Ultra-High-Density Magnetic Recording: Storage Materials and Media Designs

“The book is planned and executed well and a good addition to Developments in Data Storage, which I wrote five years ago. It covers in detail the developments in magnetic recording in the past five years. The authors have chosen from a good mix of industry and academia. The book is highly recommended for engineers and users interested in recent developments in magnetic recording.”

S. N. PiramanayagamAssociate Professor, Nanyang Technological University, Singapore

“This book summarizes recent developments in various potential technologies for the next generation of magnetic recording, especially recording media, and shares good experiences of industry practices. Some of the emerging candidates of magnetic memories for future information memory are also reviewed. It is a good reference book for researchers in magnetic recording as well as postgraduate students.”

Dr. Baoxi XuSenior Scientist, Data Storage Institute (DSI), Singapore

“Given the worldwide explosion of data generation in recent years, hard disk drives remain highly relevant devices for data storage and management. This book brings us up to date on magnetic information storage materials and technology, encompassing both hard disk drives and memories. The chapters are expertly written and cover a wide range of topics from the basics of magnetism, fundamentals of hard disk drives, and current and future magnetic media technologies to the latest in magnetic memories. It will serve beginners well and is also a welcome addition to the library of professionals in the field.”

Kumar SrinivasanSenior Principal Engineer, Western Digital, Irvine, USA

In this book, worldwide experts from universities, public research institutions, and industry have collaborated to illustrate the most recent advances and developments in magnetic recording from the media perspective, including theoretical, experimental, and technological aspects. It also provides an overview of the emerging classes of magnetic memories regarded as potential candidates for future information storage devices. Comprehensive references, together with clear and thorough figures, complement each section, making the book a useful reference for final-year undergraduates, postgraduates, and research professionals in the magnetic recording area.

Gaspare Varvaro is a research scientist at the Institute for Structure of Matter of the Italian National Research Council (ISM-CNR). He obtained his PhD in materials science (preparation and investigation of CoPt films for ultrahigh-density magnetic recording media) from La Sapienza University in 2007. His main research interest is in the study of magnetic properties of nanostructured single-phase and composite materials (thin films, multilayers, nanoparticles, nanopatterned systems) for fundamental studies and applications (information storage, energy, sensors, biomedicine). He is the author of more than 30 peer-reviewed papers and 1 book chapter on the subject.

Francesca Casoli is a research scientist at the Institute of Materials for Electronics and Magnetism (IMEM-CNR). She obtained her PhD in physics from the University of Parma in 2005, investigating magnetic thin films and multilayers with perpendicular anisotropy and exchange-spring properties. Her main research interest is in the design and study of magnetic thin films, nanostructures, and nanocomposites with new functional properties. She has published more than 50 peer-reviewed papers on the correlation between nanoscale structure and magnetic properties in materials for magnetic recording, sensors/actuators, and biomedicine.

Varvaro | CasoliGaspare VarvaroFrancesca Casoli

edited byUltrahigh-Density M

agnetic Recording

Ultrahigh-Density Magnetic RecordingStorage Materials and Media Designs

ISBN 978-981-4669-58-0V504

Ultrahigh-Density Magnetic Recording

edited by

Gaspare VarvaroFrancesca Casoli

Ultrahigh-Density Magnetic RecordingStorage Materials and Media Designs

January 22, 2016 18:6 PSP Book - 9in x 6in 00-Gaspare-Varvaro-prelims

Published by

Pan Stanford Publishing Pte. Ltd.

Penthouse Level, Suntec Tower 3

8 Temasek Boulevard

Singapore 038988

Email: [email protected]

Web: www.panstanford.com

British Library Cataloguing-in-Publication DataA catalogue record for this book is available from the British Library.

Ultrahigh-Density Magnetic Recording: Storage Materials and MediaDesigns

Copyright c© 2016 Pan Stanford Publishing Pte. Ltd.

All rights reserved. This book, or parts thereof, may not be reproduced in anyform or by any means, electronic or mechanical, including photocopying,recording or any information storage and retrieval system now known or tobe invented, without written permission from the publisher.

For photocopying of material in this volume, please pay a copying

fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive,

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required from the publisher.

