Investigations on advanced magnetic recording media 1

Table of Contents 4.3 Effect of Implantation of Boron in the Recording Layer and Soft Underlayer of CoCrPt-SiO 2 Media .... Lateral straggle can hinder the prospects of achieving high a

INVESTIGATIONS ON ION IMPLANTATION IN ADVANCED MAGNETIC RECORDING MEDIA

NIKITA GAUR

NATIONAL UNIVERSITY OF SINGAPORE

2012

INVESTIGATIONS ON ION IMPLANTATION IN ADVANCED

MAGNETIC RECORDING MEDIA

NIKITA GAUR

B.Sc Electronics (Hons), Delhi University, India

A THESIS SUBMITTED FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

DEPARTMENT OF ELECTRICAL AND COMPUTER

ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE

AUGUST 2012

Acknowledgements

Acknowledgements

Firstly, I would like to express my deep and sincere gratitude to my supervisors Prof Charanjit Singh Bhatia and Dr S N Piramanayagam for their invaluable guidance, advice and counseling during my PhD candidature It was an absolute pleasure and honor to conduct my research under their supervision Their patience and assurance during times of difficulty will always be remembered

Special thanks always goes out to all my seniors from Sri Venkateswara College, Delhi University, New Delhi, for helping and guiding me during my initial days at National University of Singapore (NUS) I would also like to express my gratitude to all my colleagues and friends in the ISML and SEL labs for their invaluable help and friendship Many thanks to the lab officer, Mr Jung Yoon Yong Robert, for all his help during my stay in SEL The experimental facilities provided to carry out this research work by the Data Storage Institute (DSI) are acknowledged I am very thankful to Mr Tan Hang Khume from DSI for all his help during my attachment with DSI I would also like to thank Mojtaba, Shreya, Taiebeh, Ajeesh, Mridul and Aditi for their lovely friendship and support I am also thankful to Dr K.K.M Pandey and Mr Le Hong Vu, with whom I have had the privilege of working during my candidature

Over and above, I would like to acknowledge the support provided by NUS Grant

No R 263-000-465-112 and NRF-CRP 4-2008-06 (NUS Grant No R

263-000-585-281) for this work Also, I am truly grateful to the National University of Singapore

for the NUS scholarship

I would like to express my appreciation towards Mr S.L Maurer, Dr R.W Nunes and Mr S.E Steen from the IBM Thomas J Watson Research Center, Yorktown

A big heartfelt thank you to everyone!

Nikita Gaur

Table of Contents

Table of Contents

ACKNOWLEDGEMENTS I TABLE OF CONTENTS III ABSTRACT VII LIST OF TABLES XII LIST OF FIGURES XIII LIST OF PUBLICATIONS, PATENTS AND CONFERENCES XIX Publications in Peer-reviewed journals XIX Patents XX Conferences XX LIST OF SYMBOLS XXI LIST OF ABBREVIATIONS XXV LIST OF EQUATIONS XXIX

