Developments in Data Storage E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 665

Veröffentlichungsjahr: 2011

Beschreibung

A timely text on the recent developments in data storage, from a materials perspective Ever-increasing amounts of data storage on hard disk have been made possible largely due to the immense technological advances in the field of data storage materials. Developments in Data Storage: Materials Perspective covers the recent progress and developments in recording technologies, including the emerging non-volatile memory, which could potentially become storage technologies of the future. Featuring contributions from experts around the globe, this book provides engineers and graduate students in materials science and electrical engineering a solid foundation for grasping the subject. The book begins with the basics of magnetism and recording technology, setting the stage for the following chapters on existing methods and related research topics. These chapters focus on perpendicular recording media to underscore the current trend of hard disk media; read sensors, with descriptions of their fundamental principles and challenges; and write head, which addresses the advanced concepts for writing data in magnetic recording. Two chapters are devoted to the highly challenging area in hard disk drives of tribology, which deals with reliability, corrosion, and wear-resistance of the head and media. Next, the book provides an overview of the emerging technologies, such as heat-assisted magnetic recording and bit-patterned media recording. Non-volatile memory has emerged as a promising alternative storage option for certain device applications; two chapters are dedicated to non-volatile memory technologies such as the phase-change and the magnetic random access memories. With a strong focus on the fundamentals along with an overview of research topics, Developments in Data Storage is an ideal reference for graduate students or beginners in the field of magnetic recording. It also serves as an invaluable reference for future storage technologies including non-volatile memories.