ISBN 978-981-4669-58-0 (Hardcover)

ISBN 978-981-4669-59-7 (eBook)

Printed in the USA


Contents

Preface xiii

1 Fundamentals of Magnetism 1Paolo Allia and Gabriele Barrera1.1 Magnetic Fields and Energies: A Macroscopic

Approach 2

1.1.1 Magnetic Fields 2

1.1.2 Magnetostatic Energy 4

1.1.3 Shape Anisotropy of a Magnetized Body 5

1.2 Basics of Magnetization Dynamics 7

1.2.1 Torques 7

1.2.2 Magnetization Dynamics and Data Writing in

Magnetic Recording 10

1.3 Elements of Microscopic Magnetism 11

1.3.1 Stable Magnetic Moments on Free Atoms/Ions 11

1.3.2 Van Vleck Paramagnetism:

Superparamagnetism 12

1.3.2.1 Paramagnetic response of

noninteracting magnetic moments 12

1.3.2.2 Superparamagnetism 15

1.3.2.3 Quenching of the angular momentum 15

1.3.3 Ferromagnetic and Antiferromagnetic

Interaction 16

1.3.4 Mean-Field Approach to Ferromagnetism 19

1.4 Magnetic Mesostructures in Bulk Materials 23

1.4.1 Magnetic Crystalline Anisotropy and Other

Anisotropies 23

1.4.2 Magnetic Domains 26

1.4.3 The Single-Domain Regime 27


vi Contents

1.4.4 Magnetic Domain Walls 28

1.4.4.1 A simple model for 180◦ domain walls

in bulk ferromagnets 28

1.4.4.2 Thin films: Neel domain walls 31

1.4.5 Back to Magnetic Domains 32

1.4.6 Magnetization Processes 33

1.4.6.1 Domain wall motion 34

1.4.6.2 Magnetization rotation 37

1.5 Magnetic Hysteresis Loops and Magnetic Losses 38

1.5.1 Magnetic Hysteresis 38

1.5.2 Ferromagnetic Materials: An Inventory 40

1.5.2.1 Soft magnetic materials 40

1.5.2.2 Hard magnetic materials for

permanent magnets 41

1.5.2.3 Magnetic nanoparticles 42

1.5.2.4 Materials for magnetic recording 44

1.5.2.5 Meeting the requirements for

magnetic recording 44

2 Hard Disk Drives: Fundamentals and Perspectives 51Gerardo Bertero, Guoxiao Guo, Shafa Dahandeh,and Anantha Krishnan2.1 Magnetic Recording Fundamentals 51