CHAPTER 1 1

1 INTRODUCTION 1

1.1 Background 1

1.2 Research Objectives 4

1.3 Organization of Thesis 7

CHAPTER 2 9

2 MAGNETIC RECORDING: LITERATURE REVIEW 9

2.1 History of Magnetic Recording 9

2.2 Basic Magnetism 10

Table of Contents

2.3 Magnetic Recording Media 17

2.3.1 Longitudinal Magnetic Recording (LMR) 17

2.3.2 Perpendicular Magnetic Recording (PMR) 18

2.4 Challenges Faced by PMR 20

2.4.1 Superparamagnetism 21

2.4.2 Magnetic Recording Trilemma 22

2.5 Future Magnetic Recording Technologies 23

2.5.1 High Anisotropy Constant Material 23

2.5.2 Heat-Assisted Magnetic Recording (HAMR) 27

2.5.3 Anisotropy Graded Media 32

2.5.4 Bit Patterned Media (BPM) 34

2.6 Ion Implantation 38

CHAPTER 3 42

3 EXPERIMENTAL AND COMPUTATIONAL DETAILS 42

3.1 Introduction 42

3.2 Samples Fabrication by Sputtering 42

3.3 Ion Implantation 48

3.3.1 Samples Processing by Ion Implantation 48

3.3.2 Simulation Using SRIM and TRIM 49

3.4 Nanoimprint Lithography (NIL) 55

3.5 Magnetic Characterization 59

3.5.1 Magneto Optical Kerr Effect (MOKE) 59

3.5.2 Alternating Gradient Field Magnetometer (AGFM) 62

3.5.3 Magnetic Force Microscopy (MFM) 66

3.6 Structural Characterization 69

3.6.1 X-Ray Diffraction (XRD) 69

3.6.2 X-Ray Photoelectron Spectroscopy (XPS) 71

3.6.3 Electron Microscope (EM) 73

3.7 Density Functional Theory (DFT) Calculations 77

CHAPTER 4 82

4 ION IMPLANTATION IN MAGNETIC MEDIA: EFFECT OF DEPTH OF IMPLANTATION 82

4.1 Introduction 82

4.2 Experimental Methods 84

Table of Contents

4.3 Effect of Implantation of Boron in the Recording Layer and Soft

Underlayer of CoCrPt-SiO 2 Media 87

4.3.1 Magnetic Properties 89

4.3.2 Crystallographic Properties 96

4.3.3 Depth Profile of the Layer Structure 98

4.3.4 Microstructural Properties 101

4.3.5 SRIM Calculations and Lateral Straggle 102

4.4 Effect of Implantation of Argon in the Recording Layer and Soft Underlayer of CoCrPt-SiO 2 Media 106

4.4.1 Magnetic Properties 107

4.4.2 Crystallographic Properties 113

4.4.3 Depth Profile of the Layer Structure 114

4.4.4 Microstructural Properties 116

4.4.5 SRIM Calculations and Lateral Straggle 119

4.5 Summary 123

CHAPTER 5 125

5 ION IMPLANTATION IN MAGNETIC MEDIA : EFFECT OF MASS OF ION SPECIES 125

5.1 Introduction 125

5.2 Patterned Media Requirements 125

5.3 Experimental Details 128

5.4 Magnetic Properties 131

5.4.1 Hysteresis Loops 131

5.4.2 Saturation Magnetization 135

5.4.3 First Order Reversal Curves (FORC) Study 138

5.5 Crystallographic Properties 141

5.6 Calculated Lateral Range and Straggle 143

5.7 Relation between Straggle and Exchange Interaction 144

5.8 Density Functional Theory Calculations (DFT) 147

5.9 Summary 149

CHAPTER 6 150

6 PATTERNED MEDIA FABRICATION 150

6.1 Introduction 150

Table of Contents

6.2 Experimental Methods 150

6.2.1 FePt Media Optimization 152

6.2.2 Ion Species and Dose Optimization 157

6.2.3 Hard Mask Fabrication 161

6.3 Characterization of Patterned Media 164

6.3.1 Magnetic Properties 164

6.3.2 Crystallographic Properties 167

6.3.3 Magnetic Domain Study 168

6.4 Summary 169

CHAPTER 7 171

7 GRADED MEDIA BY ION IMPLANTATION 171

7.1 Introduction 171

7.2 Experimental Methods 172

7.3 Magnetic Properties 176

7.3.1 Kerr Loops 176

7.3.2 Switching Field Distribution 179

7.3.3 Magnetic Domain Study 180

7.3.4 Thermal Stability Measurements 182

7.4 XPS Depth Profiles 185

7.5 Crystallographic Properties 186

7.6 Summary 188

CHAPTER 8 190

8 CONCLUSION AND SUGGESTIONS FOR FUTURE WORK 190

8.1 Conclusions 190

8.2 Suggestions for Future Work 193

BIBLIOGRAPHY 195

Abstract

Abstract

Hard disk drives (HDD) have become an integral part of our daily lives As the areal density is approaching a few tera bits per square inch (Tbits/in2) and beyond, the conventional perpendicular magnetic recording (PMR) technique has started facing thermal instability issues Graded media and bit patterned media (BPM) are considered two possible candidates which can push the areal density limit higher In this thesis, ion implantation as a potential method to produce these media is studied Bit patterned media have magnetic islands in a sea of nonmagnetic matrix, providing improved thermal stability of magnetization, in addition to providing lower noise, greater ease of writing information, etc One of the main limitations with current techniques for patterned media fabrication is the need of planarization — a process by which the lithographically modified topography is flattened A lot of research is ongoing to overcome these challenges Ion beam modification is one approach, which can help to solve the planarization problem In this method, ion implantation is done though a mask (hard mask) such that the implanted regions become nonmagnetic and masked regions retain their magnetism However, in order

to meet such a goal it is crucial to understand the role of the implantation conditions

on the saturation magnetization and lateral straggle The objective of this work is to understand and determine the ions and energies needed to meet such requirements for achieving high areal densities

One of the crucial requirements for patterned media fabrication by the ion implantation method is to be able to reduce the saturation magnetization to zero in the implanted regions so that no signal is detected from that region The second most critical parameter is lateral straggle, in simpler terms, the diffusion of ions in the

Abstract

lateral direction underneath the masked area Lateral straggle can hinder the prospects

of achieving high areal densities beyond a few tera bits per square inch in the magnetic recording media Not many studies in the literature have shown both parameters being taken into consideration in the process of fabricating patterned media Moreover, various parameters like fluence, energy and ion species types have also not been studied in order to gain a deeper understanding of ion implantation on magnetic media Therefore, here in this thesis the effects of ion implantation as a function of various parameters like fluence, energy of implantation and ion species have been studied to give insights from a fundamental point of view and to make this process suitable for high-density nanostructures

