From bulk to nano : the many sides of magnetism

The inspiration for this book can be traced back many years to two major works that in?uenced the author's outlook on applied physics: FerromagnetismusbyR. Becker,W. D¨ oring (Springer, Berlin 1939), and Ferromagnetism by R. M. Bozorth (IEEE Press, New York 1951). The former work is a collection of lectures held in the 1930s for 'technicians' attending a technical college. The German language in which the work was originally written was extremely convenient for the author of this present book, as it was for a long time the only comfortable technical language in an English speaking environment. Later on, upon encountering the work by Bozorth, it was a relief to see the clarity and eloquence of the subjects presented in English, despite the impressive thickness of the book. Bozorth's work still constitutes a practical review for anyone in a multidisciplinary industry who comes across the various manifestations of magnetism. The popularity of both works is so enduring that they are regarded as highly academic, and yet extremely readable, a reference in their own right, still attracting many readers these days in industry and academia. The ?eld of magnetism progressed immensely in the twentieth century, and shows no signs of slowing down in the present one. It has become so vast that it is quite often viewed only in its parts, rather than as a whole. In today'smyriadofapplications,especiallyonananoscale,andtheirchangeable implications mostly on a macroscale, it often seems that di?erent aspects of reported work on magnetism are scattered and unrelated.

Author Notes

Dr. Stefanita is a physicist/materials scientist who received her Ph. D. degree in Physics (Magnetic Nondestructive Testing and Materials Characterization) from Queen's University, Kingston, Ontario, Canada in 1999. Her Ph.D. thesis is entitled "Surface Magnetic Barkhausen Noise Response To Plastic Yield of Steel". Previously, she received her Diploma of Engineer in Physics (Technological Physics) from the University of Bucharest, Romania in 1989 with the thesis entitled "Apparatus for Measuring Solar Radiation Using a Thin Film Transducer". Dr. Stefanita also has a 1984 Baccalaureate in Mathematics and Physics from the German School of Bucharest in Romania.

Dr. Stefanita has over 15 years collective experience in academia and industry with international exposure on 3 continents, and 5 countries. Her most recent research experience is in nanotechnology, in self-assembled metallic and semiconductor nanowires or nanodots, spintronics, magnetotransport, Hall effect, and infrared absorption and transmission, acquired during her work at Virginia Commonwealth University in Richmond, Virginia, USA.

Her previous work in magnetism in the 1990s at Queen's University involves magnetic behavior of plastically deformed steel, microyielding phenomena, cold rolling effects on magnetic properties; magnetic nondestructive testing techniques for detecting defects in steel components. She has also developed a prototype for a medical diagnostics apparatus based on a thin film interference filter, at the University of Alberta in Edmonton, Alberta, Canada.

Dr. Stefanita's previous work as a materials scientist in South Africa, and in Germany and Romania is in the areas of failure analysis, scanning electron microscopy, energy dispersive X- ray analysis, atomic emission spectroscopy and import of chemicals and raw materials.

Dr. Stefanita is a co-founder and Senior Partner with NanoDotTek based in the Bostonarea, Massachusetts, USA. This company is dedicated to promoting nanotechnology, and allied areas such as non-destructive testing (the "NDT" in NanoDotTek) for engineering systems. With respect to the latter Dr. Stefanita is presently engaged in developing a lossy dielectric with a prescribed fractional impedance. This is in furtherance of improving fractional order control systems and pulse shaping for wireless broadband communications. This work is in collaboration with engineers in California and Mexico.

Dr. Stefanita also has experience in teaching. Her lectures have covered solid state devices, electromagnetics, and mechanics of deformables.

Dr. Stefanita is a regular referee for nanotechnology papers, as well as a published author of journal papers, conference publications, and internal reports. The latter reports are in many cases in relation to contract-based research. Dr. Stefanita's most recent journal paper in spintronics appeared in Nature Nanotechnology earlier this year.

Finally, note that Dr. Stefanita is a licensed Professional Engineer in the Province of Alberta, Canada.