2.1.1 Magnetic Recording Relevance in Information

Storage 54

2.1.2 Head Field Basics 55

2.1.3 Recording Transitions and Signal-to-Noise

Ratio 57

2.1.4 Thermal Decay and the Recording Trilemma 62

2.1.5 Sources of Media Noise 65

2.2 Magnetic Recording Medium 69

2.2.1 Magnetic Recording Media Structure and

Evolution 69

2.3 Magnetic Recording Heads 77

2.3.1 Writer Technology and Challenges 77

2.3.2 Reader Technology and Challenges 79

2.3.3 Sources of Head Noise 84

2.3.4 Magnetic Head Integration 84


Contents vii

2.4 Mechanics and Servo 85

2.4.1 HDD Servo Basic Components 86

2.4.1.1 Disk spindle pack 87

2.4.1.2 HDD mounting and vibration 87

2.4.1.3 Acoustics 88

2.4.2 Servo Fundamentals and Challenges 89

2.4.2.1 Servo system model and resonance

control 89

2.4.2.2 Servo signal generation and timing

control 90

2.4.2.3 Servo control principle: seek/settle

control 93

2.4.2.4 Servo control principle: track-

following control 97

2.4.2.5 Current and future servo challenges 107

2.5 Signal Processing Fundamentals and Challenges 108

2.5.1 The Read Channel 109

2.5.2 Peak Detection 110

2.5.3 Partial Response Maximum Likelihood 112

2.5.3.1 Noise whitening 114

2.5.4 Error-Correcting Code 115

2.5.5 Signal Processing Outlook 117

2.6 Summary 119

3 Conventional Perpendicular Magnetic Recording Media 133Hong-Sik Jung3.1 Introduction 134

3.2 Evolution and Improvement of PMR Media 138

3.2.1 Definition and Functions of Layer Structures 138

3.2.2 Magnetic Soft Underlayer 139

3.2.2.1 Roles of an SUL 139

3.2.2.2 Noise sources and solutions 143

3.2.2.3 Trend of SUL requirement and

materials 146

3.2.3 Intermediate Layer: Seed Layer and Interlayer 148

3.2.3.1 Intrinsic IL roles as an exchange

breaking layer 149


viii Contents

3.2.3.2 Template for grain growth and grain

isolation 152

3.2.3.3 Trend of IL materials 159

3.2.4 Magnetic Recording Layers 161

3.2.4.1 Initial PMR media with CoCrPt-based

oxide-containing layers only 161

3.2.4.2 Hard/soft stacked composite media:

coupled granular/continuous layer

and capping layer 169

3.2.4.3 Static and dynamic tilted magnetic

switching media 175

3.2.4.4 Magnetic switching block–controlled

(or segmented) media 180

3.3 Challenges of Conventional PMR Media 181

4 Energy-Assisted Magnetic Recording 195Edward Gage, Kai-Zhong Gao, and Jian-Gang (Jimmy) Zhu4.1 Introduction 195

4.2 Microwave-Assisted Magnetic Recording 196

4.3 Heat-Assisted Magnetic Recording 209

4.3.1 Introduction 209

4.3.2 Recording Head 212

4.3.2.1 Near-field transducer 212

4.3.2.2 Light delivery path 214

4.3.3 HAMR Media 219

4.3.4 HAMR Recording System 223

4.3.4.1 Define/achieve HAMR transition 223

4.3.4.2 HAMR performance improvement 226

4.3.4.3 HAMR recording model, design, and

understanding of recording process 229

4.3.5 Head–Disk Interface 230

4.3.6 Status and Challenges 233

4.4 Conclusions 235

5 L10–FePt Granular Films for Heat-Assisted MagneticRecording Media 245Kazuhiro Hono and Yukiko K. Takahashi5.1 Introduction 245


Contents ix

5.2 Ideal Structure for HAMR Media 249

5.2.1 A1→L10 Ordering 251

5.2.2 [001] Texture 251

5.2.3 FePt–C Granular Films 255

5.2.4 (FePt)0.9Ag0.1–C Granular Films 258

5.2.5 Exploration for a New Segregant for FePt–X

Media 261

5.3 Optimization of Nanostructure of the FePt–C System 265

5.3.1 Columnar Grain Growth 265

5.3.2 Reduction of In-Plane Magnetic Component 267

5.4 Concluding Remarks 271

6 Exchange-Coupled Composite Media 279Francesca Casoli, Lucia Nasi, Franca Albertini,and Pierpaolo Lupo6.1 Introduction 279