Leseprobe

Table of Contents

Cover

Series page

Title page

Copyright page

Dedication

PREFACE

ACKNOWLEDGMENTS

CONTRIBUTORS

1 INTRODUCTION

1.1 INTRODUCTION

1.2 BASICS OF DATA STORAGE

1.3 RECORDING MEDIUM

1.4 HEADS

1.5 MATERIALS ASPECT OF THE HEAD–DISK INTERFACE

1.6 TECHNOLOGIES FOR FUTURE HDDS

1.7 MEMORY TECHNOLOGIES

1.8 SUMMARY

2 FUNDAMENTALS OF MAGNETISM

2.1 INTRODUCTION

2.2 THE ORIGIN OF ATOMIC MAGNETIC MOMENT

2.3 MAGNETISM IN A SOLID

2.4 MAGNETIC SUSCEPTIBILITY

2.5 TYPES OF MAGNETIC MATERIALS

2.6 FERROMAGNETIC MATERIALS

2.7 HYSTERESIS LOOPS

2.8 HARD AND SOFT MAGNETIC MATERIALS

2.9 MAGNETIZATION AND CURIE TEMPERATURE

2.10 VARIOUS ENERGIES INSIDE MAGNETIC MATERIAL

2.11 MAGNETIC DOMAINS

2.12 MAGNETIZATION PROCESSES

2.13 TIME-DEPENDENT EFFECTS

2.14 MAGNETIC CHARACTERIZATION TECHNIQUES FOR DATA STORAGE

3 LONGITUDINAL RECORDING MEDIA

3.1 INTRODUCTION

3.2 BASICS OF MAGNETIC RECORDING

3.3 MAGNETISM AT NANOSCALE

3.4 RECORDING PERFORMANCE MEASUREMENTS

3.5 TYPES OF NOISE

3.6 SOURCES AND WAYS OF MINIMIZING NOISE IN LONGITUDINAL RECORDING

3.7 INFLUENCE OF GRAIN SIZE ON RECORDING PERFORMANCE

3.8 THERMAL STABILITY LIMITS

3.9 IMPROVEMENTS IN LONGITUDINAL RECORDING TECHNOLOGY

3.10 LONGITUDINAL RECORDING MEDIA MANUFACTURING

3.11 SUMMARY

4 PERPENDICULAR RECORDING MEDIA

4.1 INTRODUCTION

4.2 HISTORY OF PERPENDICULAR MAGNETIC RECORDING (PMR)

4.3 ADVANTAGES OF PMR

4.4 OVERVIEW OF PMR MEDIA

4.5 DESCRIPTION OF VARIOUS LAYERS AND THEIR FUNCTIONS

4.6 SUL IN PERPENDICULAR RECORDING

4.7 CURRENT PERPENDICULAR RECORDING MEDIA

4.8 RECENT DEVELOPMENTS AND FUTURE TRENDS

4.9 SUMMARY

5 WRITE HEADS: FUNDAMENTALS

5.1 INTRODUCTION

5.2 THE DAWN OF THE WRITE HEAD

5.3 WRITE HEAD FUNDAMENTALS

5.4 BASIC STRUCTURE OF THE WRITE HEAD

5.5 THE RING HEAD FIELD NEAR THE GAP

5.6 THE PERPENDICULAR RECORDING WRITE HEAD

5.7 ROLE OF THE HEAD FIELD IN PERPENDICULAR RECORDING

5.8 DEVELOPMENT OF THE PERPENDICULAR RECORDING HEAD

5.9 FUTURE DEVELOPMENTS

6 MAGNETORESISTIVE READ HEADS: FUNDAMENTALS AND FUNCTIONALITY

6.1 INTRODUCTION

6.2 PRINCIPLES OF THE READOUT PROCESS

6.3 A BRIEF HISTORY OF THE GMR AND TMR EFFECTS

6.4 SVS AND THE READ HEAD

6.5 TMR READ HEAD

6.6 FUTURE PERSPECTIVES

6.7 CONCLUSION

7 READ SENSORS FOR GREATER THAN 1 Tb/in.2

7.1 CURRENT-IN-PLANE (CIP) GIANT MAGNETORESISTIVE (GMR) READ HEAD

7.2 CPP-GMR READ HEAD

7.3 CPP-TMR READ HEAD

7.4 CONFINED CURRENT PATH (CCP)-CPP GMR READ HEAD

7.5 DIFFERENTIAL DUAL SPIN-VALVE (DDSV) HEAD

7.6 NOISE ISSUES

7.7 THERMAL MAGNETIZATION FLUCTUATION NOISE (MAG-NOISE)

7.8 SPIN-TORQUE NOISE

7.9 SYSTEM NOISE

7.10 FUTURE READ HEAD TECHNOLOGIES

8 THIN-FILM MEDIA LUBRICANTS: STRUCTURE, CHARACTERIZATION, AND PERFORMANCE

8.1 INTRODUCTION

8.2 LUBRICANT STRUCTURES

8.3 LUBRICATION PROCESS

8.4 LUBRICANT CHARACTERIZATION

8.5 DYNAMICS/FLOW

8.6 SLIDER EFFECTS

8.7 LUBRICANTS FOR BIT-PATTERNED MEDIA (BPM) AND HEAT-ASSISTED RECORDING (HAMR)

8.8 CONCLUSION

ACKNOWLEDGMENTS

9 OVERCOAT MATERIALS FOR MAGNETIC RECORDING MEDIA

9.1 INTRODUCTION

9.2 EVOLUTION OF HD TRIBOLOGY

9.3 DESIRED PROPERTIES OF OVERCOATS

9.4 EVOLUTION OF THE MEDIA OVERCOAT

9.5 DLC FOR MEDIA OVERCOAT

9.6 DEPOSITION OF CARBON OVERCOATS

9.7 EMERGING MEDIA OVERCOAT MATERIALS AND DESIGNS FOR THE FUTURE

9.8 SUMMARY

10 HEAT-ASSISTED MAGNETIC RECORDING

10.1 INTRODUCTION

10.2 HAMR RECORDING PHYSICS

10.3 HAMR OPTICS AND HEAD

10.4 HAMR MEDIA

10.5 HAMR RECORDING AND MAGNETIC FORCE MICROSCOPY IMAGES

10.6 SUMMARY AND OUTLOOK

ACKNOWLEDGMENTS

11 L10 FePt FOR MAGNETIC RECORDING MEDIA APPLICATION

11.1 INTRODUCTION

11.2 OVERVIEW OF FUNDAMENTAL OF L10 FePt

11.3 MAGNETIC ANISOTROPY OF L10 FePt AND CHEMICAL ORDERING

11.4 MICROSTRUCTURE CONTROL OF L10 FePt

11.4 CURIE TEMPERATURE (TC)

11.5 SUMMARY

12 PATTERNED MAGNETIC RECORDING MEDIA: PROGRESS AND PROSPECTS

12.1 INTRODUCTION