It was observed that all the above mentioned parameters were significant in reducing saturation magnetization and lateral straggle of the magnetic media with a recording layer based on the Co-alloy system (CoCrPt-SiO2) It was seen that coercivity or anisotropy decreased as a function of ion fluence Depth (energy) of implantation played an important role in reducing the coercivity or anisotropy It was observed that the reduction in coercivity was more significant when the ions were implanted into the recording layer compared to implantation at a greater depth, in this case, a soft underlayer (SUL) This was explained based on the damage profile obtained due to stopping mechanisms as can be seen from TRIM simulations and experiments like XPS and TEM in Chapter 4 Also, it was seen that the lateral straggle was reduced from 90.5 nm to 13.3 nm with decreasing energy of implantation for one particular ion species (B+) from 83 keV to 10 keV Hence the chapter showed high fluence and low energy of implantation were better for reducing the anisotropy

as well as lateral straggle

Abstract

Furthermore, in Chapter 5, lateral straggle was observed to reduce with increasing mass of ion species (ions ranging from He+ to Sb+) to around 2.3 nm (for Sb+ ion) Such a reduction in lateral straggle was explained on the basis of collisions

encountered by the species of atomic mass Saturation magnetization, M s, was seen to

be independent of mass of ion species and was found to reduce to zero for argon (Ar+) and antimony (Sb+) at different doses This has been explained based on the possibility of modification of the electronic band structure of the magnetic material or dilution, where there is replacement of magnetic atoms from the host matrix with non-magnetic atoms Such a possibility has been validated by density functional calculations (DFT) as well Even though high fluence may sputter etch the top surface

to a few nanometers, the reduction in thickness was not found to contribute much to

the reduction in M s In short, the results in this chapter indicated that the implantation

of heavier ions is a better approach for minimizing the straggle in Co-based alloy recording medium system for high densities

Based on the information obtained in previous chapters, heavy ions (Sb+) were chosen for implantation to form patterned media using highly ordered FePt medium instead of CoCrPt-SiO2 media Lighter ions (He+) were also used for reference in this

high anisotropy (K u) media This was because high anisotropy materials have been considered for high areal density Implantation of either Sb+ or He+ showed loss of the L10 phase at high doses like 1016 ions/cm2 Saturation magnetization, M s, reduced to

~0 emu/cc for Sb+ compared to He+ where the drop was much less Upon implantation with Sb+ ions through 30 nm masked regions with 80 nm pitch, it was observed that due to the small mask size a disordered FePt (with low magnetic anisotropy) was formed even in the masked region With He+ ions, however, the problem was high saturation magnetization even after high dose implantation Hence, there appears to be

Abstract

a trade-off between heavy and light ion species Thus, forming bit patterned medium using FePt media at 10 Tbits/in2 appears to be a challenge using ion implantation Besides patterned media, another way to extend the areal density barrier of conventional media technology is by increasing the anisotropy constant of the material Such a medium, however, would lead to writability issues hence some way

of switching the magnetization at a lower switching field has to be worked out Anisotropy graded medium is one such technique Such a medium consists of magnetically graded regions from hard to soft regions within each grain, where the soft regions assist the hard regions in reversal thereby reducing the overall switching field Such a medium has been reported to reduce the switching field by four times However, preparing graded media is an extremely challenging task using the conventional sputtering technique A major drawback of currently used sputtering technique is that there are a number of interfaces in graded media Hence it is more appropriate to use a multilayer media than a graded media Moreover, the gradient in anisotropy is not quadratically graded which is a requirement Hence, the objective of this work is to figure out a more controllable method of fabricating a quadratically anisotropy graded media In Chapter 8, ion implantation has been suggested as a novel way of fabricating graded media by making use of its Gaussian profile Here, the ion profile was chosen in such a manner that it had its peak at the top surface of the recording layer with a gradual tail going deep into the recording layer Thermal stability measurements have shown a reduction in the intrinsic anisotropy field while the thermal stability of grains maintained at around/above 60, which shows successful fabrication of the graded media Moreover, XPS on Co+ ions showed a profile with maximum density of cobalt on top of the recording layer and density reducing down the depth in the recording layer

Abstract

In brief, this thesis presents two different ways to achieve a single goal, enhancing areal densities beyond a few Tbits/in2

List of Tables

List of Tables

Table 2-1 Magnetocrystalline anisotropy energy densities, Ku, for materials related to

or of interest for ultra-high density magnetic recording media [65]–[67] 26Table 3-1 Optimized thermal imprinting steps and conditions (Demolding

temperature=110 ºC) 58Table 3-2 Optimized UV imprinting steps and conditions (Demolding temperature=55 ºC) 58

List of Figures

List of Figures

Figure 2-1 Hysteresis loop of a ferromagnet and various magnetic parameters 12

Figure 2-2 Schematic of LMR and PMR 20

Figure 2-3 The change in the magnetization state in case of small ferromagnetic particle or high temperature condition 22

Figure 2-4 Schematic of magnetic recording trilemma 23

Figure 2-5 Phase Diagram of FePt All (Reprinted with permission of ASM International All rights reserved www.asminternational.org) [71] 27

Figure 2-6 Comparison between regular PMR and HAMR media 29

Figure 2-7 Schematic of field reversal in ECC media by spin chain model 33

Figure 2-8 Schematic of the bits in PMR media and BPM along a track on the magnetic disk 35

Figure 2-9 Schematic of two approaches for BPM fabrication 38

Figure 2-10 Ion profile of Carbon implanted in CoCrPt-SiO2 and four moments of the ion profile 40

Figure 3-1 Schematic of diode sputtering 43

Figure 3-2 Schematic of DC magnetron sputtering 45

Figure 3-3 Thornton model for metals deposited by magnetron sputtering Reprinted with permission from [J A Thornton, “The microstructure of sputter-deposited coatings,” Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, vol 4, no 6, pp 3059–3065, 1986.] Copyright [1986], American Vacuum Society [119] 46

