recording medium with a soft underlayer and a single layer longitudinal recording medium will be investigated. For completeness, more exotic configurations such a longitudinal recording medium with a soft underlayer (keeper layer) and perpendicular recording medium without a soft underlayer will be considered as well.

shield

Recording Medium

shield

Recording Medium

Recording Medium

MR Sensor

MR Sensor

shield

MR Sensor

(b)

(a)

MR Sensor

105

MR Sensor

MR Sensor

Chapter 3 Physics of Playback

shield

Recording Medium (d)

(c)

Figure 16. Schematics of various reader designs: a) unshielded reader, b) shielded reader, c) differential

reader, d) shielded differential reader.

As mentioned earlier, the reciprocity principle is used to study playback performance of various readers [98,110,111]. According to the reciprocity principle (See Equation 4), the playback voltage of a linear playback head is equal to the convolution of the sensitivity field (function) of the reader with the magnetization pattern written into the recording layer. The sensitivity field is calculated as the field generated by the read-head, in which the read sensor is substituted with an equivalent soft magnetic material with a current carrying imaginary coil wrapped around [112]. dMR-shield dMR Shield

Shield

hshield

hMR

FH tRL

tMR

tshield

Recording Layer

Figure 17. Schematic of a shielded differential reader with the relevant dimensions outlined.

Perpendicular Magnetic Recording

106

The reader design parameters used in calculations are similar to the ones suggested by M. Mallary et. al. [113] for a 1Terabit/in2 perpendicular recording system design. Unless specified otherwise, the magnetic thickness of an MR sensor, tMR, is assumed to be 10nm. The cross-track width of an MR sensor, wMR, is 40nm. The height of the MR sensor, hMR, is 40nm. The separation between the MR sensor and the shields, dMR-shield, and the separation between the two MR sensors in a differential designs, dMR, are set to be 10nm each. The flight-height, FH, is 5nm and the media thickness, tRL, is 10nm. The shields thickness, tshield, is 100nm, the shield cross-track width, wshield, is 400nm, and the shield height, hshield, is 220nm. The dimensions mentioned above are shown in a schematic drawing of a shielded differential reader in Figure 17. In this chapter, the presence of an ideal soft underlayer is modeled with symmetric boundary conditions on the top surface of the soft underlayer. Magnetic field modeling based on boundary element approach is utilized throughout the chapter [92]. 2.1.5. Basic Reader Design Comparison The presented calculations of the playback are based on the Reciprocity Principle. The Reciprocity Principle requires the knowledge of the sensitivity functions for the playback heads [99]. Figures 18 and 19 show the z (vertical) and x (horizontal) components of the sensitivity fields along the track for the four types of heads, respectively.

No shield (sul) Shield (sul) Diff (sul) Shield Diff (sul)

6

Hz (a.u.)

4 2 0 -2 -4 -6 -200

-100

0

100

200

Distance along the track (nm) Figure 18. Vertical component, Hz, of the sensitivity field for different reader types.

Chapter 3 Physics of Playback No shield (no sul) Shield (no sul) Diff (no sul) Shield Diff (no sul)

5 4 3

Hx (a.u.)

107

2 1 0 -1 -2 -200

-100

0

100

200

Distance along the track (nm) Figure 19. Horizontal along-the-track component, Hx, of the sensitivity field for different reader types.

The playback signal off a perpendicular recording medium with a soft underlayer versus the linear density for the four reader designs is shown in Figure 20. While conventional shielded reader provides improved performance over unshielded reader, both differential reader configurations offer a major performance improvement in terms of higher playback amplitude and higher resolution. The unshielded differential reader offers higher signal amplitude at lower linear densities. The shielded differential reader offers the highest spatial resolution out of all the four designs. 35

No shield (sul) Shield (sul) Diff (sul) Shield Diff (sul)

Playback (a.u)

30 25 20 15 10 5 0 500

1000

1500

2000

2500

Linear Density (kfci) Figure 20. Playback off a perpendicular recording medium with a soft underlayer versus linear density for four reader designs.

Perpendicular Magnetic Recording

108

The playback signal off a single layer longitudinal medium versus the linear density for the four reader designs is shown in Figure 21. Similarly to the case of perpendicular recording presented above, the conventionally used shielded reader provides improved performance over the unshielded reader. Both differential reader configurations offer a major performance improvement in terms of higher playback amplitude and higher resolution over their non-differential counterparts. The unshielded differential reader offers higher signal amplitude at lower linear densities. The shielded differential reader offers the highest spatial resolution out of all the four designs.

No shield (no sul) Shield (no sul) Diff (no sul) Shield Diff (no sul)

Playback (a.u.)

15

10

5

0 0

500

1000

1500

2000

2500

Linear Density (kfci) Figure 21. Playback off a single layer longitudinal recording medium versus linear density for four reader designs.

2.1.6. Parallels between perpendicular and longitudinal recording It should be reminded that a conventional shielded reader when applied to longitudinal recording is equivalent to a differential reader when applied to perpendicular recording [114]. This is illustrated in Figure 22 where the sensitivity fields of a (shielded) differential reader and a shielded reader are compared. It should be reminded that the maximum value of the sensitivity field defines the maximum value of the playback signal. Therefore, this graph illustrates that the differential reader, regardless of whether it is shielded or not, provides a larger playback signal than the equivalent regular shielded reader.

Chapter 3 Physics of Playback

109

Sensitivity Field (a.u.)

6 4 2 0 -2

Shielded Reader (Hx) Diff Reader (Hz) Shield Diff (Hz)

-4 -6 -200

-100

0

100

200

Distance along the track (nm) Figure 22. Sensitivity fields for shielded, differential and shielded differential readers.

The normalized sensitivity fields for the readers above are shown in Figure 23. It can be observed that that normalized sensitivity functions of a shielded reader and a shielded differential reader are almost identical while the sensitivity function of a not shielded differential reader has somewhat wider tails.

Sensitivity Field (a.u.)

1.0

0.5

0.0

Shielded, Hx Differential, Hz Shielded Diff, Hz

-0.5

-1.0

-200

-100

0

100

200

Distance along the track (nm) Figure 23. Normalized sensitivity fields for a shielded, differential, and shielded differential readers.

For completeness of this description, it is instructive to compare the performance of the above mentioned reader designs as applied to perpendicular and longitudinal recording.

Perpendicular Magnetic Recording

110 35

No shield (perpendicular) Shielded (perpendicular) Differential (perpendicular) Shield Diff (perpendicular) No shield (longitudinal) Shielded (longitudinal) Differential (longitudinal) Shield Diff (longitudinal)

Playback (a.u.)

30 25 20 15 10 5 0 0

500

1000

1500

2000

2500

3000

Linear Density (kfci) Figure 24. Perpendicular and longitudinal systems playback for four reader designs.

Figure 24 compares the playback versus the linear density for a perpendicular medium with a soft underlayer and a longitudinal single layer medium with equivalent recording layers. It can be observed that for all the considered reader designs, the playback amplitude is higher for perpendicular recording (as compared to longitudinal recording), which is clearly an advantageous feature of perpendicular recording. Shielded (longitudinal) Differential (longitudinal) Shield Diff (longitudinal) Shielded (perpendicular (no sul)) Differential (perpendicular (no sul)) Shield Diff (perpendicular (no sul))

Playback (a.u.)

20

15

10

5

0 0

500

1000

1500

2000

2500

Linear Density (kfci) Figure 25. Perpendicular without a soft underlayer and longitudinal systems playback for three reader designs.

For comparison, Figure 25 shows the playback signals off a perpendicular system with a medium without a soft underlayer and a single layer longitudinal medium. Figure 26

Chapter 3 Physics of Playback

111

shows the playback signal off a perpendicular system with a medium with a soft underlayer. Similarly, the same three reader designs as in Figure 25 are considered. The comparison of Figure 25 and Figure 26 indicates that the higher playback amplitude in perpendicular recording is mostly due to the utilization of a medium with a soft underlayer. 2.1.7. Influence of Shields Number of Shields. The playback signal off a perpendicular recording medium with a soft underlayer and a single layer longitudinal recording medium versus the linear density for three cases, 1) not shielded, 2) one-side shielded, and 3) two-side shielded single MR sensor reader designs are shown in Figure 26 and Figure 27, respectively. 30

No shield z (sul) One shield z (sul) Two shield z (sul)

Playback (a.u.)

25 20 15 10 5 0 0

500

1000

1500

2000

2500

3000

Linear Density (kfci) Figure 26. Playback off a perpendicular recording medium with a soft underlayer versus linear density for not shielded, one-side shielded, and two-side shielded single MR sensor reader designs.

No shield x (no sul) One shield x (no sul) Two shield x (no sul)

Playback (a.u.)

15

10

5

0 0

500

1000

1500

2000

2500

Linear Density (kfci) Figure 27. Playback off a single layer longitudinal medium versus linear density for not shielded, one side shielded, and double shielded single MR sensor reader designs.

Perpendicular Magnetic Recording

112

Sensitivity function (a.u.)

These graphs illustrate that the addition of shields improves the resolution of a reader at higher linear densities for both perpendicular and longitudinal recording. Thick shield, Hx (no sul) Thick Shield, Hz (sul) Thin Shield, Hx (no sul) Thin Shield, Hz (sul)

6

4

2

0

-2 -200 -150 -100

-50

0

50

100

150

200

Distance along the track (nm) Figure 28. Sensitivity functions for perpendicular and longitudinal shielded readers of two extreme values for the shield thickness: thick shield – 100nm, thin shield – 10nm.

Thick shield, Hx (no sul) Thick Shield, Hz (sul) Thin Shield, Hx (no sul) Thin Shield, Hz (sul)

Playback (a.u.)

20

15

10

5

0 0

500

1000

1500

2000

2500

Linear Density (kfci) Figure 29. Playback versus linear density for perpendicular and longitudinal shielded readers of two extreme values for the shield thickness: thick shield – 100nm, thin shield – 10nm.

Chapter 3 Physics of Playback

113

Shield Thickness. Figures 28 and 29 show the sensitivity function and the roll-off curve, respectively, for double-sided shielded readers for the cases of a single layer longitudinal medium and a perpendicular medium with a soft underlayer for 100nm and 10nm thick shields.

Only a very weak dependence on the shield thickness could be observed for the cases presented above. 35

Shield (sul) Shield (no sul) Diff (sul) Diff (no sul) Shield Diff (sul) Shield Diff (no sul)

Playback (a.u.)