12.2 BIT-PATTERNED MEDIA AND DISCRETE TRACK MEDIA

12.3 FABRICATION OF PATTERNED STRUCTURES

12.4 RECORDING SYSTEMS

12.5 PRODUCING A PATTERNED MEDIA DRIVE

13 PHASE CHANGE RANDOM ACCESS MEMORY

13.1 INTRODUCTION

13.2 PCRAM PRINCIPLES

13.3 PCRAM TECHNOLOGY

14 NONVOLATILE SOLID-STATE MAGNETIC MEMORY

14.1 NONVOLATILE SOLID STATE MAGNETIC MEMORY

Index

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Library of Congress Cataloging-in-Publication Data:

Developments in data storage : materials perspective / [edited by] S.N. Piramanayagam, Tow C. Chong.

p. cm.

ISBN 978-0-470-50100-9 (hardback)

 1. Computer storage devices. I. Piramanayagam, S. N. II. Chong, Tow C.

 TK7895.M4D497 2011

 621.39'7–dc22

2011006739

oBook ISBN: 978-1-118-09683-3

ePDF ISBN: 978-1-118-09681-9

ePub ISBN: 978-1-118-09682-6

To my mentors

Prof. Shiva Prasad,

Prof. S. N. Shringi,

Prof. Mitsunori Matsumoto and

The Late Ms. Terue Kamoi

who made a positive difference in my life

S. N. Piramanayagam

To my family and all the staff of Data Storage Institute, Singapore

Tow C. Chong

PREFACE

It gives us great pleasure to present this book on the developments in data storage, from a materials perspective. This book has been designed to help the final year undergraduate and graduate students of physics, materials science, chemistry, electrical engineering, and other disciplines with an interest in the magnetic recording or other emerging alternative storage technologies. This book will also help new engineers as well as professionals in the recording industry to broaden their knowledge and serve as a useful reference.

It has been quite some time since a book on data storage was released and since then there have been several developments. The hard disk industry has moved from longitudinal recording technology to perpendicular recording. Even though the hard disk technology is more than 50 years old, the storage density of the hard disk drives has been increasing at a rate of about 30% per year. As of 2010, all the hard disk products are based on perpendicular recording technology. It is believed that some alternatives, such as heat-assisted magnetic recording or patterned media technology, are needed to maintain the growth of storage density which will enable the hard disk industry to maintain its superiority over competing technologies. Such growth has been (and will continue to be) brought on by various technological developments. We thought there was a need to condense these developments into a book that will benefit the readers for the next several years. With this aim, we requested several researchers from different backgrounds to write on the developments in their respective field. However, the book would not be complete if it did not provide the background information for a novice. With that in mind, we have also included some chapters that introduce the fundamentals. Chapters 1–3 provide the fundamentals and Chapters 4–9 provide the recent developments in several areas of hard disk drives. Chapters 10–12 cover the emerging technologies of hard disk drives.

Even though hard disk drives have enjoyed a successful advantage over the competing technologies in terms of cheaper costs and higher capacities, their superiority is threatened by the flash memory. Since there are several books on flash memory, we did not attempt to cover it here. However, flash memory will face its hurdles in the near future and several alternative memory technologies such as phase change random access memory (PCRAM) and magnetic random access memory (MRAM) are emerging as alternatives. Chapters 13–14 describe the fundamentals as well as the recent advances and challenges in these emerging memory technologies.

We believe the book will be useful to researchers and students and fulfill the aim with which we began this work.