Symbols	p. XVII
1 Introduction	p. 1
1.1 Review of Certain Historic Magnetic Concepts	p. 2
1.1.1 Magnetic Susceptibility	p. 2
1.1.2 Classification of Magnetic Materials	p. 3
1.1.3 The Concept of Magnetic Pole	p. 5
1.1.4 Magnetic Dipoles	p. 6
1.2 Origins of Magnetism on an Atomic Scale	p. 6
1.2.1 The Importance of Angular Momentum	p. 7
1.2.2 Magnetic Moment of a Sample of N Atoms	p. 8
1.2.3 Crystal Field vs. Spin-Orbit Coupling	p. 9
1.2.4 Magnetocrystalline Anisotropy	p. 10
1.2.5 Magnetostriction	p. 10
1.3 Structure-Dependent Micromagnetism	p. 11
1.3.1 Division into Magnetic Domains	p. 12
1.3.2 Formation of Domain Walls	p. 12
1.3.3 Types of Domain Walls	p. 13
1.3.4 Significance of Magnetic Domains and Domain Walls	p. 14
1.4 Towards Technological Advancements	p. 15
1.4.1 Design of New Magnetic Materials	p. 15
1.4.2 Magnetic Quantum Dots	p. 15
References	p. 16
2 Barkhausen Noise as a Magnetic Nondestructive Testing Technique	p. 19
2.1 Introduction	p. 19
2.2 A Basic Definition of Magnetic Barkhausen Noise	p. 20
2.2.1 Types of MBN Experiments	p. 20
2.2.2 Where does MBN Originate?	p. 21
2.2.3 Formation of Magnetic Domains	p. 22
2.2.4 MBN and 180[degree] Domain Walls	p. 23
2.3 Stress Effects	p. 24
2.3.1 Elastic Stress Causes Changes in Bulk Magnetization	p. 24
2.3.2 Magnetic Domains Respond to Stress	p. 24
2.3.3 Magnetic Anisotropy and MBN	p. 25
2.3.4 Some Parameters Used in MBN Analysis	p. 25
2.3.5 Elastic Stress Influences on Magnetic Anisotropy	p. 27
2.3.6 Plastic Deformation and Magnetic Anisotropy	p. 27
2.3.7 Effects of Residual Stresses	p. 28
2.3.8 Influence of Dislocations	p. 30
2.3.9 Selective Wall Energy Increases at Pinning Sites	p. 30
2.3.10 Roll Magnetic Anisotropy	p. 31
2.3.11 Limits in MBN Signal Increase with Plastic Stress	p. 32
2.4 Effects of Microstructure on MBN	p. 33
2.4.1 Variations in Grain Size	p. 33
2.4.2 Compositional and Phase Influences	p. 34
2.4.3 MBN Behavior in Different Materials	p. 34
2.5 Competitiveness of MBN in Nondestructive Evaluation	p. 36
2.5.1 Usefulness of MBN for MFL	p. 36
2.5.2 Need for Calibration of MBN as NDT	p. 37
References	p. 38
3 Combined Phenomena in Novel Materials	p. 41
3.1 The Interest in Magneto-optical Media	p. 41
3.1.1 Conventional vs. Continuous Media	p. 42
3.1.2 The Basis of Magneto-optical Effects	p. 43
3.1.3 Composite Films Used in Magneto-optical Recording	p. 43
3.1.4 Magnetic Recording and Optical Readout	p. 44
3.1.5 Quality of Magnetic Recording	p. 44
3.1.6 Overcoming Noise Problems	p. 45
3.1.7 The MO Sony Disk	p. 46
3.1.8 Magnetically Induced Super Resolution	p. 47
3.1.9 Nondestructive Optical Readout	p. 47
3.1.10 Double and Multilayer MO Disks	p. 48
3.1.11 Domain Wall Displacement Detection	p. 49
3.1.12 Magnetic Bubble Domains	p. 50
3.1.13 Generation of a Bubble Bit of Memory	p. 50
3.1.14 Driving Force for Wall Displacement	p. 50
3.2 Magnetoelectric Materials	p. 51