30 25 20 15 10 5 0 0

500

1000

1500

2000

2500

Linear Density (kfci) Figure 30. Comparison of playbacks of three reader designs (shielded, differential, and shielded differential) for the cases of perpendicular media with and without a soft underlayer.

2.1.8. Soft Underlayer Versus No Soft Underlayer Figures 30 and 31 show the roll-off curves for perpendicular and longitudinal systems, respectively, for three reader designs for media with and without a soft underlayer. It can be observed that while in the perpendicular system the addition of a soft underlayer substantially increases the playback amplitude, in the longitudinal system the addition of a soft underlayer (keeper layer) leads to a substantial drop in the playback signal. The physical explanation of the phenomenon is illustrated in Figure 32 where imaging properties of a soft underlayer film are outlined for the cases of perpendicular and longitudinal recording. In perpendicular recording, the addition of a soft underlayer effectively doubles the recording layer thickness and thus increases the amplitude of the stray field. In longitudinal recording, the addition of a soft underlayer film creates an effective layer underneath the recording layer with the magnetization oriented opposite to the magnetization written into the recording layer. As a result, the net stray field decreases.

Perpendicular Magnetic Recording

114 16

Shield (no sul) Diff (no sul) Shield Diff (no sul) Shield (sul) Diff (sul) Shield Diff (sul)

Playback (a.u.)

14 12 10 8 6 4 2 0 0

500

1000

1500

2000

2500

Linear Density (kfci) Figure 31. Comparison of playbacks of three reader designs (shielded, differential, and shielded differential) for the cases of longitudinal media with and without a soft underlayer (keeper layer).

2.1.9. Differential Reader Optimization and Single MR Differential Readers

The performance of differential readers can be further optimized by adding a soft magnetic material bridge that magnetically couples the two sensors, as shown in Figures 33a and b. Additional configurations of a differential reader that are worth considering are shown in Figure 33c and d, where one of the MR sensors is substituted with an equivalent soft magnetic material. The latter is simpler to manufacture as building differential readers represents technological challenges associated with manufacturing of double-MR elements with the outputs connected to form a differential circuit.

SUL perpendicular

SUL longitudinal Figure 32. Illustration of the imaging properties of soft underlayer for the cases of perpendicular and longitudinal recording schemes.

Recording Medium

MR Sensor

shield

MR Sensor

MR Sensor

115

shield

Recording Medium (b)

MR Sensor

(a)

MR Sensor

MR Sensor

Chapter 3 Physics of Playback

shield

Recording Medium

shield

Recording Medium (d)

(c)

Figure 33. Schematics of bridged differential readers: a) not shielded, b) shielded, c) not shielded with one MR element, d) shielded with one MR element.

Figures 34 and 35 show z and x components of the sensitivity function, respectively, for three configurations of an unshielded differential reader. It can be observed that the addition of the bridge connecting two MR elements substantially increases the magnitude of the sensitivity function. Also, it can be noted that all the readers have asymmetric profiles along the track. Similarly, Figure 36 and 37 show z and x components of the sensitivity functions, respectively, for the three configurations of a shielded differential reader. Again, it can be noted that the addition of the bridge connecting the two MR elements substantially increases the magnitude of the sensitivity function. As in the case of the unshielded reader, single MR element based shielded readers have asymmetric profiles along the track. Bridged (sul) Diff (sul) Half Diff (sul)

10

Hz (a.u.)

5

0

-5

-10 -200

-100

0

100

200

Distance along the track (nm) Figure 34. Vertical component of the sensitivity function for three types of differential readers: diff – conventional differential reader, bridged – differential reader with two MR elements connected by a soft magnetic bridge, half diff – bridged with one of the MR elements substituted with soft magnetic material.

Perpendicular Magnetic Recording

116

10 Bridged (no sul) Not bridged (no sul) Half Diff (no sul)

8

Hx (a.u.)

6 4 2 0 -2 -200

-100

0

100

200

Distance along the track (nm) Figure 35. Horizontal (along the track) component of the sensitivity function for three types of differential readers: not bridged – conventional differential reader, bridged – differential reader with two MR elements connected by a soft magnetic bridge, half diff – bridged with one of the MR elements substituted with soft magnetic material.

10

Hz (a.u.)

5

0 Bridged (sul) Diff (sul) Half Diff (sul)

-5

-10 -200

-100

0

100

200

Distance along the track (nm) Figure 36. Vertical component of the sensitivity function for 3 modifications of shielded differential readers: diff – conventional differential reader, bridged – differential reader with two MR elements connected by a soft magnetic bridge, half diff – bridged with one of the MR elements substituted with soft magnetic material.

Chapter 3 Physics of Playback

117

8 Bridged (no sul) Diff (no sul) Half Diff (no sul)

Hx (a.u.)

6 4 2 0 -2 -200

-100

0

100

200

Distance along the track (nm) Figure 37. Horizontal component of the sensitivity function for 3 modifications of shielded differential readers: diff – conventional differential reader, bridged – differential reader with two MR elements connected by a soft magnetic bridge, half diff – bridged with one of the MR elements substituted with soft magnetic material.

Figures 38 and 39 compare the roll-off curves for double MR element differential readers and modified single MR element differential readers as shown in Figure 33 for the cases of perpendicular and longitudinal recording, respectively. It can be seen that in both perpendicular and longitudinal recording, single MR sensor differential readers give better performance at lower linear densities than double MR sensor differential readers. The performance at higher linear densities is approximately the same for all the considered types of readers.

Playback (a.u.)

50

Diff (sul) Shield Diff (sul) Half Diff (sul) Shield Half Diff (sul)

40 30 20 10 0 0

500

1000

1500

2000

2500

3000

Linear Density (kfci) Figure 38. Playback off a perpendicular recording medium with a soft underlayer versus the linear density for double-MR sensor and single-MR sensor differential readers.

Perpendicular Magnetic Recording

118

Diff (no sul) Shield Diff (no sul) Half Diff (no sul) Half Shield Diff (no sul)

Playback (a.u.)

20

15

10

5

0 0

500

1000

1500

2000

2500

Linear Density (kfci) Figure 39. Playback off a single layer longitudinal medium versus linear density for double-MR sensor and single-MR sensor differential readers.

2.1.10. Parallels Between Playback in Perpendicular and Longitudinal Magnetic Recording: Revisited Although perpendicular recording is one of the closest alternatives to conventional longitudinal recording, the implementation of this technology is subject to resolving a number of technical issues not encountered in longitudinal recording. As mentioned above, relatively little attention has been given to the read process [115,85]. Currently, the read heads used in perpendicular recording system prototypes remain largely unchanged from their original longitudinal versions [116]. It is not clear that such read heads provide optimal playback performance. In addition, the application of longitudinal readers in perpendicular recording leads to undesirable phenomena associated with adjacent track reading [117,118] and calls for modification of the existing read channels [119,120, 121].

The subject of this Section is a read head design for a perpendicular recording system equivalent in its playback characteristics to a conventional longitudinal reader used in longitudinal recording. Overview of Reader Designs. A diagram of a conventional longitudinal read head (LRH) - shielded reader consisting of a read element (such as MR, GMR etc.) surrounded by two magnetic shields - is shown in Figure 40 [122].

Chapter 3 Physics of Playback

119

z x S1

S2 tSS

tMR

Figure 40. A side view diagram of a shielded reader.

Designed for the use in longitudinal recording, a typical LRH is configured so that it predominantly senses the stray fields emanating from the bit transitions in a longitudinal recording medium. In longitudinal media, stray fields near bit transitions are mostly of perpendicular orientation and the magnitude of the perpendicular component of the stray field, Hz, decays rapidly away from a bit transition. Therefore, a typical LRH is designed to preferably sense the longitudinal field component, Hz. If designed to read the longitudinal component of the stray field, Hx, the reader would sense stray fields relatively far from a bit transition, because the rate of the decay of the Hx is substantially slower, as shown in Figure 41a.

Hz

Hx -0.10 -0.05 0.00 0.05 0.10 Distance along the track (Pm)

(a)

Hx and Hz (arb. units)

1000 Hx 500

0 Hz

-500 -0.10 -0.05 0.00 0.05 0.10 Distance down the track (Pm)

(b)

Figure 41. The z- and x-components of the stray field at a 5nm flying height near the transition (X=0) in case of (a) a longitudinal and (b) perpendicular medium.

Perpendicular Magnetic Recording

120

In the case of a perpendicular medium, the vertical and longitudinal components of the stray field are interchanged compared to the case of a longitudinal medium: the stray field near the bit transitions is predominantly longitudinal, while the stray field away from the transitions is mostly perpendicular, as shown in Figure 41b. This is opposite to the longitudinal medium for which the stray field near the transitions is predominately perpendicular while the stray field away from the transitions is mostly longitudinal, as shown in Figure 41a. Therefore, in a magnetically equivalent perpendicular system, not only should the medium orientation change from longitudinal to perpendicular, the readhead configuration should also be changed so that it preferably reads the longitudinal component, Hx, instead of the perpendicular component, Hz. To develop such a perpendicular read head (PRH) design, conformal mapping (CM) could be utilized [123]. It should be remembered that conformal mapping is a 2-D mathematical tool. However, as shown below, this convenient conceptual instrument allows to avoid any unnecessary guess work in the design of an adequate perpendicular magnetic head. The concepts developed for the 2-D case could be adequately extended in the 3-D case. Below, 3-Dboundary element modeling (BEM) is utilized to apply the conceptual findings to design a perpendicular magnetic playback head which is magnetically equivalent to a magnetic playback head used in longitudinal recording.

Hof H tMR

tSS

Figure 42. Transformation of a general geometry into a shielded reader at Hof.

Conformal Mapping. The LRH geometry, as shown in Figure 40, is a limiting (H of) case of a more general geometry, as shown in Figure 42. The complementary object to the general LRH geometry is a 180-degree rotated dual head (DH), as shown in Figure 43. Moreover, because the LRH and DH are complementary objects in a complex variable plane, Z = X +iY, according to the Symmetry Principle, these objects could be obtained as a result of correlated transformations of the positive and negative imaginary semi-planes of the complex variable plane, W = U + iV, by two complementary functions, F ( w ) and F ( w ) , respectively. Function F ( w ) could be found according to the Schwartz-Christoffel Transformation [124].

Chapter 3 Physics of Playback

Z=X+iY

W=U+iV

F (w)

SR Y=0

121

ImW > 0

ImW < 0

DH

F (w) Figure 43. The LRH and DH designs are the results of the transformation by complimentary functions, F ( w ) and F ( w ) , of the top and bottom imaginary semi-planes.