Figure 3-4 Schematic of a typical ion implanter [125] 48

Figure 3-5 Ion ranges of carbon at 5.8 keV in a CoCrPt-SiO2 media sample 54

Figure 3-6 Lateral distribution plot for carbon implanted into the RL at 5.8 keV 55

Figure 3-7 Schematic of types of NIL, thermal and UV imprint lithography 56

Figure 3-8 (a)The first order reversal curve (FORC) (b) a set of first order reversal curves (FORCs) and (c) The FORC contour plot for the conventional CoCrPt-SiO2 media 65

Figure 3-9 AFM and MFM on AC demagnetized patterned media [154] (Copyright © 2012 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim) 68

List of Figures

Figure 3-10 Schematic of X-ray interaction with the crystal plane 69 Figure 3-12 Common computational approaches used for electronic structure

calculations [172] 80 Figure 4-1 Schematic of media layer structure 84 Figure 4-2 Ion profiles in recording media target obtained by TRIM for boron

implantation in (a) RL and (b) SUL 88 Figure 4-3 Kerr loops of samples implanted with boron in (a) RL (corresponding to energy of implantation, 10 keV) and (b) SUL (corresponding to energy of

implantation, 83 keV) with the doses of 1011 and 1016 ions/cm2 89 Figure 4-4 MFM on AC demagnetized as-deposited (unimplanted) sample and

samples implanted with boron in RL with the doses of 1011 and 1016 ions/cm2 93 Figure 4-5 MFM on AC demagnetized as-deposited (unimplanted) sample and

samples implanted with boron in SUL with the doses of 1011 and 1016 ions/cm2 94 Figure 4-6 In-plane loops of the granular media implanted with boron ions in the SUL

at both the doses of 1011 and1016 ions/cm2, respectively 95 Figure 4-7 XRD wide θ-2θ scan of as-deposited (reference) CoCrPt-SiO2 sample 96 Figure 4-8 XRD θ-2θ scans of samples implanted with boron in RL and SUL with the doses of 1011 and 1016 ions/cm2 97 Figure 4-9 XPS depth profiles of samples implanted with boron in RL with the doses

of 1011 and 1016 ions/cm2 99 Figure 4-10 XPS depth profiles of samples implanted with boron in SUL with the doses of 1011 and 1016 ions/cm2 100 Figure 4-11 Cross-section TEM of samples implanted with boron in RL with the doses of 1011 and 1016 ions/cm2 102 Figure 4-12 SRIM-2008 simulation of nuclear and electronic stopping powers of boron in recording media target with respect to implantation energy 103 Figure 4-13 Damage profiles obtained from TRIM calculation for boron implantation into RL and SUL 104 Figure 4-14 (a) The lateral distribution profile obtained from TRIM for boron

implanted into recording media at 10 keV, (b) the lateral range and straggle plotted as

a function of energy of implantation of boron ions 106 Figure 4-15 Ion profiles in recording media target obtained by TRIM for argon

implantation in (a) RL and (b) SUL 107

of 1011 and 1016 ions/cm2 115 Figure 4-22 XPS depth profiles of samples implanted with argon in SUL with the doses of 1011 and 1016 ions/cm2 116 Figure 4-23 Cross-section TEM of samples implanted with argon in RL with the doses of 1011 and 1016 ions/cm2 117 Figure 4-24 Plan-view TEM on as-deposited samples and samples implanted with argon at 1011 and 1016 ions/cm2 doses 118 Figure 4-25 SRIM-2008 simulation of nuclear and electronic stopping powers of argon in recording media target with respect to implantation energy 119 Figure 4-26 Damage profiles obtained from TRIM calculation for argon implantation into RL and SUL 121 Figure 4-27 Coercivity versus ions species at 1016 ions/cm2 when implantation is done

in the RL and SUL 122 Figure 4-28 The lateral range and straggle plotted as a function of energy of

implantation of argon ion 122 Figure 4-29 The lateral range and straggle plotted as a function of ion species

implanted 123 Figure 5-1 Schematic of granular media and BPM for 10 Tbits/in2 126 Figure 5-2 Schematic of magnetic islands of (a) ideal BPM and BPM fabricated by ion implantation with (b) minimal lateral straggle and (c) very high lateral straggle 127 Figure 5-3 Plan-view TEM of conventional PMR showing grain boundary ~1-2 nm 128

List of Figures

Figure 5-4 Schematic of media layer structure 130 Figure 5-5 Ion profile in the recording media target obtained by TRIM for carbon ions (similar profiles for other ions as well) 130 Figure 5-6 Kerr loops on the sample implanted with various doses and implanted species – 4He+, 12C+, 14N+, 40Ar+, 59Co+ and 121Sb+ 132 Figure 5-7 Switching field distribution (SFD) plotted as a function of various ion fluences 134 Figure 5-8 Coercivity plotted as a function of fluence for all the implanted species – 4He+, 12C+, 14N+, 40Ar+, 59Co+ and 121Sb+ 135