3.2.1 The Magnetoelectric Effect	p. 51
3.2.2 Oxides, Boracites, Phosphates, etc.	p. 52
3.2.3 Layered Composite Materials	p. 52
3.2.4 Product, Sum and Combination Properties	p. 53
3.2.5 PZT and Magnetostrictive Materials	p. 53
3.2.6 Avoiding Ferrites	p. 54
3.2.7 Undesired Effects of Sintering	p. 54
3.2.8 Variations in Signal Due to Mechanical Coupling	p. 55
3.2.9 Laminated Composites	p. 55
3.2.10 Voltage Coefficient [alpha]	p. 56
3.2.11 Obtaining Improved Voltage Coefficients	p. 57
3.2.12 ME and Nanostructures	p. 57
3.2.13 Effects on a Nanoscale	p. 58
3.2.14 Residual Stresses and Strains in Nanostructures	p. 60
3.2.15 Multiferroics	p. 61
3.2.16 Using Terfenol-D	p. 61
3.2.17 Multiferroic Transformers	p. 61
3.2.18 Multiferroic Sensors for Vortex Magnetic Fields	p. 63
3.2.19 Enhancing Multiferroicity through Material Design	p. 63
3.2.20 Identifying Multiferroics	p. 64
References	p. 64
4 Magnetoresistance and Spin Valves	p. 71
4.1 Introduction	p. 71
4.2 A Simple Way of Quantifying Magnetoresistance	p. 72
4.3 What is Responsible for GMR?	p. 72
4.4 Deskstar 16 GP	p. 73
4.5 "Spin-down" vs. "Spin-up" Scattering: Magnetic Impurities	p. 73
4.6 Fabrication of GMR Multilayers: Thin Films and Nanostructures	p. 74
4.7 Spin Valves	p. 75
4.8 The Role of Exchange Bias	p. 75
4.9 Ni-Fe Alloys	p. 76
4.10 Ternary Alloys	p. 77
4.11 Ni-Fe Alloys with Higher Fe Content	p. 77
4.12 Basic Principles of Storing Information Magnetically	p. 78
4.13 Materials for spin valve Sensors	p. 80
4.14 The Need for Proper Sensor Design	p. 81
4.15 Magnetic Tunnel Junctions	p. 82
4.16 Anisotropic Magnetoresistive Sensors	p. 82
4.17 Extraordinary Magnetoresistance	p. 83
4.18 GMR Sensors with CPP Geometry	p. 83
4.19 Dual Spin Valves	p. 84
4.20 Some GMR Multilayer Material Combinations	p. 85
4.21 Ferromagnetic/Nonmagnetic Interfaces	p. 86
4.22 The Nonmagnetic Spacer	p. 86
4.23 Magnetic Tunneling	p. 87
4.24 The Magnetic Tunnel Transistor	p. 87
4.25 Some Special Types of Ferromagnets	p. 88
4.26 Colossal Magnetoresistance	p. 89
4.27 CPP Geometry Preferred in Sensors	p. 90
4.28 Spin Valves in Commercial Applications	p. 91
References	p. 93
5 Some Basic Spintronics Concepts	p. 99
5.1 Encoding Information: Emergence of Spintronics	p. 99
5.2 Spin Injection	p. 100
5.2.1 Minority vs. Majority Spin Carriers	p. 100
5.2.2 Spin Injection Rate	p. 100
5.2.3 Spin Polarization and Spin Transfer	p. 101
5.2.4 CPP vs. CIP Geometry	p. 102
5.2.5 Spin Accumulation, Spin Relaxation, and Spin Diffusion Length	p. 103
5.2.6 No Spin Accumulation in CIP Geometry	p. 103
5.2.7 Half-Metallic Ferromagnets	p. 104
5.2.8 Some Epitaxial Growth Techniques	p. 104
5.2.9 ME Materials and Spintronics	p. 105
5.2.10 Spontaneous Band Splitting	p. 106
5.2.11 Spin Valves	p. 106
5.2.12 Poor Injection Efficiency	p. 107
5.2.13 Additional Layer Between Ferromagnet and Spacer	p. 107
5.2.14 III-V Magnetic Semiconductors	p. 107