S. N. PIRAMANAYAGAM

TOW C. CHONG

ACKNOWLEDGMENTS

Data storage—even if it has to be written purely from a materials perspective—is a multidisciplinary. It involves physics, chemistry, and materials science at the fundamental level; and mechanical, chemical, and electrical engineering at the application level. A book of such a multidisciplinary nature cannot be accomplished in an authoritative fashion without significant contributions of several kind hearts. First of all, we would like to thank all of the contributing authors: Shiva Prasad (IIT Bombay, India), Kumar Srinivasan (Western Digital, USA), Naoki Honda (Tohoku Institute of Technology, Japan), Kiyoshi Yamakawa (Akita Industrial Technology Center, Japan), Bruno Marchon (HGST, USA), Ganping Ju, William Challener, Yingguo Peng, Mike Seigler, and Ed Gage (Seagate Technology, USA); Jingsheng Chen (NUS, Singapore), Bruce Terris (HGST, USA), and Thomas Thomson (U. Manchester, UK), who represented authors from different parts of the globe; and Rachid Sbiaa, Guchang Han, Viloane Ko, Zaibing Guo, Hao Meng, Allen Poh, Thomas Liew, Jiangfeng Hu, Luping Shi, Rong Zhao, Randall Law, and Sunny Lua from Data Storage Institute (DSI), Singapore. They were very glad to contribute chapters and very cooperative in revising and improving the chapters. Our sincere thanks are due to these authors.

There are several others who provided the help and support at the initial stages which motivated us to take this job. Catherine Faduska (formerly of IEEE) and Liesl Folks (HGST, USA) were instrumental in helping with the review and selection of the book proposal. There are several authors who were interested in writing a chapter but could not write because of certain unavoidable circumstances. We acknowledge their support. We also would like to thank IEEE Press and Wiley for their support to publish this work.

Personally, S.N.Piramanayagam would like to thank his family members Preeti, Priya, and Pramesh, and colleagues and students in DSI for their support, understanding, and patience during this period. Tow C. Chong would like to dedicate this effort to his family and all the DSI staff.

S. N. P.

T. C. C.

CONTRIBUTORS

William Challener, Seagate Technology, USA

Jingsheng Chen, National University of Singapore, Singapore

Tow C. Chong, Data Storage Institute, Singapore

Ed Gage, Seagate Technology, USA

Zaibing Guo, Data Storage Institute, Singapore

Guchang Han, Data Storage Institute, Singapore

Naoki Honda, Tohoku Institute of Technology, Japan

Jiangfeng Hu, Data Storage Institute, Singapore

Ganping Ju, Seagate Technology, USA

Viloane Ko, Data Storage Institute, Singapore

Randall Law Yaozhang, Data Storage Institute, Singapore

Thomas Y. F. Liew, Data Storage Institute, Singapore

Sunny Y.H. Lua, Data Storage Institute, Singapore

Bruno Marchon, Hitachi GST, USA

Hao Meng, Data Storage Institute, Singapore

Yingguo Peng, Seagate Technology, USA

S. N. Piramanayagam, Data Storage Institute, Singapore

Allen Poh Wei Choong, Data Storage Institute, Singapore

Shiva Prasad, Indian Institute of Technology-Bombay, India

Rachid Sbiaa, Data Storage Institute, Singapore

Mike Seigler, Seagate Technology, USA

Luping Shi, Data Storage Institute, Singapore

Kumar Srinivasan, Western Digital, USA

Bruce D. Terris, Hitachi GST, USA

Thomas Thomson, The University of Manchester, UK

Kiyoshi Yamakawa, AIT, Akita Industrial Technology Center, Japan

Rong Zhao, Data Storage Institute, Singapore

1

INTRODUCTION

S. N. Piramanayagam

Data Storage Institute, Agency for Science, Technology and Research (A*STAR), Singapore

1.1 INTRODUCTION

What is the earliest form of data storage used by human beings? When this question is posed, answers such as stone, paper, tape, and so on often come up before someone suggests “the brain.” Although the brain is the data storage system that nature has provided us with, it is not sufficient for all purposes. Even though the brain can be used for storage of certain kinds of information, how reliably one can retrieve the information depends on the individual and the circumstances. Moreover, information stored in a person’s brain cannot be transferred to others after the life of that person. We need data or information storage systems for at least two purposes: (1) to reliably preserve data and information for retrieval when it is needed, and (2) to spread or communicate information/knowledge to others. When humans realized this, they started inventing other means of storing information, such as using stone, clay, paper, and so on as media for data storage.