In analogy with LRH, it can be noted that in the limiting case, H of, the DH geometry transforms into geometry, consisting of two infinitely tall poles, as shown in Figure 44.

Hof H tGAP

tGAP + 2tPOLE

Figure 44. Infinite-throat-height limit (Hof) of the DH geometry.

According to the Reciprocity Principle [125], the sensitivity field could be used as a measure of the playback performance. According to this theory, the playback signal is a convolution of the sensitivity field and the magnetization in the recording layer. It should be remembered that the use of the Reciprocity Principle assumes that no saturation processes occur in the system. [126]. The sensitivity field, as the field generated by a unit electric current in imaginary coils wrapped around the magnetoresistive element of the read head, is an intrinsic property of the read system. The boundary conditions for the LRH are chosen such that the main pole and the shields have two different values of the scalar potential, 1 and –1, respectively, as shown in Figure 45a. The transformation of the boundary surface into the W-plane is a straight line, ImW=0, with three intervals of specified scalar potentials, as shown in Figure 45a. Similarly, the boundary conditions for the DH are chosen such that the two poles have different scalar potential values, 1 and –1, as shown in Figure 45b. The transformation

Perpendicular Magnetic Recording

122

of the boundary surface of the DH into the W-plane is the same straight line, W=0, with two intervals of specified scalar potential, respectively. The intervals with specified scalar potentials are used as the boundary conditions to solve the Laplace’s Equation in the W-plane in the two cases, as shown in Equations 5 and 6, for the LRH and DH, respectively. A-3

F (w)

Z=X+iY A-2 A2 A3

W=U+iV ImW > 0

M = -1

H Y=0

M=1

a-4 a-3 a-2 a-1 a1 a2 a3 a4

A5 = f A-4 A-1 A1 A4 A5 = f M = 1 M = -1 (a) M = -1 Z=X+iY M = -1 A-3 A-2 A2 A3

F (w )

ImW < 0 M=1

H Y=0

(b)

W=U+iV M = -1

M=1

a-4 a-3 a-2 a-1 a1 a2 a3 a4

A5 = f A-4 A-1 A1 A4 A5 = f

Figure 45. Diagrams showing the “charge” configuration in (a) longitudinal and (b) perpendicular recording.

bnd 2 f · §  bnd 1 ¨  1dt 1dt  1dt ¸ ¸,  Im¨  \ t W S ¨ t W t W ¸ ¸ ¨ ¹ © f bnd 1  bnd 2

(5)

bnd 1 § bnd 2 ¨ 1 dt dt Im¨ \  S ¨ t W t W ¨ ©  bnd 1 bnd 2

(6)

1

³

³

³

³

³

· ¸ ¸, ¸¸ ¹

As the last step to calculate the magnetic field components in the Z-space, it is fairly illustrative to use the representation, as given by Expression 7 [127].

Chapter 3 Physics of Playback

H y i H x



d\ dW dZ

1

,

123 (7)

dW

where dZ/dW is the derivative of the above described transformation functions, F ( w ) and F ( w ) , for the two cases of interest, respectively. Considering the complementary nature of the two cases, it can be noted that Hx and Hy for the LRH are equivalent to –Hy and Hx for the DH, respectively. Such a 90-degree rotation of the sensitivity field, as compared to the LRH, is exactly what is expected from the magnetically equivalent PRH, as described above. Therefore, a pole tip configuration of the DH type satisfies the requirement, providing the LRH and DH dimensions are related as tMR = tGAP, tSS = tGAP + 2 tPOLE, where tMR and tSS are the sensor thickness and the shield-to-shield separation of the LRH, respectively, and tGAP and tPOLE are the gap and the thickness of each pole of the DH, respectively (See Figure 42 and Figure 44). Use of a Soft Underlayer. A typical perpendicular system includes a soft underlayer (SUL), the presence of which is not directly addressed in the arguments presented in Section 0. In the presence of the SUL, the net sensitivity function of a reader consists of the sensitivity function of the reader itself and of the sensitivity function of the reader’s image (with respect to the SUL boundary). Due to the symmetry, the sensitivity function of the reader in the presence of the SUL is given by

H(z)

H R (z)  H I (z)

H R(z)  H R(z  t) ,

(8)

where HR(z) and HI(z’) are the sensitivity functions of the reader and its image, respectively, z and z’ are the distances from the ABS of the reader and its image, respectively, and tRL is the recording layer thickness. The playback is then given by

tFH tRL tFH tRL 3 1 1 S~ M ( z ) ˜ H R ( z ) dr  M ( z  t ) ˜H R ( z  t ) dr 3 , I t³ I t³ FH FH

(9)

where M(z) is the recording layer magnetization and tFH is the flight height. The integration boundaries are shown for the z coordinate. The above equation can be rewritten as

S~ 1 I

t FH  2t RL 3 ³ M( z ) ˜H R ( z ) dr , t FH

(10)

Perpendicular Magnetic Recording

124

where M(z) for z> tFH +tRL is defined as a mirror image of the recording layer magnetization with respect to the SUL boundary. The latter equation describes the playback in the absence of a SUL with the recording layer thickness doubled. Therefore, a DH reader used in a perpendicular recording system with a SUL is magnetically equivalent to a LRH used in longitudinal recording, in which the recording layer thickness is twice as thick as the recording layer thickness in the perpendicular system. 3D BEM Calculation. As a direct application of the developed concept, 3D-BEM calculations were performed [92]. As an example of a PRH with the DH type pole configuration, a yoke type MR or GMR head is considered, as shown in Figure 46. MR element GMR

PT G Yoke

ABS

Figure 46. Side view diagram of a yoke type magnetoresistive head.

The sensitivity field components corresponding to the two head types, LRH and PRH, at a 5nm flying height are shown in Figure 47.

Hx and Hz (normalized)

1 0 -1

Perpendicular

PRH:Hx Hx PR: PRH: Hz PR: Hz

0.5 0.0

-0.5

Longitudinal

LRH: Hx LR: Hx LRH: Hz LR: Hz

-1.0 0.00 0.02 0.04 0.06 0.08 0.10 Down the track (um) Figure 47. Sensitivity field components for the LRH and PRH at a 5nm flying height.

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125

Normalized Playback Amplitude

Assuming 3-D geometry, a different coordinate system was chosen: unlike in the system implied in the 2D CM theory, the y-coordinate stands for the direction across the track, and the z-coordinate stands for the perpendicular direction. Both the standard LRH and the yoke head of the PRH type were modeled to have the same trackwidth, W = 200 nm. Also, the following ratio was kept between the LRH and PRH parameters: tMR = tGAP = 10nm, tSS = tGAP + 2tPOLE = 50 nm. In agreement with the theoretical conclusions above, it can be noticed that Hx and Hz for LRH look similar to -Hz and Hx for the PRH. According to the Reciprocity Principle, the playback signal is convolution between the sensitivity field and the magnetization in a recording medium. Therefore, it can be concluded the two designs provide similar waveforms, if they are used with longitudinal and perpendicular media, respectively. It should be remembered that the arguments presented in the previous section is an approximation. Consequently, the equivalency of a LRH and a PRH is also an approximation. The normalized roll-off curves for a LRH and a PRH used with longitudinal and perpendicular media, respectively, are shown in Figure 48. It can be noticed, that while both curves are similar, they are not identical.

1.0 LRH: longitudinal medium PRH: perpendicular medium

0.8 0.6 0.4 0.2 0.0 0

500

1000 1500 2000 Linear Density (kfci)

2500

Figure 48. Normalized playback amplitude versus linear density for a LRH and a PRH used with longitudinal and perpendicular media, respectively.

The implementation of the design above is subject to the fine control of the magnetic domain noise within the yoke structure [128]. A design that addresses the issue of domain noise control in a DH, is a dual element magnetoresistive reader [129,130], a schematic of which is shown in Figure 49. Because the sensitivity field is defined largely by the yoke structure employed in the reader design, both the yoke type (G)MR sensor shown in Figure 46 and the dual (G)MR sensor shown in Figure 49 are magnetically equivalent.

126

Perpendicular Magnetic Recording

MR1

PT G

Yoke

MR2

ABS

Figure 49. Side view diagram of a dual (G)MR element reader.

Conclusions on Study of Parallels between Playback in Perpendicular and Longitudinal Recording. Conformal mapping theory was developed to show the playback equivalency between the conventional shielded GMR read-head configuration used in conjunction with a longitudinal medium and the DH configuration used in conjunction with a perpendicular medium. The approach chosen to design a PRH, which is magnetically equivalent to a LRH, was identifying the head configuration sensitive predominantly to the longitudinal stray field, thus significantly reducing reading away from the transitions. The requirements for maintaining the playback waveform equivalency between these two head configurations are: tMR=tGAP and tSS=tGAP+2tPOLE, where tMR and tSS are the thickness and shield to shield separation of the LRH, respectively, and tGAP and tPOLE are the gap and the thickness of each pole tip of the PRH, respectively. This concept was extended to 3D and supported by 3D BEM calculations of the sensitivity fields for the two types of heads. As examples of the PRH’s in 3D case, a yoke type (G)MR reader and a dual (G)MR reader were considered.

Chapter 4 Perpendicular Recording Media

Chapter 4

Perpendicular Recording Media

1. Introduction

As magnetic data storage industry is facing its fundamental limit due to thermal instabilities in the longitudinal recording media [131], perpendicular magnetic recording is becoming the center of attention in the industry [132,133]. The use of perpendicular magnetic recording media instead of conventional longitudinal media is the main reason why perpendicular recording is considered to be the technology capable of deferring the superparamagnetic limit to areal densities much beyond 100 Gbit/in2. As described above, a typical perpendicular medium consists of two main magnetic layers [134,135]: 1) the recording (magnetically “hard”) layer [136] and 2) the magnetic “soft” underlayer (SUL) [137,138, 139]. Such double-layer perpendicular medium is usually used together with a single pole recording head, as described above in Chapter Physics of Writing. 1.1.CHAPTER OVERVIEW In this chapter, the results of theoretical and experimental study of some of the key issues related to perpendicular magnetic media are presented. To stress the specific aspects of the recording physics native to perpendicular recording, a comparison between longitudinal and perpendicular recording media is carried out throughout the entire chapter. Specific attention is given to the role of the soft underlayer as a new component in the recording process. Among the discussed issues are the guidelines and the underlying physics to choose the optimized parameters of the recording layer and the soft underlayer and the integration of these two components. The noise due to the soft underlayer and means to minimize the noise are discussed. In addition, it is described how Kerr microscopy could be utilized to study the dynamics of perpendicular recording with a soft underlayer.

