Nanomagnetism (eBook)

Ultrathin Films, Multilayers and Nanostructures

J.A.C. Bland, D.L. L. Mills (Herausgeber)

eBook Download: EPUB

2006 | 1. Auflage
348 Seiten
Elsevier Science (Verlag)
978-0-08-045717-8 (ISBN)

Infos

Nanoscience is of central importance in the physical and biological sciences and is now pervasive in technology. However nanomagnetism has a special role to play as magnetic properties depend uniquely on both dimensionality and lengthscales. Nanomagnetism is already central to data storage, sensor and device technologies but is increasingly being used in the life sciences and medicine. This volume aims to introduce scientists, computer scientists, engineers and technologists from diverse fields to this fascinating and technologically important new branch of nanoscience. The volume should appeal to both the interested general reader but also to the researcher wishing to obtain an overview of this fast moving field.

The contributions come from acknowledged leaders in the field who each give authoritative accounts of key fundamental aspects of nanomagnetism to which they have themselves made a major contribution. After a brief introduction by the editors, Wu first surveys the fundamental properties of magnetic nanostructures. The interlayer exchange interactions within magnetic multilayer structures is next discussed by Stiles. Camley then discusses the static, dynamic and thermal properties of magnetic multilayers and nanostructures, followed by an account of the phenomenon of exchange anisotropy by Berkowitz and Kodama. This latter phenomenon is widely in current read head devices for example. The transport properties of nanostructures also are spectacular, and again underpin computer technology, as we see from the discussion of giant magnetoresistance (GMR) and tunnelling magnetoresistance (TMR) presented by Fert and his colleagues. Beyond GMR and TMR we look to the field of spintronics where new electronic devices are envisioned and for which quantum
computing may depend as discussed in the chapter by Flatte and Jonker.

The volume concludes with discussion of the recently discovered phenomenon of current induced switching of magnetization by Edwards and Mathon.

* Subject is in the forefront of nanoscience
* All Section authors are leading figures in this key field
* Presentations are accessible to non specialists, with focus on underlying fundamentals

Inhalt

Front cover 1
Title page 4
Copyright page 5
Table of contents 6
List of contributors 8
Series Preface: Contemporary Concepts of Condensed Matter Science 10
Volume Preface 12
1 The Field of Nanomagnetism 16
Introduction 17
The Ultrathin Ferromagnetic Film 19
Magnetic Nanostructures: Spin Configurations and Magnetization Reversal 28
Experimental Techniques 32
Notes 39
References 39
2 Fundamental Properties of Magnetic Nanostructures: A Survey 44
Introduction 44
Enhancement of Magnetization 45
Magnetic Anisotropy 50
Magnetic Ordering 52
Magnetic Transport 55
Summary and Future Outlook 57
Acknowledgments 58
References 58
3 Exchange Coupling in Magnetic Multilayers 66
Introduction 66
Quantum Well Model 69
Model for Transition Metal Ferromagnetism 69
Spin-Polarized Quantum Well States 71
Interlayer Exchange Coupling 73
Critical spanning vectors 74
Coupling strength 77
Torques and spin currents 80
Measurement of Interlayer Exchange Coupling 80
Growth and Disorder 80
Measurement Techniques 82
Biquadratic Coupling 85
Summary 86
References 88
4 Static, Dynamic, and Thermal Properties of Magnetic Multilayers and Nanostructures 92
Introduction 92
Theoretical Treatment of Magnetic Multilayers 93
Examples of Magnetic Multilayer Structures 96
The Dynamic Response of Magnetic Multilayers: Collective Spin Wave Modes 109
A Single Computational Method that Provides Static and Dynamic Results 118
Summary 126
Notes 126
References 126
5 Exchange Anisotropy 130
Introduction 130
Meiklejohn and Bean’s Research 131
Early Thin Film Research 134
Introduction to more Recent Research 135
Antiferromagnetic Systems 136
AFM Oxides 136
Metallic AFM 141
AFM Fluorides 143
Probing Spin Structures 143
Neutron Diffraction 143
Linear and Circular Magnetic Dichroism 145
X-ray Photoelectron Emission Microscopy 146
Mössbauer Spectroscopy 148
X-ray Absorption Spectroscopy 148
Some Comments on Spin-Probe Findings 150
Theory 150
Interfacial Uncompensated Spins (IUS) 150
Outlook and Current Work 155
Applications of Exchange Anisotropy 156
Acknowledgments 161
References 161
6 Spin Transport in Magnetic Multilayers and Tunnel Junctions 168
Introduction 168
Spin-Dependent Conduction in Ferromagnetic Metals 169
GMR: Experimental Survey 172
Models of GMR and Discussion 178
Physics of GMR 178
Models of CIP-GMR 182
Free-Electron Semi-Classical Models of CIP-GMR 182
Free-Electron Quantum-Mechanical Models of CIP-GMR 185
Models of CIP-GMR Based on Realistic Band Structure Calculations 187
Quantum Channelling in CIP-GMR 189
Models of Spin Accumulation and CPP-GMR 191
Concepts of Interface Resistance and Spin Accumulation 191
The Valet-Fert Model of CPP-GMR 194
Interpretation of Experimental Results on CPP-GMR 196
Physical Data from the Interpretation of CPP-GMR Experiments 201
Influence of Temperature on GMR 205
Angular Dependence of GMR 207
Basics of Spin-Dependent Tunnelling 208
Jullière’s Pioneering TMR Experiment and Model 212
TMR: Experimental Survey 213
TMR with Transition Metal Electrodes and Alumina Barrier 213
Search for Highly Spin-Polarized Ferromagnets 216
Dependence of the TMR on the Barrier and Electrode/Barrier Interface 217
TMR: Bias Voltage Dependence 220
TMR: Temperature Dependence 222
Spin Filtering by Ferromagnetic Barriers 223
Models of TMR 224
Free-Electron Models 224
Bonding at the Ferromagnet/Insulator Interface 225
First-Principle Calculations of TMR and Symmetry Effects 226
Models for Disordered Junctions 230
Applications of GMR and TMR 231
References 231
7 Electrical Spin Injection and Transport in Semiconductors 242
Introduction 242
Basic Requirements for Semiconductor Spintronics 244
Material Properties Influencing Spin Injection 245
Coupling between Light and Electron Spin, and Optical Spin Excitation 245
Spin Lifetimes 246
Spin Currents versus Charge Currents 249
Drift Effects on Spin Currents 253
Electrical Spin Injection into Semiconductors from Magnetic Materials 254
Detection of Spin-Polarized Carriers: The Spin-LED 254
Magnetic Semiconductors: Paramagnetic or Semimagnetic Materials 257
Magnetic Semiconductors: Ferromagnetic Materials 260
Ferromagnetic Metals 267
Conductivity mismatch 268
Tunnel barrier-based spin injection 271
Tailored Schottky tunnel barriers 271
Discrete layers as tunnel barriers 274
Temperature Dependence of Spin Polarization 275
Drift Effects on Spin Injection 277
Electrical Spin Injection into Semiconductors from Non-magnetic Materials 278
Summary and Outlook 280
Acknowledgments 281
References 281
8 Current-Induced Switching of Magnetization 288
Introduction 288
Phenomenological Treatment of Current-Induced Switching of Magnetization 292
Origin of Spin-Transfer Torque 304
General Principles 304
Discussion of Previous Work and SM Concepts 307
Generalized Landauer Method for Calculating the Spin Current 311
Keldysh Formalism for Fully Realistic Calculations of the Spin-Transfer Torque 316
References 324
Indices 328
Author Index 328
Subject Index 346