Figure 5-9 M s plotted as a function of fluence for all the implanted species – 4He+, 12C+, 14N+, 40Ar+, 59Co+ and 121Sb+ 136 Figure 5-10 (a) SIMS depth profile of the as-deposited sample (b) The change in Mswith etching time (related to the etched depth) 138 Figure 5-11 FORC contours for samples implanted with (a) 4He+and (b) 121Sb+at various doses 140 Figure 5-12 Hu plotted as a function of fluence for all the implanted species – 4He+, 12C+, 14N+, 40Ar+, 59Co+ and 121Sb+ 141 Figure 5-13 XRD θ-2θ scan plotted as a function of fluence for all the implanted species – 4He+, 12C+, 14N+, 40Ar+, 59Co+ and 121Sb+ 142 Figure 5-14 The lateral range and straggle as a function of ion species 144 Figure 5-15 Correlation between straggle and ΔHu as measured by FORC curves 145 Figure 5-16 Schematic of straggle/movement of ions in (a) light ion (b) heavy ion implantation with time 147 Figure 5-17 Moment calculated from first principle calculations on Co-18 cluster and doped cluster 149 Figure 6-1 Alloy composition of the deposited FePt film 151 Figure 6-2 Schematic diagram of FePt media stack 152 Figure 6-3 XRD θ-2θ plot on CrRu film deposited at power varying between 150, 200 and 250W 153 Figure 6-4 (a) Out-of-plane and in-plane coercivities (b) XRD θ-2θ plot of the FePt media stack 154 Figure 6-5 (a) Out-of-plane and (b) In-plane hysteresis loops from the FePt media with MgO deposition at temperatures of 200 ºC and 400 ºC 155

List of Figures

Figure 6-6 (a) Out-of-plane and (b) In-plane hysteresis loops of the FePt media with CrRu deposited at 1.5 mTorr and MgO and FePt layers deposited at either 3 mTorr or 1.5 mTorr of Ar pressure 156 Figure 6-7 (a) Out-of-plane s (b) In-plane hysteresis loops for FePt samples implanted with 121Sb+ ions at various dose 159 Figure 6-8 Dependence of Ms as a function of various ion fluences 160 Figure 6-9 (a) Out-of-plane s (b) In-plane hysteresis loops for FePt samples implanted with 4He+ ions at various dose 161 Figure 6-10 Ta metal deposited on FePt media samples with thicknesses of 20, 30 and

45 nm 162 Figure 6-11 Cross-section SEM of FePt media sample with 45 nm Ta when etched for

1 min 30 sec 163 Figure 6-12 Ta metal on FePt media samples with thicknesses of 20, 30 and 45 nm etched in CF4 for 1 min 30 sec to obtain Ta pillars of ~20 nm 164 Figure 6-13 (a) Out of Plane and (b) In-plane hysteresis loops on unpatterned and patterned samples of different pillar heights implanted with a dose of 5×1016 ions/cm2 165 Figure 6-14 Dependence of Ms for various types of patterned samples 167 Figure 6-15 θ-2θ scan on unpatterned samples implanted with various doses of

121Sb+ 168 Figure 6-16 AFM and MFM of the patterned sample implanted with Sb+ and He+ at a dose of 5×1016 ion/cm2 169 Figure 7-1 Schematic of media layer structure 173 Figure 7-2 Schematic of ion profile required to fabricate graded media by means of ion implantation 174 Figure 7-3 Ion profiles obtained from TRIM program for various ions – 14N+, 16O+and 59Co+ 175 Figure 7-4 Kerr loops obtained on samples implanted with nitrogen (14N+), oxygen (16O+) and cobalt (59Co+) 177 Figure 7-5 Coercivity plotted as a function of ion fluence for samples implanted with nitrogen, oxygen and cobalt 179 Figure 7-6 Switching field distribution plotted as a function of ion fluence for samples implanted with nitrogen, oxygen and cobalt 180 Figure 7-7 MFM on samples in remanent state as the nucleation field of as-deposited samples and samples implanted with nitrogen, oxygen and cobalt 181

List of Figures

Figure 7-8 (a) SF and (b) H 0 plotted as a function of dose for nitrogen, oxygen, and cobalt ions 183 Figure 7-9 SF is plotted as a function of H0 184 Figure 7-10 XPS depth profiles on as-deposited sample and samples implanted with oxygen, nitrogen, and cobalt ions at high doses 186 Figure 7-11 XRD θ-2θ scans of samples implanted with nitrogen, oxygen and cobalt with the doses from 1015 to 1016 ions/cm2 187 Figure 8-1 Steps in fabricating graded patterned media by means of ion implantation 194

List of Publications, Patents and Conferences

List of Publications, Patents and Conferences

Publications in Peer-reviewed journals

1 S Kundu, N Gaur, M S M Saifullah, H Yang and C S Bhatia, ‘Spacer-less,

decoupled granular L10 FePt magnetic media using Ar–He sputtering gas’, Journal

of Applied Physics, 112, 113916 (2012)

2 N Gaur, S N Piramanayagam, S L Maurer, S E Steen, H Yang and C S

Bhatia, ‘First order reversal curve investigations on the effect of ion implantation

in magnetic media’, IEEE Transactions on Magnetics, 48, 2753 (2012)