127

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2. Perpendicular Recording (“Hard”) Layer

The primary approach to the design of a perpendicular recording layer is in many ways similar to the design of a conventional longitudinal media. Major goals relevant to the development of both longitudinal and perpendicular recording layers are: achieving sufficiently small grain size and grain size distribution, texture control, optimization of the inter-granular quantum exchange de-coupling, etc. However, some aspects of the recording layer design are specific only to the perpendicular recording modes [140]. Understanding the fundamentals of the aspects inherent to perpendicular recording media is the subject of this chapter. 2.1. TYPES OF MEDIA Among the large variety of today’s perpendicular magnetic recording media, CoCr-based alloys, Co/Pt-based mutlilayers, L10 phases of FePt, BaFe, and others, one could separate two large categories which have been most thoroughly explored for this purpose: (1) CoCr-based alloy media and (2) media based on magnetic multilayers, such as Co/Pt, Co/Pd or others [141,134,142,143,144,145].

Co Pd Figure 1. A schematic diagram of the cross-sectional view of a Co/Pd-multilayer-based recording layer.

Material-wise, perpendicular CoCr-based alloy [146,147,148] recording layers are similar to the conventional longitudinal CoCr-based media, with the major difference being the orientation of the magnetic easy axis [149]. Therefore, a significant amount of information accumulated in the course of the longitudinal media development could be used to control the critical parameters such as the grain size and the inter-granular quantum exchange coupling in the perpendicular media. At the same time, the development of CoCr-based perpendicular media has some unresolved issues not encountered in the development of the longitudinal media [150]. For example, it is not yet clear whether it is feasible to engineer a CoCr-based medium with sufficiently high anisotropy to avoid thermal instabilities at ultra-high areal densities. It has also proved not to be trivial to engineer CoCr-alloy-based perpendicular recording layers with a remanent squareness of 1. It is believed that a remanent squareness of 1 is necessary for the low-density bit pattern stability. Also, remanent squareness of < 1 can lead to substantial amounts of DC noise. Various magnetic alloys such as L10 phases of FePt, CoPt, etc., are being studied as highest anisotropy alternatives for the recording layer [151,152,153]. Selected material properties, such as the anisotropy density, Ku, the saturation magnetization, Ms, the anisotropy field, Hk, and the minimum stable grain size,

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a, as defined earlier in Chapter Fundamentals of Perpendicular Recording, for different alloy systems, including Co-based alloys, L1 phases, and rare earth materials, are shown in Table 1. Table 1 Selected material properties (the anisotropy density, Ku, the saturation magnetization, Ms, the anisotropy field, Hk, and the minimum stable grain size, a, as defined earlier in Chapter Fundamentals of Perpendicular Recording) for different alloy systems: Cobased alloys, L1 phases, and rare earth materials. Alloy System

Co-alloy

Material

CoCrPtX Co Co3 Pt

FePd FePt CoPt MnAl Rare Earth Nd2 Fe 14 B SmCo5 L10 -phase

Anisotropy

Saturation Magnetization

Anisotropy Field

Minimum stable grain size

Ku (10 7erg/cc)

Ms (emu/cc)

Hk (kOe)

a (nm)

0.20 0.45 2.00 1.8 6.6-10 4.9 1.7 4.6 11-20

200-300 1400 1100 1100 1140 800 560 1270 910

15-20 6.4 36 33 116 123 69 73 240-400

8-10 8.0 4.8 5.0 2.8-3.3 3.6 5.1 3.7 2.2-2.7

Figure 2. Top view TEM images of (a) a CoCrPtTa-alloy-based and (b) Co/Pd multilyaer –based recording layers, respectively.

The multilayer-based recording layers (See Figure 1) typically have significantly larger anisotropy (coercive fields of above 15 kOe have been reported) and thus promise to be extendable to significantly higher recording densities [154]. In these materials, the magnetic anisotropy is controlled through the (surface) interfacial interaction between the magnetic layer, (Cobalt) and a highly polarizable spacer layer (Palladium or Platinum). In

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contrast to the alloy media, the multilayers typically display a very weak texture. Top view TEM images of CoCrPtTa-alloy-based and Co/Pd-multilayer-based recording layers are shown in Figure 2a and b, respectively. The alloy-based and multilayer-based media were sputter-deposited on Ti and ITO-based seed layers, respectively. A (thickness) cross-sectional TEM image of the Co/Pd-based medium is shown in Figure 3. This image clearly indicates a columnar-type texture with an average column size of approximately 20 nm and with no ordered structure across the thickness. In other words, despite the strong perpendicular anisotropy of the multilayer medium (due to the surface energy, as discussed earlier), typically no matching is detected between the sets of easy axes in the adjacent Pd sub-layers.

Figure 3. A cross-sectional TEM image of a C/Pd-multilayer-based medium.

Another advantage of the magnetic multilayers is the fact that typically these materials have a remanent squareness of 1. The squareness of 1 indicates that the anisotropy field, Hk, which keeps the magnetization in the perpendicular to the disk direction, is larger than the maximum demagnetization field, 4SMs. Consequently, because the demagnetization field reaches its maximum in the low-density limit, a medium with a squareness of less than 1 tends to be unstable at low densities. In this case, the relatively strong demagnetization field substantially increases the chance of the magnetic moment to be reversed as a result of thermal fluctuations [155,156]. To compare basic magnetic properties of CoCr-alloy and multiplayer based recording layers, typical M-H loops [157] by a Kerr magnetometer [158,159] for a 50 nm thick perpendicular CoCr thin film and a 52 nm thick Co/Pd structure (a stack of 40 sets of adjacent 3 and 10 A thick layers of Co and Pd, respectively) are shown in Figures 4a and b, respectively. It can be noted that in addition to the remanent squareness of 1, the Co/Pd structure exhibits nucleation fields in excess of 3 kOe, a useful characteristic to avoid data self-erasure due to stray fields. Meanwhile, the CoCr material shown in Figure 4a has squareness of 0.75. The CoCr and Co/Pd recording layers have coercive field and magnetization of approximately 3 and 9 kOe and 300 and 200 emu/cm3, respectively.

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Figure 4. A M-H loop of a 50 nm thick (a) CoCr-alloy layer and (b) Co/Pd multilayer.

The direct consequence of remanent squareness of < 1 is shown in Figure 5, which compares the spectral SNR distributions for the two media types [160]. The CoCr medium exhibits a significant amount of noise at lower linear densities. This is mainly due to the fact that the dominant contribution to the noise at low linear density in the CoCr-base medium comes from the DC noise that results from the relatively low value of remanent squareness, as described below in more details. 2.2. CONTINUOUS MEDIA Also, it should be mentioned that there is another type of a magnetic recording medium, which, similarly to a typical magneto-optical recording medium, due to relatively strong exchange coupling between grains, acts as a magnetically continuous media [161]. In these so-called continuous magnetic materials, the bit separation is determined not by the grain size, but rather by the domain wall width. The domain wall width (in these relatively high anisotropy magnetic materials) could be as thin as few Angstroms. The coercivity field for these materials strongly depends on the mechanism and strength of the pinning of the domain walls to naturally or artificially created defects. Today, because of many open questions continuous medium recording is not considered as the nearfuture alternative to longitudinal recording, and research activities in this area are still fairly rare. Therefore, the continuous materials are not covered in this chapter. 2.3. MAGNETIC FIELD CALCULATION In this chapter, two approaches are used to calculate the magnetic fields. The analytical solution of the Laplace’s Equation is used to calculate the stray and demagnetization field for the cases of periodic bit patterns written into a perpendicular recording medium, as shown in Equations 1 and 2, respectively, where G and Ms are the hard layer thickness and saturation magnetization, and a and b are the bit length and width, respectively [162,163].

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Figure 5. SNR versus the linear density for a CoCr-alloy (hollow circles) and a Co/Pd multilayer (hollow squares).

The origin of the reference coordinate system was chosen to be located at a corner of a bit at the top side of the recording layer. To study the dependencies on the BAR and the areal density (AD) the transformation equations representing the bit length and width, a and b, via BAR and AD, can be used, as shown by System of Equations 3. Note that the BAR is defined as the ratio of the track width to the bit length, b/a, and the effect on the fields of the use of a SUL is equivalent to a two-fold increase of the recording layer thickness. Therefore, the same expressions can be used to model an ideal SUL just via replacing G with 2G.

Figure 6. An illustration of the mirror imaging by an ideal SUL.

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133

f 32 M s sin §¨ S n x ·¸ sin §¨ S k y ·¸ ¦ © a ¹ © b ¹ n 1k 1 S n k odd odd § n ·2 § k ·2 § n ·2 § k ·2  S ¨¨¨ ¸¸¸  ¨¨¨ ¸¸¸ z  S ¨¨¨ ¸¸¸  ¨¨¨ ¸¸¸ G ©a¹ ©b¹ ©a¹ ©b¹ u" (1  " ), z ! 0

(1)

f f 32M s §Sn · §Sk · sin ¨ y¸ x ¸ sin ¨ ¦ ¦ © a ¹ © b ¹ n 1 k 1 S nk odd odd 2 2 2 ª S §¨ n ·¸  §¨ k ·¸2 z S §¨ n ·¸  §¨ k ·¸ (G  z ) º «" © a ¹ © b ¹  " », G  z  0 ©a¹ ©b¹ » « »¼ «¬

(2)

H z stray

f ¦

H z demag

a

BAR ; AD

b

1 BAR ˜ AD

(3)

Three-dimensional (3D) boundary element modeling (BEM) using a commercial field solver, Amperes, is used to calculate the magnetic field when bit patterns are written into a longitudinal medium as well as to evaluate the magnetic field generated by magnetic recording heads [164]. It should be noted that within the precision of the calculations, the BEM applied to periodic bit patterns in perpendicular media gives the results identical to the results, which were calculated using the analytical solution [136]. The approximation of an “ideal” SUL is used in all calculations presented in this chapter [165]. It should be reminded that the effect of the presence of an ideal SUL on the stray and demagnetizing fields generated by a recording layer is equivalent to the perfect mirror-imaging of the recording layer with respect to the SUL boundary, as illustrated in Figure 6. The fields above the SUL boundary are equal to the sum of the fields generated by the real recording layer and by its imaginary counterpart located below the SUL boundary. If there is no separation between the recording layer and the SUL (in other words, no buffer/exchange-decoupling layer is present), the use of the SUL is equivalent to a two-fold increase of the recording layer. Unless specified otherwise, it is assumed

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that the thickness of the buffer layer is substantially smaller than the thickness of the recording layer and, therefore, can be neglected.