5.2.15 Obtaining Spin-Polarized Magnetic Semiconductors	p. 108
5.2.16 Light vs. Electric-Field-Induced Carrier Enhancement	p. 108
5.2.17 Giant Planar Hall Effect	p. 109
5.2.18 Maintaining Spin Polarization	p. 109
5.2.19 The Future of Spin Injection	p. 111
5.3 Control of Spin Transport	p. 111
5.3.1 The Need for Long Spin Relaxation Times	p. 111
5.3.2 Organic Semiconductor Spacers	p. 112
5.3.3 Spin Transport in Organic Semiconductor Spin Valves	p. 113
5.3.4 Nanoscale Effects at Ferromagnet/Organic Semiconductor Interface	p. 113
5.3.5 Carbon Nanotubes	p. 114
5.3.6 GMR vs. TMR	p. 114
5.3.7 The Parallel Resistor Model	p. 116
5.3.8 Effects at Adjacent Interfaces in GMR	p. 116
5.3.9 Scattering at Bloch Walls	p. 117
5.3.10 Importance of Materials Choice	p. 118
5.3.11 Spin Control Through Electric Fields	p. 118
5.4 Spin Selective Detection	p. 119
5.4.1 Detecting Single Spins	p. 119
5.4.2 Detecting Spin Polarization of an Ensemble of Spins	p. 119
5.4.3 The Datta and Das Spin Field Effect Transistor	p. 121
5.4.4 The Future of Spintronics Devices	p. 121
References	p. 121
6 Trends in Magnetic Recording Media	p. 129
6.1 The Popularity of Magnetic Tapes	p. 129
6.1.1 Quality of Magnetic Tapes	p. 130
6.1.2 The Pressure for Higher Capacity Magnetic Tapes	p. 131
6.1.3 Constraints Imposed by Thermal Stability	p. 131
6.1.4 Forming a Bit	p. 132
6.1.5 Influence of Magnetic Anisotropy	p. 133
6.1.6 Choice of Materials	p. 133
6.2 Bit Patterned Magnetic Media	p. 134
6.2.1 Bit-Cells	p. 134
6.2.2 Minimizing Errors	p. 135
6.2.3 Some Disadvantages of Patterned Bits	p. 136
6.2.4 Solutions for Patterning Bits Efficiently	p. 136
6.2.5 Materials for Bit Patterned Magnetic Media	p. 137
6.2.6 Maintaining Competitiveness	p. 138
6.2.7 Going Nano and Beyond	p. 138
6.3 Self-assembly and Magnetic Media	p. 139
6.3.1 Alumina Templates	p. 139
6.3.2 Guided Self-assembly as a Solution to Long-Range Ordering	p. 142
6.3.3 Chemically vs. Topographically Guided Self-assembly	p. 144
6.3.4 Biological Self-assembled Templates	p. 144
6.3.5 The Versatility of Block Copolymers	p. 144
6.3.6 Inorganic Templates May Still Be Competitive	p. 145
6.4 Present Alternatives for Discrete Media Production	p. 145
6.4.1 Patterning with Stampers and Masks	p. 145
6.4.2 Cleanliness Concerns	p. 146
6.4.3 Obtaining High Aspect Ratios	p. 147
6.4.4 Types of Nanopatterning Processes	p. 147
6.4.5 Emerging Fabrication Techniques	p. 148
6.4.6 Discrete Track Media	p. 149
6.4.7 Identifying Track Locations	p. 149
6.4.8 Parallel Writing of Data	p. 150
6.4.9 Magnetic Lithography for Mass Data Replication	p. 150
6.4.10 Magnetic Disk Drives vs. Semiconductor Processing	p. 151
6.4.11 Head Performance	p. 151
6.4.12 Spin Valves and Giant Magnetoresistive Heads	p. 152
6.4.13 Looking Back and into the Future	p. 152
References	p. 153
7 Concluding Remarks	p. 161
Reference	p. 161
Index	p. 163

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Table of Contents