6.2 Soft–Hard Composite Systems 281

6.2.1 The Exchange-Spring Magnet 281

6.2.2 Composite Media Advantages for Magnetic

Recording 284

6.3 ECC Media Based on Conventional Media Materials 290

6.4 ECC Media Based on L10–FEPT 293

6.4.1 Fe/L10–FePt Composite Media 293

6.4.2 Composite Media with a Soft Phase Other

Than Fe 301

6.5 Anisotropy-Graded Media 310

6.5.1 Experimental Realization of Graded Media 310

7 Bit-Patterned Magnetic Recording 327Denys Makarov, Philipp Krone, and Manfred Albrecht7.1 Introduction 327

7.2 Magnetization Reversal of a Single-Domain Particle

with Uniaxial Anisotropy 329

7.3 BPM: Array of Magnetic Nanodots (Bits) 338

7.3.1 SFD: Dependence on the Magnetic Anisotropy

Value 341

7.3.2 SFD: Dependence on Dipole–Dipole Interaction

between Bits 345


x Contents

7.3.3 SFD: Role of the Angular Orientation of the

Magnetic Anisotropy Axes in BPM 347

7.4 Experimental Realization of BPM via Lithographic

Patterning 348

7.4.1 Sample Preparation and Experimental

Techniques 349

7.4.2 Magnetization Reversal Process and SFD in

BPM 350

7.5 Exchange-Coupled Composite BPM Concept 354

7.5.1 Correlation of Magnetic Anisotropy

Distributions in Layered ECC BPM 361

7.5.2 Magnetic Recording on ECC BPM 364

7.6 Experimental Realization of BPM via Self-Assembly 366

7.6.1 Magnetic Probe Recording on Nanocaps 368

7.6.2 Magnetic Probe Recording on Cap Arrays:

Micromagnetic Simulations 370

8 Magnetic Characterization of Perpendicular RecordingMedia 385Gaspare Varvaro, Alberto Maria Testa, Elisabetta Agostinelli,Davide Peddis, and Sara Laureti8.1 Introduction 385

8.2 Perpendicular Granular Media 388

8.2.1 Investigation of Primary Magnetic Properties 389

8.2.1.1 Magnetization and remanence curves 389

8.2.1.2 Demagnetizing fields 392

8.2.1.3 Magnetic anisotropy 395

8.2.2 Determination of Switching-Field Distribution 400

8.2.3 Study of the Reversal Mechanism 410

8.2.4 Thermal Effects and Time-Dependent

Measurements 414

8.3 Exchange-Coupled Composite Media 419

8.3.1 Reversal Mechanism 420

8.3.2 Thermal Stability and Writability 423

8.4 Bit-Patterned Magnetic Recording Media 423

8.4.1 Switching-Field Distribution and

Switching-Field Probability 426

8.4.2 Reversal Mechanism 430


Contents xi

8.5 Energy-Assisted Magnetic Recording Media 431

8.5.1 Heat-Assisted Magnetic Recording Media 432

8.5.1.1 Determination of TC and D(TC) 433

8.5.1.2 Determination of SFD 436

8.5.2 Microwave-Assisted Magnetic Recording Media 437

8.5.2.1 Frequency dependence of Hsw 438

9 New Trends in Magnetic Memories 457Riccardo Bertacco and Matteo Cantoni9.1 Introduction 457

9.2 State of the Art of Magnetic Random Access Memories 461

9.2.1 Basics of MRAM Working Principles 461

9.2.2 Current Writing 463

9.2.2.1 Stoner–Wolhfarth astroid 464

9.2.2.2 Current lines 466

9.2.2.3 Thermally assisted switching 467

9.3 New Strategies for Writing Ferromagnetic Electrodes 468

9.3.1 Spin Transfer Torque 468

9.3.2 Spin–Orbit Torque 473

9.3.3 Electric Writing of Magnetic Information 476

9.3.3.1 Voltage-controlled magnetic

anisotropy 477

9.3.3.2 Magneto-electric coupling 480

9.3.4 Optical Writing 482

9.4 Increasing Density and Stability 489

9.4.1 Domain Wall–Based Memories 489

9.4.2 Antiferromagnet Spintronics 491

9.5 Beyond Binary Static Memories 494

9.5.1 Spin Transfer Torque Memristors 496

Index 511


Preface

A stunning amount of data, of the order of a few zettabites

(1021 bytes), is presently stored on hard disk drives (HDDs), data

storage devices based on magnetic recording technology. HDDs

are present in desktop computers, multiple-user computers, digital

video recorders, and automotive vehicles. In laptop computers,

they are being replaced by solid-state memories, which are lighter,

quieter, and faster. However, HDDs remain the desirable option

whenever the price per bit, per device recording capacity, and

endurance are important requirements, as in massive secondary

data storage.

Magnetic recording technology has maintained the lead in data

storage since IBM first introduced the RAMAC drive operating at

2 kbit/in2 in 1956, continuing to the present date, with the latest

commercially available modern drives approaching the 1 Tbit/in2

target. Despite the impressive growth of 500 million times in

recording areal density in 60 years, increasing the recording density

beyond 1 Tbit/in2 requires a radical technological improvement,

since the current magnetic recording technology is unable to achieve

storage densities above this value.