Figure 7. The demagnetization field versus the distance down the track along the central planes of 10 and 20 nm thick recording layers for (a) longitudinal recording, (b) perpendicular recording without and (c) with a SUL.

Figure 8. MFM images of tracks recorded into 30 nm thick CoCr alloys with a magnetization of (a) 200 emu/cm3 and (b) 400 emu/cm3.

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It should be remembered that the use of a SUL is not equivalent to the effect of mirrorimaging when net energy is to be evaluated. Therefore, the magnetic mirror-imaging should be used with caution when applied to the problems that deal with the bit energy. For example, one cannot combine the energy of a bit with the “energy” of its magnetic image to estimate the thermal stability of recorded information [166].

Figure 9. The maximum demagnetization field along the central line of 10, 20, and 40 nm recording layers without a SUL at two values of linear density, 50 and 316 kfci.

2.4. DEMAGNETIZATION FIELD IN PERPENDICULAR RECORDING LAYER The calculated normalized demagnetization field near a single ideal transition along the central plane of a recording layer is shown for longitudinal and perpendicular recording layers with and without a SUL at two different values of the recording layer thickness, 10 and 20 nm, are shown in Figures 7a-c, respectively [162]. In these calculations, a relatively wide trackwidth is assumed. First, it can be noted that, unlike in longitudinal recording, the demagnetization field in perpendicular recording decreases as the thickness increases, thus promoting a larger thickness. If a perpendicular medium with a SUL is used, the SUL effectively further increases the recording layer thickness. Also, unlike in the longitudinal medium, in both types of perpendicular media, the demagnetization field reaches its minima at the transitions, thus promoting high-density recording. In this respect, it is common to notice that although perpendicular recording promotes high densities, the stronger influence of the demagnetization field at lower densities is a disadvantage of perpendicular recording. One of the direct consequences of the strong demagnetization fields at low densities is a relatively strong dc-noise from perpendicular media with a squareness of <1, as mentioned above. MFM images of wide (with respect to the media thickness) tracks recorded into two 30 nm thick CoCr alloy films with a coercive field, Hc of

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Perpendicular Magnetic Recording

approximately 2 kOe and magnetization values of 100 and 400 emu/cm3 are shown in Figures 8a and b, respectively. The presence of smoother and more uniform tracks in the case of the smaller magnetization indicates less dc-noise than in the other case.

Figure 10. The demagnetization field versus the areal density at the center of a bit (a) in the middle between the top and bottom surfaces and (b) near the top surface of the recording layer for 10 nm thick recording layers as a function of the areal density for different values of BAR.

The maximum demagnetization field at the center of a bit for three values of the recording layer thickness with and without a SUL versus the track density for two values of the linear density, 50 and 316 kfci, respectively, is shown in Figure 9. If considering only the magnetics of the recording process, perpendicular recording is symmetric with respect to the directions along and across the track. For example, the demagnetization field and stray field contours look similar (if rotated 900 in the plane) for the two sets of the following values for the linear and track densities: (1) 316 kfci and 50 ktpi, and (2) 50 kfci and 316 ktpi. Therefore, to avoid repetition, only one of the two data sets is presented. In addition, it should be noted that under the assumption of zero spacing between the recording layer and the SUL, 10 and 20 nm cases with a SUL are equivalent to 20 and 40 nm cases with no SUL, respectively. As shown in Figure 9, the demagnetization field decreases with the increase of the track density. The rate of the density roll-off is determined by the effective recording layer thickness and by the value of the linear density. (It should be reminded that the linear and track densities are determined by the bit length and width, respectively.) It can be noted that to avoid issues with thermal instability and dc-noise, a larger thickness and a higher track density are preferred. The SUL increases the effective recording layer thickness and thus reduces the demagnetization field. However, it should be noted that the demagnetization field cannot be reduced through the reduction of the trackwidth if the adjacent tracks mimic the main track. Therefore, a special encoding might be necessary to avoid the unfavorable bit patterns at low densities. It is important to point out that at present, the available encoding schemes (to accomplish the above-mentioned task) result in a substantial loss in the recording density.

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137

Hstray + +

(a)

+ +

M

charges in the transition

Hstray (b)

(c) Medium image

++++++++ -------------------

+++++++

Charges on two surfaces

++++++++ ----------

----------

+++++++

Underlayer boundary

Figure 11. Diagrams showing the "charge" distribution in (a) longitudinal media and perpendicular media (b) without and (c) with a SUL.

An additional insight into the nature of the demagnetization field in the perpendicular recording layer can be obtained if the demagnetizing field is plotted against the areal density for different values of the bit aspect ratio (BAR). The demagnetizing field at the center of a bit in the middle between the top and bottom surfaces and near the top surface of the recording layer is shown in Figures 10a and b, respectively. It can be seen that a higher value of BAR at a given areal density results in a lower demagnetizing field. This is in contrast to longitudinal recording where the demagnetizing field increases as the value of BAR is increased. 2.5. STRAY FIELD FROM PERPENDICULAR RECORDING MEDIA It should be mentioned that in perpendicular recording stray magnetic fields (the fields sensed by a reader) emanate not from the magnetic “charge” in transitions, as in longitudinal recording, but from the “charge” at the top and effective (due to the use of the SUL) bottom sides of the recording layer, as shown in Figures 11a-c [140]. Outside the recording layer the fields from the top and effective bottom “charge” are of opposite directions; hence, they cancel each other if the recording layer thickness is significantly

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smaller than the characteristic bit sizes. As shown in Figure 11c, the presence of the SUL effectively doubles (in case of perfect imaging) the recording layer thickness.

Figure 12. The stray field above the center of a bit in 10, 20, and 40 nm thick recording layers without a SUL at a 5 nm flying height versus the linear density for two values of the track density, 50 and 316 ktpi.

The net stray field emanating from the center of a bit in a periodically written bit patterns at two values of the track density, 50 and 316 ktpi, versus the linear density for media with a recording thickness of 10, 20, and 40 nm with no SUL, are shown in Figure 12. It should be stressed again that under the assumption of zero spacing between the recording layer and the SUL, 10 and 20 cases with SUL are equivalent to 20 and 40 nm cases with no SUL, respectively. In these calculations, a flying height of 5 nm is assumed. It can be noted that for the narrower trackwidth, the signal could be substantial even at relatively low linear bit densities because no total compensation of the fields generated by the top and effective bottom charges occurs. It is not unnatural that at sufficiently high densities, the stray field drops because the bit cell surface area containing the effective magnetic “charge” and, thus, also the net magnetic “charge” becomes relatively small. Regardless of the effective recording layer thickness, the characteristic bit length at which this happens cannot be smaller than approximately the flying height. At the same time, regardless of the linear and track density values, the stray field drops with the thickness reduction because of the above-mentioned field cancellation effect due to the magnetic “charge” at the top and effective bottom surfaces of the disk. (The latter effect is equivalent to the stray field reduction at some finite thickness as the bit dimensions become substantially larger than the thickness.) In the intermediate case, the stray field generated by the effective “charge” at the top surface is sufficiently large to generate a

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139

non-negligible field and at the same time cannot be compensated by the “charge” from the effective bottom side. Therefore, the stray field’s dependence on the areal density has a maximum, as illustrated in Figure 12.

Figure 13. The stray field above the center of a bit in a 10 nm thick recording layer without a SUL at a 5 nm flying height versus the linear density for different values of the spacing between a recording layer and a SUL.

From the above arguments, it is also clear that unless a special encoding is not performed to limit the maximum physical bit dimensions, the stray field becomes negligibly small at sufficiently low densities. Assuming that a special encoding is performed to avoid the low-density degradation of the stray field, the limiting lower boundary’s value for the recording layer thickness is determined by the highest values of the required linear and track densities. For example, aiming for 200 Gbit/in2 at a 2:1 bit aspect ratio (BAR), the track and linear densities would be 316 ktpi and 632 kfci, respectively. For these values of the track and linear densities, the signal rapidly drops as the thickness becomes smaller than approximately 10 nm. By now, it was assumed that there is no separation between the recording layer and the SUL. The effect of a non-zero spacing between the recording layer and the SUL is shown in Figure 13. It can be noted that the difference in the values of the stray field between a 0 nm spacing and a 4 nm spacing can be as large as 20 %. Although this effect does not alter any of the conclusions presented in this chapter, it should be taken into account when optimizing the design of a perpendicular recording system.

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Figure 14. Perpendicular component of the stray field versus the areal density for the perpendicular mode with and without a SUL.

Figure 15. Perpendicular component of the stray field versus the areal density for the longitudinal recording mode.

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141

It gives an additional insight into the difference between perpendicular and longitudinal recording if one compares the stray fields emanating from perpendicular and longitudinal media as functions of the areal density at different values of bit aspect ratio (BAR). The perpendicular component of the stray field at a 5 nm distance from the center of a bit in a periodically written pattern for a 10 nm thick hard layer at 3 values of BAR, 1:1, 4:1, and 8:1, is shown for perpendicular recording with and without a SUL in Figure 14. Since in the case of longitudinal recording the directions along the track and across the track are not equivalent, one needs to consider the BAR values corresponding to both types of bits, elongated along the across the track. The perpendicular component of the stray field at a 5 nm distance from the center of the transition in a periodically written pattern of a 10 nm thick longitudinal thick longitudinal hard layer at 6 values of BAR, 1:4, 1:2, 1:1, 2:1, 4:1, and 8:1, is shown in Figure 15. The pronounced peak in the perpendicular cases, as seen in Figure 14, can be explained by the above-mentioned cancellation effect of the fields generated by the top and bottom magnetic “charge”. It can be noted that there exists a certain characteristic density of ~ 100 and ~ 200 Gbit/in2 for the cases with and without a SUL, respectively, above and below which the stray field depends differently on the areal bit density. This characteristic density increases with the reduction of the recording layer thickness.

Figure 16. The peak value of the perpendicular component of the stray field versus the recording layer thickness for the perpendicular mode without a SUL.