3 S Kundu, R Ganesan, N Gaur, M S M Saifullah, H Hussain, H Yang and C

S Bhatia, ‘Effect of angstrom-scale surface roughness on the self-assembly of a block copolymer’, Scientific Reports, 2, 617 (2012)

4 N Gaur, K K M Pandey, S L Maurer, S N Piramanayagam, R W Nunes, H

Yang and C S Bhatia, ‘Magnetic and structural properties of CoCrPt–SiO2-based graded media prepared by ion implantation’, Journal of Applied Physics, 110,

083917 (2011)

5 N Gaur, S N Piramanayagam, S L Maurer, R W Nunes, S Steen, H Yang

and C S Bhatia, ‘Ion implantation induced modification of structural and magnetic properties of perpendicular media’, Journal of Physics D: Appl Phys.,

44, 365001 (2011)

6 K.K.M Pandey, N Gaur and C S Bhatia, ‘Interface mediated control of

microstructure and magnetic properties of FePt–C thin films’, Journal of Magnetism and Magnetic Materials, 323, 2658 (2011)

List of Publications, Patents and Conferences

Patents

1 C S Bhatia, K K M Pandey, N Gaur, S L Maurer and R W Nunes ‘Method

of Fabricating Graded Media’ (PCT/SG2012/000028) with Intellectual Property

Office of Singapore (2012)

Conferences

1 International Magnetics Conference (INTERMAG), Vancouver, Canada (2012)

Oral: ‘First Order Reversal Curve Investigations on the Effect of Ion Implantation

in Magnetic Media’, N Gaur, S N Piramanayagam, S L Maurer, S E Steen,

H Yang and C S Bhatia -Shortlisted as one of the six finalists for the Student

Best Presentation Award in INTERMAG conference (2012)

2 11th Joint MMM-Intermag, Washington DC, USA (2010)

Poster: ‘Effect of Ion-Implantation on Perpendicular Anisotropy of Recording Media’, N Gaur, S L Maurer, R W Nunes, S N Piramanayagam and

C.S Bhatia

3 International Conference on Materials for Advanced Technologies (ICMAT), Singapore (2009)

Poster: ‘Ion-Implantation Studies on Perpendicular Media’, N Gaur, S L

Maurer, R W Nunes, S N Piramanayagam and C S Bhatia

B Magnetic field induction

μ0 Permeability of the magnetic material

Sj, Si Spins of two electrons i and j

Jij Exchange integral due to two spins S i and S j

Htotal Total write field

List of Symbols

H mag Magnetic field

x Fe(Pt) Atomic fraction of Fe(Pt)

yFe(Pt) Fraction of Fe(Pt) sites

γFe(Pt) Fraction of Fe(Pt) sites occupied by the correct atom

Z1 Atomic number of the projectile ion

M 1 Mass of the projectile ion

Z2 Atomic number of the target atoms

M2 Mass of the target atoms

k Lindhard correction factor

re Fresnel amplitude reflection coefficient

List of Symbols

H0 Intrinsic anisotropy

H gradient Gradient field applied to the sample

M(Hr, Ha) Magnetization measured at each field step, H a and H r

Δf Frequency shift due to magnetic force

θ Angle between incident beam with respect to the film surface

d Spacing in the crystal lattice

E b Binding energy of electrons

Ekin Kinetic energy of ejected electrons

β Semi-angle of collection of the magnifying lens

δ Smallest distance that can be resolved

r Position coordinate

List of Symbols

V ext (r) External potential (nuclear potential)

Exc Exchange correlation energy

N i Number of electrons over all the molecular orbitals

Φi(r) Electron wave function

RI Position vector of nucleus

εi Lagrange multipliers

V XC Exchange-correlation potential

List of Abbreviations

List of Abbreviations

HDDs Hard disk drives

RAMAC Random access method of accounting and control

Gbits/in2 Giga bits per square inch

LMR Longitudinal magnetic recording

PMR Perpendicular magnetic recording

HAMR Heat-assisted magnetic recording

MAMR Microwave-assisted magnetic recording media

ECC Exchange coupled composite

BPM Bit Patterned Media

kbits/in2 Kilo bits per square inch

SFD Switching field distribution

hcp Hexagonal close packed

SNR Signal-to-Noise ratio

AFC Antiferromagnetically coupled

SUL Soft underlayer

List of Abbreviations

Tbits/ in2 Tera bits per square inch

fct Face centered tetragonal

fcc Face centred cubic

GMR Giant magnetoresistance

ECC Exchange coupled composite

ESM Exchange spring media

S-W Stoner-Wohlfarth

EBL Electron beam lithography

NIL Nanoimprint lithography

HSQ Hydrogen silsesquioxane

SRIM Stopping and range of ions in matter

TRIM Transport of ions in matter

MOKE Magneto-optical Kerr effect

AGFM Alternating gradient field magnetometer

MFM Magnetic force microscopy

XRD X-ray diffraction

XPS X-ray photoelectron spectroscopy

TEM Transmission electron microscopy

List of Abbreviations

SEM Scanning electron microscopy

DFT Density functional theory

UHV Ultra high vacuum

BCA Binary collision approximation

LSS Lindhard, Scharff & Schiott

DUV Deep Ultraviolet lithography

UV-NIL UV nanoimprint lithography

IPS Intermediate polymer stamp

SFD Switching field distribution

FWHM Full width at half maximum

FORC First-order reversal curve

SPM Scanning probe microscopy

AC Alternating current

JCPDS Joint Committee on Powdered Diffraction Standard

List of Abbreviations

DFT Density functional theory

GGA Generalized gradient approximation

LDA Local density approximation

NLSD Nonlocal spin-density approximation

BLYP Becke, Lee, Yang, and Parr

Pa Pascal (1.013 × 106 dyn/cm2)