Magnetic recording was invented more than a century ago by Valdemar Poulsen [1]. It took about 30 years for magnetic tapes to be successfully commercialized [2]. Even though magnetic tapes were good for archival or sound recording, they did not possess random access capability, and hence access times were longer compared to other forms of recording available during that period, such as punch cards. To overcome the random-access problem of magnetic recording, IBM invented the first hard disk drive (HDD), which combined the advantages of magnetic recording (multiple read/erase cycles) with random acess capability, and suitably named it RAMAC (random access memory accounting system or random access method of accounting and control). The RAMAC (introduced in 1956) had a capacity of 5 MB, which was achieved using 50 magnetic disks with a diameter of 24 in.—each offering an areal density of 2 kilobits per square inch (kb/in.2). Since then, HDDs have come a long way and now pack 1000 GB in two magnetic disks with a diameter of 2.5 in., each offering an areal density of over 600 gigabits per square inch (Gb/in.2) as of 2011. An areal density increase of the order 108 times in a period of close to 50 years is simply remarkable and was possible because of the tremendous efforts to develop the technology behind each component of the HDD. Although most chapters of this book will cover in detail the technology behind the development of the HDD, this chapter will provide an overview of HDD technology, briefly covering the technology from a materials perspective, in line with the theme of this book. This chapter also provides a brief overview of memory technologies that are emerging as alternatives for future memory/storage applications.

1.2 BASICS OF DATA STORAGE

Any data storage system/device needs to satisfy certain basic criteria. The first basic requirement is a storage medium (or media). On this storage medium, the data will be written. The other requirements are that there should be ways to write, read, and interpret the data. For example, let us look at this book as a form of data storage containing the chapters written by the contributing authors. In the printed version of the book, paper is the storage medium. Writing the information (printing) is completed using ink, and reading is carried out with the user’s eyes. Interpretation of the data and sometimes even error correction is carried out in the user’s brain. Components with similar functions exist in an HDD, too.

Figure 1.1 shows the components of a typical HDD used in desktop personal computers (PCs). Some of the key components that make up an HDD are marked; an HDD has disk media, heads, a spindle motor, an actuator, and several other components. A disk is a magnetic recording medium that stores information, similar to the pages of a book. A head performs two functions, writing and reading information, corresponding to a pen and an eye in our example. A spindle motor helps to spin the disk so that the actuator, which moves along the radial direction, can carry the head to any part of the disk and read or write information. An HDD also has several circuitries in a printed circuit board that serve as its brain, controlling its activity, and receiving and conveying meaningful information from or to the computer or whatever device that uses the HDD.

Figure 1.1. Picture of a hard disk drive and various components.

Several disks (also called platters) may be stacked in an HDD in order to multiply the capacity. In almost all HDDs, the information is stored on both sides of the disks. Figure 1.2 shows the way the data are organized on magnetic disks. The data are stored in circular tracks. The number of tracks that can be packed closely within a given length is called track density and is expressed in tracks per inch (TPI). The number of bits that can be stored along the track is measured in terms of bits per millimeter (bits/mm) or bits per inch (bits/in.) and is called linear density. For a particular track density, media with better performance can achieve larger linear density than the inferior disk. The areal density, which is the number of bits that can be stored in a given area, is a product of the track density and the linear density, and is often expressed as bits per square inch (bpsi). Within the tracks there are addressed sectors in which the information can be written or read. The randomness in access or storage of information from or in an address provided by a central processing unit (CPU) comes from the ability to move the head to a desired sector. In state-of-the-art HDD, the total length of tracks on one side of a 65 mm disk covers a distance of 42 km, almost a marathon run. As of 2011, in each track the bits are packed at a density of 1.5–2 million bits in an inch.

Figure 1.2. Illustration of hard disk media, various tracks, and the way the bits are arranged in tracks. The contrasting lines indicate the magnetic field emanating from the media.