The presence of the SUL is equivalent to the increase of the recording layer thickness by a factor of two. The observed essential sensitivity to the recording layer thickness can be explained by the above-described nature of the magnetic “charge” in perpendicular recording. As the areal density increases, the magnitude of the stray field becomes

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independent of the recording layer thickness. As illustrated in Figure 16, the peak value of the stray field strongly depends on the recording layer thickness and is independent of the BAR. Roll-off curves show no peak in the case of longitudinal recording. Rather, they continuously drop for all values of BAR. The slowest roll-off is observed for a 1:1 value of BAR. Although the stray field roll-off curves are rather complex and strongly depend on the value of BAR, it can be noted that in perpendicular recording, the stray field roll-offs substantially slower than in longitudinal recording. This is due to the fact that the stray field is composed of the fields from the top and effective bottom “charge” of the recording layer, respectively. The two fields are oppositely directed with respect to each other, thus the field generated by the bottom “charge” reduces the field generated by the top “charge” and in overall reduces the net field. As the bit size becomes smaller, the relative contribution of the bottom “charge” decreases thus slows the overall decrease of the field amplitude with the areal density increase. 2.6. WELL-DEFINED PERPENDICULAR EASY AXIS: THICKER RECORDING LAYER?

Figure 17. Schematic diagrams illustrating (a) a longitudinal medium with randomly oriented easy axes and (b) a perpendicular medium with aligned easy axes, respectively.

In perpendicular recording, the easy axis is relatively well aligned in one direction (perpendicular), unlike in a conventional longitudinal medium, in which the easy axes are randomly oriented in the 2D plane of the disk, as shown in Figure 17. X-Ray “rocking” curve with a peak corresponding to the perpendicular easy crystalline axis (which coincides with the perpendicular magnetic “easy” axis) for CoCrPtTa alloy medium grown on a Ti seed layer is shown in Figure 18. The “rocking” curve indicates a texture

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spread of less than 6.3 degree. Typically, the texture spread varies from 1 to 10 degrees, depending on the deposition condition, seed layer(s), etc. A well-defined easy axis potentially relaxes the stringent requirement to the trailing and side writing field gradient necessary to achieve sharp transitions, thus enabling the use of thicker media. Consequently, this also explains the excellent overwrite in perpendicular recording. Even, for sub-100-nm trackwidth recording, overwrite of the order of 40 dB could be easily achieved [63].

X-ray rocking curve

80000

FWHM=6.30

70000 60000

ideal

50000 40000 30000

non-ideal

20000 10000 0

0

(a)

10

20

Z

30

40

(b)

Figure 18. (a) X-Ray rocking curve of a perpendicular CoCrPtTa alloy medium. (b) A schematic diagram showing ideally and non-ideally aligned magnetic media.

The intrinsically better alignment of perpendicular media helps to record narrow tracks with well-defined transitions even into a relatively thick recording layer, in contrast with longitudinal recording. A MFM image of two adjacent tracks with a 65 nm trackpitch written into a 50 nm thick CoCr recording layer using a 50 nm wide single pole head is shown in Figure 19.

Figure 19. A MFM image of two adjacent 65-nm wide tracks.

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144

In this respect, it should be remembered that previously it was shown that, although wellaligned perpendicular media might have a relatively small average angle between the magnetization and the perpendicular recording field, the torque between the magnetization and the field is still sufficiently large to relatively rapidly switch the magnetization [167]

3. Soft Underlayer

Figure 20. Mirror image representation of the SUL from the write head perspective.

One of the key features of perpendicular recording that makes it superior compared to longitudinal recording with respect to the superparamagnetic instabilities is the use of media with a soft underlayer (SUL) [137,168,169]. Integration of a single pole head and media with a SUL enables recording magnetic fields in excess of 80% of 4SMs, where 4SMs is the saturation magnetization of the head material. For comparison, state-of-theart longitudinal systems are only capable of recording fields of less than 2SMs. Such a substantial increase in the recording field in perpendicular recording due to the use of a SUL opens the possibility to record on media with substantially higher anisotropy and thus leads to improved thermal stability. As mentioned earlier (See Figure 6), because of the un-ambiguity of the solutions of the Laplace’s Equation (used to calculate the magnetic field) with respect to the adequate boundary conditions, ideally, the SUL can be represented as a magnetic mirror and the effect of the SUL in the static sense is equivalent to the two-field increase of the thickness of the recording layer. The effective increase of the recording layer leads to both increase of the stray magnetic field sensed by

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a read head and reduction of the demagnetizing fields with a potential to further improve thermal instability. The described view of the mirror imaging of the recording layer in the presence of the SUL is the perspective of the read head. From the perspective of the write head (another mirror imaging perspective), the SUL can be equivalently replaced with a mirror image head placed on the other side of the SUL symmetrically with respect to the SUL’s side adjacent to the recording layer, as shown in Figure 20. However, while it is expected that the use of a SUL should make it possible to defer the superparamagnetic limit to areal densities beyond 1 Terabit/in2, the SUL also introduces a number of technical challenges and issues. These issues should be resolved before perpendicular recording can be fully implemented. 3.1. SATURATION MOMENT The importance of the relation between the moment of the SUL’s material and the moment of the recording pole tip’s material has been previously discussed [170]. It has been shown that saturation of the SUL can lead to a dramatic deterioration of the trailed field gradients. According to the Maxwell’s laws, div B = 0 and ’ u H = 0, to avoid the SUL’s saturation under the pole tip, the following inequality has to hold, 4SMS SUL u ASUL effective t 4SMS Pole Tip u AABS Pole Tip ,

(4)

where ASUL effective is the effective area of the SUL into which the magnetic flux emanating from the pole tip enters and AABS Pole Tip is the area of the air bearing surface (ABS) of the pole tip.

Figure 21. A schematic diagram of the recording head pole tip/SUL combination.

Figure 21 shows a schematic diagram of the combination of the recording pole tip and SUL. When the separation between the ABS of the pole tip and the SUL (d ~ 10-20 nm)

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Perpendicular Magnetic Recording

is substantially smaller than the lateral dimensions of the pole tip (lPole Tip ~ 300 – 1000 nm, wPole Tip ~ 50 – 150 nm), the effective area of the SUL, ASUL effective, is approximately equal to the ABS area of the pole tip, AABS Pole Tip. It follows that to avoid saturation in the SUL, the magnetization of the SUL’s material should be equal or higher than the magnetization of the recording tip’s material.

Figure 22. Trailing field from a single pole perpendicular write head made of FeAlN with a SUL made of two different materials, FeAlN and Permalloy.

Although it is possible to generate strong recording fields (with magnitude approaching 4SMs of the pole tip) even if the SUL has lower magnetization compared to the pole tip, saturation of the SUL will lead to a substantial deterioration of the trailing field gradients. The results of the boundary element modeling (BEM) for two different head/SUL combinations are presented in Figure 22. The two cases include the SUL made of 1-Pm thick Permalloy and FeAlN, respectively. Permalloy and FeAlN have saturation magnetization of approximately 10 and 20 kGauss, respectively. As shown below, the thickness was chosen to be sufficiently large to avoid the geometrically caused saturation. It can be observed that the trailing field gradient for the Permalloy-based SUL is substantially lower than the trailing field gradient for the FelAlN-based SUL. 3.2.

THICKNESS OF SOFT UNDERLAYER

The following approximate equation gives the value of the minimum thickness of the SUL necessary to achieve the maximum recording field with the maximum trailing field gradient, with assumption that the pole tip length is substantially larger than its width, lPole Tip ! wPole Tip (See Figure 21),

Chapter 4 Perpendicular Recording Media

M t SUL MIN ~ 1 S PoleTip wPole Tip . 2 M S SUL

147 (5)

.

The validity of the above equation could be proven using arguments similar to the arguments used in the previous section and is a consequence of the conservation of the magnetic flux.

Figure 23. The amplitude and the gradient of the trailing field generated by a single pole head made of FeAlN with a FeAlN-made SUL versus the SUL's thickness.

Figure 23 shows the results of the boundary element modeling of the magnetic field and the field gradient for the recording field generated by a recording head with a 0.4 x 2 Pm2 pole tip cross-section with SULs of different thicknesses. Both the SUL and the pole tip are assumed to be made of FeAlN with a 4SMs of 20 kG. It can observed that as the SUL’s thickness becomes smaller than approximately 0.2 Pm, the field gradient rapidly drops and thus the performance of the recording system deteriorates. Media with almost identical recording layers and different SULs were fabricated to test the validity of the theoretical results outlines above. The pole tip was made of Ni45Fe55 alloy with a 4SMs of 16 kG and the SUL was made of FelAlN with a 4SMs of 20 kG. The

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recording trackwidth (~ wPole Tip) was approximately 0.8 Pm. Then, according to Equation 5, tSUL MIN ~ 0.3 Pm. Figure 24 shows the dependence of the saturation current on the thickness of the SUL. As the SUL’s thickness is reduced below 0.3 Pm, the saturation current slowly increases, which can be explained by the partial saturation of the SUL. The substantially more rapid saturation with the thickness reduction below approximately 100 nm can be explained by the total saturation of the SUL.

Figure 24. Saturation current versus the SUL's thickness. FeAlN-made SuL and 0.8-Pm wide Ni45Fe55 single pole head were used.

Figure 25 presents roll-off curves for four different values of the SUL’s thickness, 01, 0.2, 0.3, and 0.4 Pm, obtained with a 0.8-Pm wide Ni45Fe55-made recording head. In agreement with the theoretical consideration and conferring with the result in Figure 24, the resolution of the recording system starts to deteriorate as the SUL’s thickness is reduced below 0.3 Pm. One of the consequences of the above discussion is the fact that to achieve a sufficiently efficient recording process it is not necessary to make a SUL excessively thick. SUL with thickness values of above 1 Pm have been routinely reported in literature. Often, it is not trivial to fabricate such a relatively thick SUL without loosing on the short-range uniformity. This is not an acceptable feature, especially at high areal densities. To illustrate an example of how the above-described concept can be utilized, a recording system for an areal density of 100 Gbit/in2 can be considered. Assuming a 4:1 bit aspect ratio (BAR), at 100 Gbit/in2 density, a bit cell would have a 160 x 40 nm2 cross-section. Assuming a SUL made of a high moment material such as CoFe composition with a

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149

saturation magnetization of 24 kG, for a FeAlN-made recording head of the single pole type, as analyzed above, with a saturation magnetization of 20 kG, the SUL can be as thin as ~ 70 nm. With allowing for such a thin SUL, it makes the deposition of the SUL to be a fairly straightforward process even with conventional media deposition tools.