COC Carbon overcoat

keV Kilo electron volt

HRTEM High resolution TEM

List of Equations

List of Equations

Equation 2 1 Equation for the magnetic field produced by an

infinitesimal length of current-carrying wire at a distance, r

11

Equation 2 3 Equation defining magnetic induction, B 11 Equation 2 4 Equation for exchange energy between two spins 11 Equation 2 5 Equation for coercive squareness calculation (S*) 13 Equation 2 6 Equation for thermal stability of media bits 21 Equation 2 7 Equation for chemical ordering of L10 FePt films 25 Equation 2 8 Equation for total write field gradient for HAMR 29

Equation 3 1 Universal relationship for nuclear stopping, by

using differential scattering cross-sections

50

Equation 3 2 Calculation for electronic stopping 51

Equation 3 5 Periodic force exerted on the sample placed in

static field due to alternating gradient field

61

Equation 3 9 The condition for a diffraction (peak), Bragg’s

equation

69

Equation 3 10 Scherrer Formula for Crystallite size

measurement from Rocking curve

70

Equation 3 12 Rayleigh criterion in electron microscope 74 Equation 3 13 Total energy of a cluster which is a functional of 77

is growing at the rate of 70% per year and is expected to continue for the next few years This demand can be met if the areal density also grows at such a rate or if the production is increased

The world’s first-ever hard disk drive was random access method of accounting and control (RAMAC) introduced by IBM in 1956 for total storage capacity of 5 MB (megabytes) at a recording areal density of only 2 kbits/in2 [2] The HDDs have improved over time and compared to RAMAC, HDDs with a storage capacity of 4 TB (terabyte) are available and areal density has shot up to 700 Grits/in2 [3] HDDs are also used in portable devices like digital cameras, MP3 players, digital video recorders etc owing to the increase in areal density There has been a growth of 35 –

40 % in the areal density over the past decade, but it has now reduced due to various

Chapter 1 Introduction

challenges overcoming superparamagnetism [4] In HDD, the information is stored in

a magnetic layer and the recording technique is called magnetic recording

The idea of magnetic recording was conceived and described by Oberlin Smith in

1878, and later demonstrated conceptually by Valdemar Poulsen in 1898 [5] In magnetic recording, the poles emit a magnetic field which can be sensed to read information The poles in the magnetic recording media can be arranged and rearranged by using an electromagnet as a writer If a hard magnetic material is used, the information can be stored in a non-volatile manner RAMAC, which was based on this concept, was made of magnetic particulate media where magnetic particles were coated onto a rigid polymeric substrate As technology progressed, the particulate medium could not meet the demands of higher signal, higher coercivity, smoother surface and so on Therefore, recording technology based on thin film media was introduced

With the advent of thin film deposition techniques, longitudinal magnetic recording (LMR) technology has made significant progress In longitudinal recording, the magnetizations that lie parallel to the disk surface were used for storing information and the bits could be scaled down due to several understanding that has happened in the micromagnetics of media Grain segregation was achieved using dopants such as Ta, B to reduce the grain size and the transition noise Oriented media were used to reduce the dc erase noise However, even at an areal density of about 10 Gbits/in2, thin film media showed thermal fluctuation of bits Area densities as high as

130 Gbits/in2, were demonstrated [6] Nevertheless, some other approach was required to take over it on scaling the bit size beyond 130 Gbits/in2, where the growth

in longitudinal recording became stagnant

Chapter 1 Introduction

Perpendicular magnetic recording (PMR) was suggested as an alternative and came

in practical existence in 2006 In this technology, the magnetizations lie perpendicular

to the disk surface instead of parallel This technology is still being used and has further extended the areal density limits to 700 Gbits/in2

Beyond this areal density, the perpendicular media are threatened by a combination

of limits known as the magnetic recording trilemma, which includes the competing requirements of small grain size (for large signal-to-noise ratio), writability and thermal stability [7] When considering an optimum constant value of SNR, which is proportional to the number of grains per bit, the number of grains in the medium needs to be kept constant as well For higher areal density the bit size is supposed to

be scaled down, resulting in a small grain size to maintain SNR This would lead to thermal instability of grains based on superparamagnetism This instability can be overcome in two ways: either by increasing the magnetocrystalline anisotropy

constant (K u) or by increasing the volume of the magnetic unit of the recording material

Since the anisotropy constant and writing fields are proportional, one of the key

challenges for the realization of high K u materials based media for industrial application is to reduce the writing field [8] Such a challenge can be resolved by using different technologies such as heat-assisted magnetic recording (HAMR), microwave-assisted magnetic recording media (MAMR) [9],[10] and exchange coupled composite (ECC) media [11] or graded media [12] In HAMR, the magnetic recording media is temporarily and locally heated during the writing process, close to the Curie temperature, which reduces the magnetic anisotropy In ECC media scheme, recording media consist of magnetically hard and soft regions within each grain Here, soft regions assist the hard regions to reduce the switching field Furthermore, it has