1.3 RECORDING MEDIUM

There has to be a medium for storing information in a data storage device. In magnetic recording, a disk that comprises several magnetic and nonmagnetic layers serves as the recording medium [3]. Whether the medium is tape or disk, magnetic recording relies on two basic principles. First, magnets have north and south poles out of which its magnetic field emanates and can be sensed by a magnetic-field sensor. The sensing of a magnetic field by a magnetic-field sensor provides a way of reading information. Second, the polarity of the magnets can be changed by applying external magnetic fields, which is usually achieved using an electromagnet. This provides a way of writing information. Earlier magnetic recording media such as audio tapes, video tapes, and so on were mostly used for analog applications. HDDs are digital devices, which make use of strings of 1 and 0 to store information.

Figure 1.3 illustrates the recording process using longitudinal recording technology. In this technology the polarities of the magnets are parallel to the surface of the hard disk. When two identical poles are next to each other (S–S or N–N), a strong magnetic field will emerge from the recording medium, but no field will emerge when opposite poles (S–N) are next to each other. Therefore, when a magnetic-field sensor (a giant magnetoresistive [GMR] sensor, for example) moves across this surface, a voltage will be produced only when the GMR sensor goes over the transitions (regions where like poles meet). This voltage pulse can be synchronized with a clock pulse. If during the clock window the GMR sensor produces a voltage, the voltage is represented as “1.” If no voltage is produced during the clock window, the absence of voltage is represented by “0.” This is a simple illustration of how 1s and 0s are stored in hard disk media. The fundamentals of magnetism and the details of longitudinal recording technology, which will lay the foundation for most of the chapters in the book, will be discussed in Chapters 2 and 3, respectively. In perpendicular recording technology, which is the current way of recording information on HDDs, the magnetizations lie out of plane [3, 4]. In this technology the magnetic field emanates from the center of the bit cells rather than the transitions. More details about perpendicular recording media will be provided in Chapter 4.

Figure 1.3. Illustration of the recorded pulses from magnetic transitions and the recording principle.

1.4 HEADS

The head is a tiny device (as shown in Fig. 1.1) that performs the read–write operation in an HDD. Head technology has undergone tremendous changes over the years. In the past, both reading and writing operations were carried out using an inductive head. Inductive heads are transducers that make use of current-carrying coils wound on a ferromagnetic material to produce magnetic fields. The direction of the magnetic field produced by the poles of the ferromagnetic material can be changed by changing the direction of the electric current. This field can be used for changing the magnetic polarities of the recording media (writing information). Chapter 5 discusses the physics of write heads and the materials used for this purpose.

Inductive heads can also be used for reading information, based on Faraday’s law, which states that a voltage will be generated in a coil if there is a time-varying flux (magnetic field lines) in its vicinity. When a magnetic disk with information rotates, the field emanating from the recording media bits will produce a time-varying flux, which will lead to a sequence of voltage pulses in the inductive head. These voltage pulses can be used to represent 1s or 0s. Inductive head technology was the prevailing technology for reading information until the early 1990s. However, in order to increase the bit density, the size of the bit cells had to be reduced. Moreover, the Mrδ (remanent moment-thickness product) also was reduced as technology progressed in order to reduce the medium noise, which resulted in a decrease in magnetic flux from the bits. The inductive heads were not sensitive enough to the increasingly reduced magnetic field from the smaller bits as technology progressed. To address this problem, more advanced read sensors were introduced into the head design. Modern HDDs have heads with two elements: one is a sensor for reading information (similar to an eye when reading a book), and the other is an inductive writer for writing information. Such components where the sensor and writer are integrated are called integrated heads or, simply, heads.

The HDDs used magnetoresistive (MR) heads for some time (early to late 1990s) before switching to the prevailing GMR sensors. Unlike inductive heads, MR and GMR heads work on the basis of change in the resistance of the sensor in the presence of a magnetic field. The GMR sensor is in fact made of several magnetic and nonmagnetic layers. GMR devices make use of the spin-dependent scattering of electrons. Electrons have “up” and “down” spins. When an electric current is passed through a magnetic material, the magnetic orientation of the magnetic material will favor the movement of electrons with a particular spin—up or down. In GMR devices, the magnetic layers can be designed in such a way that the device is more resistive or less resistive to the flow of electrons, depending on the direction of the field sensed by the sensors. Such a change in resistance can be used to define 1 or 0 for digital recording. Although write-head research is mostly limited to the companies that manufacture heads, read-sensor research is carried out widely. This is especially so because read sensors are not only technologically challenging but are also academically interesting. Therefore, this book has two chapters on read sensors: Chapter 6 focuses on the fundamentals of read sensors, and Chapter 7 provides an overview of future research and technologies for read sensors.