Figure 25. Roll-off curves for FeAlN-made SULs with different thickness values.

3.3. SUL-TO-ABS SEPARATION Apart from the clear advantages (for a better recording field gradient and others) of having the SUL as close to the air-bearing surface (ABS) of the recording head as possible, certain consideration concerning the playback of the recording system should be made while designing a perpendicular recording medium with a SUL. Figure 26 shows the dependence of the half-width, PW50, and the amplitude of the read sensitivity function of a 30-nm thick reader as a function of the SUL-to-ABS separation. Two effects could be identified from Figure 26. As expected, the amplitude of the read sensitivity function decreases with the increase of the separation between the SUL and the ABS, i.e., the closer the SUL to the ABS is, the higher playback signal could be expected. This trend suggests minimizing the SUL-to-ABS separation. The other observation, which is crucial for optimizing the system’s resolution, is that there exists a maximum in the value of the PW50 as the SUL-to-ABS distance is varied. From Figure 26, it is obvious that a special care should be taken to correctly choose SULto-ABS distance. Shown below experimental data indeed indicate that this effect can substantially deteriorate the playback performance of a perpendicular recording system with a SUL, thus making it, potentially, even worse than the playback performance of a perpendicular recording system in which media without a SUL is used.

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Figure 26. The dependence of the amplitude of the read sensitivity function and of the half-width PW50 on the SUL-to-ABS separation.

Figure 27. Roll-off curves for media with and without a SUL.

Figure 27 compares roll-off curves for equivalent media with and without a SUL [171]. In both cases, the recording layer was made of CoCr-based alloy. The drastic effect of using perpendicular media with a SUL (the SUL-to-ABS distance is not optimized) is clearly observed. 3.4. ANISOTROPY: MICROMAGNETICS OF SUL

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This section addresses the micromagnetics of a SUL in the presence of stray fields generated by a recording layer [166,172,173,174]. It is confirmed that relatively high anisotropy SUL materials need to be utilized to optimize the performance of the recording system. As was elaborated in Reference [166], when the characteristic bit size in the recording layer becomes comparable to the characteristic in-plane length, G, in the SUL film (which is of the order of the domain wall thickness), the imaging ability of the SUL deteriorates affecting the playback performance. This characteristic length is defined via the following expression:

G ~ A / KU

A / 2SH K M S

,

(6)

and its value along with other selected material properties for four popular SUL material candidates is summarized in Table 2. The four materials are: 1) Ni81Fe19 (Permalloy), 2) FeAlN (one of the high moment nitrides), Ni45Fe55 (high moment composition with high stress induces anisotropy), and Fe65Co35. Here, Hk and Ku are the anisotropy energy density and the anisotropy field, respectively, 4SMs is the saturation moment, A is the quantum exchange constant, and LD is the linear density.

Table 2 Selected materials properties for four types of SUL: Ni81Fe19 (Permalloy), FeAlN, Ni45Fe55 (with high stress induces anisotropy), and Fe65Co35. Hk is the anisotropy field, 4SMs is the saturation moment, A is the quantum exchange constant, and LD is the linear density.

The inability of the SUL to image high frequency spatial variations of the magnetization in the recording layer causes effective formation of a magnetically “dead” layer at the top of the SUL leading to effective increase of the SUL-to-ABS separation. As shown in Figure 26, according to 3-D finite element calculations, the increase of the SUL-to-ABS separation indeed leads to accelerated roll-offs.

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152

The following magnetic force microscopy (MFM) experiment (courtesy of Isabel Trindade) shows an effect of the existence of a “dead” layer in the SUL deposited on top of a demagnetized 20-nm thick Co/Pd perpendicular recording layer. To generate a magnetic domain pattern, each Co/Pd thin film used in this experiment was demagnetized before the SUL was deposited on top of it. An MFM image of the surface of the demagnetized recording layer (before the SUL was deposited on top of it) is shown in Figure 28a. Then, two SUL films made of Permalloy were deposited on two of the demagnetized thin films. The SUL films were different in their thickness: they were 80 and 10 nm thick, respectively. MFM images of the two SUL films are shown in Figures 28b and c, respectively. It could be noted that the 80-nm-thick SUL is sufficient to screen the demagnetization pattern in the recording layer while the 10-nm-thick SUL apparently fails to do so. This directly indicates that the 10-nm-thick SUL is magnetically “dead”. Other experimental data presented below demonstrate that the micromagnetic behavior of the SUL leading to the accelerated roll-offs, indeed, takes place and strongly depends on the anisotropy of the SUL’s material. Co/Pd-multilayer-based media with close to identical recording layers and different SULs were tested using spin-stand recording. The similarity of the recording layers was confirmed via measuring M-H-loops and microstructure of the films. All the measured parameters for the recording layers, such as coercivity, saturation magnetization, nucleation field, grain size, grain size distribution, etc., are within a 1-2 % error bar for all the recording layers.

(a)

(b)

(c)

Figure 28. MFM images of (a) a demagnetized Co/Pd perpendicular recording layer, and SUL thin films of two thickness values, (b) 80 and (c) 10 nm, respectively, deposited on top of the recording layer.

Figure 29 shows the roll-off curves for three media with different SULs. It is clear that the roll-off of the playback signal is substantially more dramatic if a SUL’s material with a lower anisotropy field, Hk, is used.

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It should be emphasized that the decrease in SUL’s relative permeability from 2000 for Permalloy to 320 for high anisotropy Ni45Fe55 cannot explain the observed behavior. Boundary element based macroscopic calculations shown that lowering relatively permeability of the SUL from infinity down to approximately 50 has virtually no effect on the playback characteristic of the recording system.

Figure 29. Roll-off curves for media with identical recording layers and different SULs.

In summary, as outlined above and in Reference [10], the inability of a SUL with low magnitude of Hk to perfectly image the magnetic bits in the recording layer at a high linear density may lead to distortions in the playback signal. The deteriorated imaging results in the loss of resolution (increased PW50 and decreased amplitude of the read sensitivity function) and, subsequently, in the deterioration of the recording system’s performance overall. 3.5. MAGNETIC BIASING Among the main technical challenges introduced by the presence of a SUL is the fact that the SUL contributes an additional source of noise [175]. A not properly optimized SUL material can introduce a significant amount of noise into the playback signal. The noise is caused by the effective magnetic “charge” created in the numerous domain walls inside the SUL. A MFM image of a 100-nm-thick SUL with visible domain walls in one of the diagonal directions is shown in Figure 30. The magnetic charge in these walls is believed

154

Perpendicular Magnetic Recording

to be the source of the noise. A solution to this problem is the magnetic biasing of the SUL.

Figure 30. A MFM image of a 100-nm thick soft SUL with magnetic domain walls creating a wave ripple in on of the diagonal directions.

Figure 31 shows a schematic diagram of the experimental setup to study the noise effects of the magnetic biasing of the SUL. The magnetic biasing can be achieved using two NdFeB-based permanent magnets places in the vicinity of the recording media. The placement of the magnets is chosen so that it allows to achieve complete saturation of the SUL underneath the reader. Special care should be taken to arrange the magnets sufficiently far from the recording head not to affect properties of the read element.

Figure 31. A schematic diagram of the experimental to magnetically bias SUL films.

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Figure 32 shows the playback signals from media with magnetically (a) non-biased and (b) biased SUL’s, respectively. As mentioned above, the substantial noise level in the first case is attributed to the presence of a large number of domain walls. A drastic reduction of the noise (by at least 10 dB) is clearly observed in the second case (when the SUL is biased). The explanation of this biasing effect is the following. Magnetic biasing saturates the SUL and thus forces the entire disk into a pseudo-single domain state. In other words, the magnetic biasing effectively sweeps away the domain walls, which in turn results in the elimination of the SUL’s noise.

Figure 32. Playback signal from two media with different SULs: (a) SUL with a large number of stripe domains. (The presence of stripe domains was also confirmed with magnetic force microscopy.) (b) Biased SUL with domain walls swept away from the SUL material.

3.6. DYNAMICS OF PERPENDICULAR RECORDING Performance of perpendicular recording at high frequencies is a subject that has not been sufficiently explored [176,177]. The demand for high data rate systems results from the constantly increasing linear density [178]. Major open questions regarding the data rates in perpendicular recording are believed to be the magnetization switching in the perpendicular recording layer (due to the relatively low “torque” because of the small angle between the magnetization and the field) and the influence of the soft underlayer [167]. Previously, it was shown that the characteristic time for the magnetization switching in the recording layer is the orders of magnitude smaller than the characteristic time for the magnetization switching in soft materials of a recording system [179]. In the case of perpendicular recording, the soft materials are the head and soft underlayer materials. Therefore, the head material and the soft underlayer are expected to dominate the (time) frequency dependent roll-off. The write driver speed raises a serious concern at data rates at which the characteristic time response is of the order of one nanosecond or less. Nevertheless, the question of the write driver speed limitation is not going to be a subject of this Chapter. Because this

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156

question is as critical in perpendicular recording as it is in longitudinal recording, several solutions have already been proposed previously [180]. The purpose of this Chapter is to study the intrinsic dynamic response of a perpendicular system with a soft underlayer and compare it with the intrinsic dynamic response of a typical longitudinal system. Realizing that the principal difference is caused by the presence of the soft underlayer in perpendicular recording, the study accentuates on the role of the soft underlayer.