Chapter 1 Introduction

been theoretically predicted and experimentally observed that multilayer media or more accurately the graded media in which anisotropy varies continuously along the film growth direction is more effective for switching field reduction than the bilayer ECC media [12],[13]

Bit patterned media (BPM) concept belongs to the approach of using high volume Here, each bit is stored on one isolated, lithographically formed single-domain magnetic island rather than a combination of few tens of grains like conventional recording media There are two approaches to fabricate BPM One comprises lithography and pattern transfer to the recording media followed by a filling and planarization process In another technique, instead of creating physical patterns magnetic patterns are created by means of ion implantation through mask [14], [15]

In this case, additional steps like filling and planarization are not required Also, the cost of manufacturing media at a mass scale can be cut down In this thesis, ion implantation is investigated as an alternative method for preparing the graded media and BPM as mentioned in the research objectives

Chapter 1 Introduction

1 Saturation magnetization is important as it is a measure of signal strength

detected from the region When implanting through the magnetic medium sample masked in certain areas, the ion species and energy should be chosen

such that no signal or zero M s value is obtained from the unmasked areas

2 For high areal density applications, such as 10 Tbits/in2, when the bit size reduces to 4 nm for a bit aspect ratio (BAR) of one, the bits or patterns need to

be well-defined To obtain such well-defined patterns when fabricating patterned media by means of ion implantation through mask, the ions need to

be localized only within the unmasked regions Any diffusion of ions beneath the mask will subsequently alter the magnetic properties of the magnetic patterned region This will consequently hinder the maximum achievable areal density Hence the lateral diffusion of ions in the recording medium needs to

be as low as possible

In the literature, studies have been solely focused on saturation magnetization and fabrication techniques of BPM of the feature sizes like 150 nm or more, which are nowhere near the areal densities of a Tbits/in2 Moreover, not many studies in the literature have shown lateral straggle being taken into consideration while fabricating patterned media Systematic studies on various parameters like fluence, energy and ion species types in order to gain a deeper understanding of ion implantation on magnetic media have also not been reported Therefore, in this thesis, the effects of ion implantation as a function of various parameters like fluence, depth of implantation and ion species have been studied to determine their influence on saturation magnetization and lateral straggle, and to make this process suitable for high-density nanostructures

Chapter 1 Introduction

Coercivity or anisotropy decreased with increasing ion fluence in magnetic media with a recording layer based on the Co-alloy system (CoCrPt-SiO2) The reduction in coercivity was most significant when the ions were implanted into the recording layer compared to implantation at a greater depth, which might cause more damage in the medium stack Furthermore, lateral straggle was observed to reduce with lowering energy or depth of implantation and increasing mass of ion

Saturation magnetization, M s, was seen to be independent of the mass of ion species and was found to reduce to zero for argon (Ar+) and antimony (Sb+) at different doses Furthermore, implantation has been studied in FePt, the future media for the fabrication of BPM by implanting ions into the recording layer in such a manner that maximum damage occurs in the recording layer (the magnetic property of the implanted region is damaged) only in selective regions which are unmasked It was observed that an optimum ion species is required to provide the

lowest lateral straggle and maximum reduction in saturation magnetization, M s, so that no signal is detected from the regions

All the above observations have been explained based on the ion profiles obtained from simulations and various experimental characterizations The

changes in saturation magnetization, M s, has been explained based on the possibility of modification of the electronic band structure of the magnetic material based on density functional calculations (DFT)

2 To investigate the graded media fabricated using ion implantation: Graded

medium is a way of increasing the writability of high magnetocrystalline anisotropy material wherein each grain comprises of continuously graded anisotropy from top to bottom Here, the soft magnetic region of grains helps in

Chapter 1 Introduction

switching the hard magnetic region at a much lower field Literature has shown various studies based on multilayer deposition of magnetic material with changing composition at certain length scales by means of doping However, as suggested

by Aharoni, the graded media concept will be useful in reducing the switching field by four times only when a continuous quadratic gradient is formed Thus the multilayer media does not fulfill the essence of the graded media concept As a new approach of fabricating graded media, ion implantation has been suggested This can be done by implanting ions into the recording layer in such a manner that the doping profile has its peak at the top surface of the recording layer and a gradual tail goes deep into the recording layer, hence creating a gradient in anisotropy across the recording layer The method developed to fabricate graded media in this work is particularly useful for resolving the current crisis of hard disk industries in pushing the areal density of commercial media beyond 1Tbits/in2, as well as being able to solve many issues which are associated with the conventional method of fabrication of graded structure using sputtering It has the advantage that the entire disk can be ion-implanted at the same time, and no issue related with throughput arises In this thesis, graded media have been successfully fabricated by ion implantation as suggested by the thermal stability measurements

1.3 Organization of Thesis

Chapter 1 gives a brief review of history of magnetic recording and some future technologies which provide the motivation and objectives of the work in this thesis Chapter 2 gives a detailed overview of conventional and the future media Bit