1.5 MATERIALS ASPECT OF THE HEAD–DISK INTERFACE

In an HDD, the head flies in close proximity to the media in order to read and write information. The component that carries the read sensor and the write head is called a slider. The slider has air bearings that provide the relevant aerodynamics for flyability at a specific height for which it has been designed. The flying height of the sliders has been reduced over the years to sub-10 nm levels [5]. In recent years, the sliders even comprise a technology called “thermal flying height control.” This technology uses a microheater embedded in the slider that can be heated using a current to cause nanometer-level expansion near the reader and writer, allowing the possibility of reducing the flying height to sub-5 nm levels, especially when reading and writing operations are carried out [6]. When the head flies at close proximity to the disk medium, there may be intermittent contacts between the head and disk, which might cause damage to the head and/or hard disk medium, resulting in data loss. In order to minimize the damage involved, the hard disk medium is usually coated with a thin lubricant layer, which among many other advantages provides a way to reduce the friction and wear during sporadic contacts. However, there are many challenges in lubricant technology. Chapter 8 provides a detailed discussion of lubricants.

In addition to the lubricant layer, the hard disk medium also has an overcoat layer, which has been some form of carbon film for several years. The carbon overcoat protects the medium from corrosion and wear. The hardness of the carbon overcoat prevents the medium from wear, and the uniformity of the carbon coating helps the medium from being corroded. The overcoat also provides a surface that is suitable for the lubricant to adhere to. In the past, carbon overcoats were very thick (several hundred nanometers). However, tremendous improvement has been made in carbon overcoat technology, resulting in overcoats with thicknesses of about 2 nm and yet providing superior wear and corrosion protection. For future recording applications, it is necessary to obtain even thinner overcoats as an enabler for smaller magnetic spacing (the spacing between the top magnetic layer of the medium and the bot.tommost magnetic part of the read sensor) [7]. There are several challenges associated with the overcoat. These are covered in Chapter 9.

1.6 TECHNOLOGIES FOR FUTURE HDDS

In addition to the different aspects of technologies related to current and future HDD technology covered in Chapters 3–9, it is also essential to look at some technologies on the horizon that are unique and different from the existing technologies. It is widely accepted that the future HDDs may use heat-assisted magnetic recording technology, patterned media technology, or a combination of the two. The need for these technologies arises from the media trilemma issue to be discussed in detail in Chapter 4. However, in brief, the media trilemma is the difficulty faced in trying to optimize the signal-to-noise ratio (SNR), thermal stability, and writability. The SNR obtained from a recording medium should be kept high for reading information reliably, which requires small grains in the recording medium. However, the small grain size of the medium will cause thermal stability issues, whereby the magnetization of the grains may be susceptible to undergoing thermal reversals leading to data loss. The thermal stability problem may be overcome by using recording media with a high anisotropy constant, but this will result in writability issues. The trilemma is unavoidable at a certain stage, and hence researchers have to look at ways to overcome or delay them. Longitudinal recording technology reached its limit a few years back, and hence in 2006, perpendicular recording technology was introduced. However, perpendicular recording technology in its current form will also reach its limit soon. Therefore, alternative technologies such as heat-assisted (or energy-assisted) recording and patterned media are considered as they provide certain advantages.

In heat- or energy-assisted recording, the recording media material makes use of a high anisotropy material with a high thermal stability even for small grain sizes, thus providing a high SNR. Writing information on a high anisotropy material will not be possible with the existing write-head materials. Heat-assisted recording addresses this problem by making use of thermal energy to minimize the energy barrier for reversal at the time of writing. This may be achieved, for example, by focusing a small beam of laser to locally heat the samples. Since the disk is rotated away from the laser beam after the writing process, and the laser beam is off at times other than the writing time, the high anisotropy constant of the recording media material makes the information stable. Chapter 10 discusses heat-assisted recording technology from a system perspective, and Chapter 11 focuses on the materials aspect of heat-assisted recording. Together, these two chapters provide detailed information on heat- or energy-assisted magnetic recording.