Coil Magnetic flux

Trailing pole tip

Yoke (a)

Closed path definition

(b) Soft underlayer

Figure 33. A diagram showing a closed magnetic flux path in (a) a longitudinal system and (b) a perpendicular system with a soft underlayer

Three-dimensional (3D) finite element modeling, including micromagnetics, could be developed to define the design guidelines for high-speed perpendicular systems [181]. Kerr imaging microscopy could be utilized to directly study the magnetic flux dynamics of a recording system with a soft underlayer [158,182,183]. 3.6.1. Design of a High-Data-Rate Perpendicular System Diagrams showing a typical longitudinal system and a perpendicular system with a soft underlayer are shown in Figures 33a and b, respectively. As mentioned earlier, assuming that magnetically “hard” materials are typically orders of magnitude faster than “soft” materials in a system, the dominant contribution to the frequency roll-off is going to be caused by the head inductance. The head inductance consists of two parts, the write coil inductance and the inductance of the yoke plus the inductance of the soft underlayer in case of perpendicular recording. Effective inductance calculations, incorporating the eddy current’s effect (resulting in the “skin” effect), could be carried out using 3D finite element modeling (FEM). The narrow portions of the recording systems, such as the leading and the trailing pole tips in the longitudinal mode and the trailing pole tip and the soft underlayer in the perpendicular mode, could be modeled micro-magnetically using Landau-Lifschitz-Gilbert (LLG) Equation [184]. In the modeling described below, the head and the soft underlayer were assumed to be made of a high-moment FeAlN

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157

compound with a saturation magnetization, Bs, of 20 kGauss and an anisotropy field, Hk, of 10 Oe. A longitudinal ring head design with a trackwidth of 100 nm and a gap length of 60 nm was modeled. An equivalent perpendicular head modeled had a 100 nm wide trailing pole with a 30 nm separation between the air bearing surface (ABS) and the soft underlayer. The calculated switching time versus the closed magnetic path length of the perpendicular system with a set of different coil turns is shown in Figure 34. The closed path is defined as the yoke length (measured at the inner yoke surface, as shown in Figure 33a) plus the length of the soft underlayer under the head, as shown in Figure 33b. The soft underlayer was modeled to be 300 nm thick. The characteristic switching time, Wsw, was defined as the time necessary to generate the magnetic field sufficient to overcome the anisotropy field of the recording layer, Hk. In this work, Hk. of 7 kOe was considered. It is evident that the inductance of the system should decrease with the reduction of the number of coil turns. However, as the number of turns is reduced, the recording field generated is also reduced. Fortunately, current state-of-the-art ultra-compact recording systems, being intrinsically more efficient, require a smaller number of write coil turns [185]. This is especially true for a perpendicular system, which is even more efficient due to the use of a soft underlayer [186]. For the particular system design, the minimum number of turns necessary to generate a recording field of a value larger than the hard layer anisotropy field, Hk, was calculated to be two (Two turns generate approximately a 14-kOe-recording field). Although, the total circuit impedance could be optimized externally, the number of turns around the yoke is an important factor to determine the impedance of the head itself. It could be observed that even a five-turn system is capable of a switching time of less than one nsec if the magnetic path length is less than 20 Pm (see Figure 34).

Switching time (nsec)

10 20 turn 10 turn 5 turn 1

2 turn

15 20 25 30 35 40 45 Magnetic Closed Path Length (um) Figure 34. The switching time versus the magnetic closed path length for perpendicular systems with 4 different numbers of coil turns: 2, 5, 10 and 20.

Perpendicular Magnetic Recording

158

Switching time (nsec)

The switching time versus the soft underlayer thickness for a 5-turn coil system with a 20 Pm long flux path is shown in Figure 35a. To compare longitudinal recording and perpendicular recording, the switching time versus the magnetic closed path for an equivalent longitudinal system with a 5-turn coil is shown in Figure 35b. The switching time for the longitudinal system was determined as the characteristic time necessary to reverse the magnetization in the longitudinal medium with a coercivity field of 7 kOe. It could be noted that for the perpendicular system, comparable switching times could be achieved by adjusting the flux path length.

4.0 3.5 3.0 2.5 2.0 1.5 1.0 200 400 600 800 1000 Soft underlayer thickness (nm)

Switching time (nsec)

(a) 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 15 20 25 30 35 40 45 Magnetic closed path length (um)

(b)

Figure 35. (a) The switching time versus the soft underlayer’s thickness for a 5-turn coil system with a 20 Pm flux path length. (b) The switching time versus the magnetic closed path length for a 5-turn coil longitudinal system.

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159

3.6.2. Kerr Microscopy Kerr microscopy was developed to directly study the dynamics of a recording system both with and without a soft underlayer [158]. In this experimental setup, a Kerr image was taken from a relatively small region under the recording pole tip. In the case of a perpendicular system, the image was taken from the region in the soft underlayer under the pole tip, as shown in Figure 36. Kerr Imaging Location

Soft underlayer

Recording layer

Head

Figure 36. A system to study the dynamics of a perpendicular system with a soft underlayer.

A focused ion beam (FIB) image of a focused-ion-beam (FIB)-trimmed perpendicular head specially fabricated to study the dynamics of a perpendicular system is shown in Figure 37 [186,187].

Trailing edge

P2

Gap trench

Leading Pole, P1

Figure 37. A FIB image (ABS view) of a FIB fabricated perpendicular head.

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160

In this experiment, a square pulse with a rise time of less than 1 nsec was applied to study the head and soft underlayer response. A time sequence of the polar Kerr images taken after the field step pulse was applied to a perpendicular system (with a soft underlayer) of the above-described type is shown in Figure 38a. For comparison, a similar time sequence after the field step pulse was applied to an equivalent longitudinal system of the type described above is shown in Figure 38b.

0 nsec

0.5

0.75

1 nsec

1.5

2 nsec P2 Gap P1

(a)

0 nsec

0.5

0.75

1

1.5

2 nsec P2

(b)

P1

Figure 38. A sequence of polar Kerr images taken at increasing time intervals after a field step pulse was applied to (a) a perpendicular system with a soft underlayer and (b) an equivalent longitudinal system.

To summarize the results, Figure 39 shows the normalized write current pulse and the Kerr angle response at the point of the maximum field change for the studied longitudinal and perpendicular systems. It can be observed that the switching times for the two perpendicular and longitudinal systems are approximately in the same range, 0.75 to 1 nsec.

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161

Normalized Signal (Current and Kerr angle)

The above described Kerr imaging experiment clearly illustrates that a perpendicular system, if optimally designed, should not demonstrate dramatically degraded dynamic performance compared to an equivalent longitudinal system.

1.0 0.5 0.0 Write Current Perpendicular System Longitudinal System

-0.5 -1.0 0.0

0.5

1.0

1.5

2.0

2.5

Time (nsec) Figure 39. Normalized current of the write driver and the Kerr angle for the studied perpendicular and longitudinal systems as a function of time.

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162

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173

INDEX Adjacent tracks, 9, 19, 41, 72, 92, 133 Air bearing surface (ABS), 35 AC demagnetized media, 77, 92 Areal density, 3 Barrium ferrite (BaFe), 128 Biasing of SUL noise, 13, 153 Bit (data) rate, 155 Bit transition, 3, 9 Boundary element modeling (BEM), 28, 74, 89, 120 Censtor heads, 1, 38 Channel (of data), 17 Closed (flux) loop (path), 29, 156, 157 CoCr- based media, 4, 9, 18 Co/Pt-multilayer based media, 4, 18 Coding (encoding), 2, 136 Coercivity, 22 Coercivity squareness, 75, 128 Conformal Mapping, 120 Data rate (see Bit Rate) DC erase, 74 DC noise, 19. Demagnetization field, 4, 11, 140 Demagnetized media (see AC demagnetized media) Detection (See Signal) Differential equations, 31, 86, 101 Differential reader, 104 Domain, 13, 41, 152, 155 Domain noise, 125 Domain wall, 12, 153, Domain width (thickness), 41, 131, 151 Dynamics (of recording), 127, 156, 159

Flying height, 15, 28, 42, 70 Focused ion beam (FIB), 8, 38, 40, 68 Frequency roll-off (See Roll-off) Gap, 5, 8, 27 Giant magnetoresistive (GMR) head, 16, 67, 118, 124 Gradient (of the magnetic field), 5, 8, 14, 23, 29, 42 Grain size, 3, 18, 128, 131, 152 Heat-assisted magnetic recording (HAMR), 22 Head (see Recording head, Single pole head (SPH), Ring head (RH), Read head, Magnetoresistive (MR) head, GMR head, Write head, Shielded head) High-density recording, 1 Kerr (imaging), 19, 127, 130, 156, 159 L10 phases of FePt and others, 19, 128 Laplace’s Equation, 31, 101, 122, 131, 144 Linear density, 20, 54, 86, 100 Longitudinal field, 4, 5, 27, Longitudinal recording, 1

Eddy current, 156 Efficiency, 6, 24, 28, 43, 49, 63, Encoding (See Coding) Erasure, 20, 130 Erase band, 8

Magnetic flux, 14, 30, 35, 43, 57, 74 Magnetic force microscopy (MFM), 9, 27, 39, 53, 143 Magnetic image, 31, 72, 101 Magnetic moment, 3 Magnetization, 2 Magnetometer, 19, 75 Magneto-optical media, 131 Magnetoresistive (See Magnetoresistive head) Magnetoresistive head, 2, 121, 125, 171 Maxwell’s Equations, 31, 101, 145 MH-loop, 19 “Mirror” imaging, 7, 31, 73, 101 Multiplicative (magnetic reflection), 74

FeAlN (high moment nitrides), 14, 68, 146, 151 Finite-element calculation, 156 Fly height (See Flying height)

Noise (See DC noise, AC noise, Signal-to-noise ratio) Non-linear transition shift (NLTS), 72

174 Overwrite, 143 Particle (grain) size, 3, 18, 128 Patterned media, 22, Perpendicular recording, 1 Playback head (See Read head) Principle of Reciprocity, 86, 92, 98, Read head, 15, 54, 67, 86, 99 Reciprocity Principle (See Principle of Reciprocity) Ring head, 6, 9, 24, 27, 56, 83 Roll-off curve, 21, 85, 92, 149 Saturation magnetization, 4, 19, 24, 35, 86, 128 Scalar (magnetic) potential, 99, 121, Scaling law, 3, 42 Scanning electron microscopy (SEM), 38, 67 Schwartz-Christoffel Transformation, 120 Self-erasure (See Erasure) Sensitivity field, 98, Shielded head, 79, 99 Shields (magnetic), 23, 80 Side (track) reading, 54 Single domain wall (See Domain) Signal (waveform, half-width PW50), 3, 11, 16, 71 Signal-to-noise ratio (SNR), 3, 20, 131 Single pole head (SPH), 5, 24, 31, 38, 42 “Skin” effect, 156 Soft (magnetic) underlayer (SUL), 5, 12, 15 Spinstand (measurements), 21 Spacing loss, 103 Squareness (See Coercivity squareness) Stability (thermal) (See Thermal stability) Superparamagnetic limit (See Thermal stability) Surface (magnetic) charge, 17, 32, 70 Thermal stability, 1, 3, 8 Time sequence, 160 Track density, 9 Transmission electron microscopy (TEM), 3, 129 Trapezoidal (write pole), 55 Vibrating sample magnetometer (VSM), 75 Vertical recording (see Perpendicular recording) Stoner-Wohlfarth mode, 37 X-Ray diffraction, 142