In this book, worldwide experts from university, public research

institutions, and industry collaborate to give an exhaustive overview

of the technology and deepen its future prospects. A comprehensive

approach is followed with the aim of offering a complete guide to

a wide readership. Two introductory chapters serve as the basis

for a better understanding of the following ones, deepening several

aspects related to current magnetic recording technology, to its

possible evolutions, and to magnetic memories in general. Four

chapters are specifically dedicated to recording media and outline

their evolution from the commercialization of the perpendicular


xiv Preface

technology in 2005 to state-of-the-art drives in 2015 to the most

promising alternatives for next-generation recording technologies.

Chapter 1 describes the fundamental aspects of magnetism,

which can assist readers new to the topic. The chapter is a complete

compendium of magnetism, conceived with an original viewpoint by

academy experts on the subject.

The second chapter gives a complete overview of the basic

principles and technologies that allowed HDDs to reach today’s

information densities. This chapter takes advantage of the expertise

of industry specialists on the subject in order to highlight recent

materials and structure breakthroughs in perpendicular magnetic

recording.

The third chapter describes the evolution of conventional

media used in perpendicular recording, from the introduction of

the technology to the market to date. Written by an industry

expert on the topic, the chapter is full of technical details and

represents an exhaustive guide on conventional perpendicular

media.

Chapter 4 is dedicated to energy-assisted magnetic recording;

it includes heat-assisted and microwave-assisted recording, two

technologies that modify the media’s magnetic properties or

switching dynamics to allow higher recording densities compared

to conventional perpendicular recording. The chapter is primarily

dedicated to heat-assisted magnetic recording (HAMR), which is one

of the most promising technologies for the future and will likely

appear in HDD products in 2018, allowing an areal density growth

up to 4 Tbit/in2.

After the realization that the L10-FePt alloy is the best candidate

for next-generation perpendicular recording media, a big effort

has been devoted to the optimization of the morphostructural and

magnetic properties of this material for recording applications. The

most significant progresses on FePt-based media for HAMR are

carefully discussed in Chapter 5.

The following chapter, that is, Chapter 6, focuses on exchange-

coupled composite media (ECC media), which exploit the old

concept of an exchange-spring magnet to improve the writability

of perpendicular recording media. The chapter reviews the results


Preface xv

obtained by exploiting the ECC principle on conventional media,

FePt-based media, and anisotropy-graded media.

Chapter 7 concentrates on bit-patterned magnetic recording

(BPMR), where one single bit of information is stored in one

lithographically defined magnetic dot. The chapter deepens the

reader’s understanding of the fundamental aspects of the magne-

tization reversal process in BPMR media and considers FePt-based

BPMR systems employing the ECC media concept. The extremely

challenging requirements for the realization of BPMR media has

pushed this technology out on the HDD industry road map to 2020

or beyond.

Chapter 8 makes an exhaustive analysis of the techniques

and methods used for magnetic characterization of perpendicular

recording granular media, ECC media, bit-patterned media, and

energy-assisted recording media. The chapter can serve as a

complete and clear guide to the reader approaching the magnetic

characterization of magnetic recording media.

Finally, Chapter 9 goes beyond magnetic recording matter, to

give a full perspective on magnetic memories for data storage in

general—from the different classes of magnetic RAMs, to domain-

wall-based memories, and beyond binary static memories, as a final

stimulus, to memristors.

Overall, this book presents a thorough treatment of the core

principles and future directions of magnetic recording, providing

a balanced coverage of theoretical, experimental, and technological

aspects. It addresses a broad readership, from final-year undergrad-

uate and postgraduate students to experienced researchers active

in the magnetic recording area, both in the academy and industry.

Chapters include many illustrations and practical examples, making

the book a clear and useful guide for both the experienced

reader and the reader approaching the treated topics for the first

time.

Finally, we would like to express our sincere gratitude to all the

authors: none of this would have been possible without their exper-

tise and commitment. We are grateful to the reviewers, who have

rigorously evaluated all the chapters and given helpful feedback to

improve the book. Our special thanks also go to the editorial office


xvi Preface

at Pan Stanford Publishing for their constant assistance in preparing

the book and to Nicoletta Marigo and Julie Karel for their help

with proofreading. Furthermore, our deepest gratitude goes to the

president of the Italian Association of Magnetism, Dino Fiorani, for

his continuous support and encouragement.

Ultra-High-Density Magnetic Recording: Storage Materials and Media Designs

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