Another unconventional scheme to tackle the media trilemma is to increase the volume (V) of the magnetic unit to tackle thermal stability problems. In conventional recording many magnetic grains store one bit, and the bit boundary is decided mainly by the grain boundary. Therefore, when the volume of the magnetic unit is increased, the bit boundary will be broader, limiting the areal density. However, bit-patterned media recording makes use of well-defined bit boundaries that could be made of nonmagnetic materials or voids created by lithography or other procedures. In this case, the grains in the magnetic unit could be exchange coupled strongly to act as a single domain with high thermal stability. In conventional magnetic recording, strong exchange coupling could lead to bit boundaries that are very wide. However, the strong exchange coupling in the magnetic entity of a bit-patterned media does not affect the bit boundary because the bit boundary is defined by the lithography process. Since the volume of the magnetic entity in patterned media for a particular areal density is much larger than that of a grain in conventional recording, thermal stability and writability are not sacrificed. Chapter 12 gives a detailed coverage on patterned media technology. Whether heat- or energy-assisted recording or patterned media recording technology will take over perpendicular recording is not clear at this moment, but it is quite likely that the two technologies may be integrated at certain point of time.

1.7 MEMORY TECHNOLOGIES

HDDs enjoyed unmatched advantages over their competitors for several decades. Because of the high areal density (bits per area) growth achieved in early 2000s, it was possible for HDDs to be used for several applications, such as portable digital music and video players. HDDs with 1-in. disk media were made with compact-flash (CF) and custom-made interfaces, offering higher capacities at cheaper prices than that is possible with semiconductor memories. Apple™ made iPods using such a 1-in. small form factor (SFF) and 1.8-inch HDDs. However, the invasion of HDDs into areas occupied by semiconductor memory did not last long. iPod-Nano MP3 players, featuring semiconductor memories, were released in the next few years as a sign of the threat faced by HDDs. HDDs with 1-in. disk media were phased out in the next few years, and it seems that 1.8-in. HDDs are under threat from solid-state-memory-based storage. Although HDDs still enjoy a significant advantage in 2.5-in. and 3.5-in. disk drives because of their higher capacity and cheaper price, they face a steady threat from solid-state memory devices.

Flash memory—the current competitor for HDDs in certain areas—is also facing technological challenges beyond sub-22 nm scaling. It has been proposed that phase change memory or phase change random access memory (PCRAM) and/or magnetic random access memory (MRAM) may emerge as alternatives for flash memory. It has been proposed that PCRAM can be potentially scaled down to 5 nm, but the question remains as to whether the associated semiconductor technology can also be scaled down to that level. Not to fail in comparison, MRAM also has the potential (based on the thermal stability of FePt materials to be discussed in Chapter 11) to be scaled down to sub-5 nm, but several questions need to be answered before reaching such limits. Nevertheless, the potential scalability of these two candidates makes them good alternatives to flash memory in the long run. As there have been several books on semiconductor-based memories, this book does not cover flash memory. However, two chapters have been dedicated to PCRAM and MRAM. Chapter 13 reviews the developments and challenges of PCRAM, and Chapter 14 provides an overview of the developments and challenges of MRAM.

1.8 SUMMARY

To summarize, the book has been organized as follows: Chapters 2 and 3 discuss the fundamentals of magnetism, magnetic recording, and media technology, and lay the foundation to understand Chapters 4–12. Chapter 4 provides a discussion of the fundamentals and advances in perpendicular recording media technology. Chapter 5 discusses write-head technology, and Chapters 6 and 7 discuss the fundamentals of read technology and the challenges and advances in read technology. Chapters 8 and 9 deal with the head–disk interface aspects of HDDs, focusing on lubricants and overcoats, respectively. Chapters 10–12 concern the emerging technologies for HDDs, namely, heat-assisted magnetic recording and patterned media technology. Chapters 13 and 14 provide an overview of the fundamentals, challenges, and prospects of memory technologies such as PCRAM and MRAM, which are emerging as potential candidates for storage of information.

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