Textauszug

Volume Preface

D.L. Mills, J.A.C. Bland

Publisher Summary

This chapter discusses the exposition of new structures and the associated physics in an important area of modern nanoscience. This is the field of nanomagnetism, where very small-scale structures, such as ultrathin films of ferromagnetic material, often incorporated into superlattices or multilayer structures have been found to have magnetic and transport properties qualitatively and dramatically different than realized in bulk magnetic matter. Giant magnetoresistance (GMR) of magnetic multilayers has been exploited to increase the capacity of hard disks by over a factor of a hundred in a small number of years. Other exciting applications are envisioned, through the use of the systems and new concepts discussed in this chapter.

During the past two decades, we have witnessed marvelous advances in our ability to synthesize nanoscale structures of all sorts, as well as the development of novel experimental methods that allow us to explore their physical properties. This is exciting for two reasons. First, new forms of matter with no counterpart in nature have been fabricated, and these have unique physical properties not found in bulk materials. This is so because either a large fraction of their atomic constituents reside in surface or interface sites of low symmetry, or their physical size is so small they are completely quantum dominated. Second, we have now realized nanostructures that open new avenues for the development of very small devices. This has already had a remarkable, qualitative impact on technology, as we will see from remarks in the next paragraph. It is our view that soon nanoscience and the nanotechnology derived from it will have an impact on human affairs comparable to the industrial revolution, when combined with the submicron technology of the past 10–15 years. We can appreciate this from the remarkable influence of modern information technology on our lives. This volume is devoted to the exposition of new structures and the associated physics in an important area of modern nanoscience.

This is the field of nanomagnetism, where very small-scale structures such as ultrathin (few atomic layer) films of ferromagnetic material, often incorporated into superlattices or multilayer structures have been found to have magnetic and transport properties qualitatively and dramatically different than realized in bulk magnetic matter. More recently, we have seen a new generation of studies of the magnetism of patterned arrays ranging from micron-sized discs and wires down to dimers and single atoms adsorbed on substrates. Physicists have been intrigued by the new phenomena uncovered as these novel materials have been fabricated and their unique properties explored, materials scientists continue to present us with new structures, and by the time of this writing we have witnessed the enormous impact of ultra high-density magnetic data storage on computer technology. Here it is the remarkable phenomenon of giant magnetoresistance (GMR) of magnetic multilayers that has been exploited to increase the capacity of hard discs by over a factor of a hundred in a small number of years. Other exciting applications are envisioned, through the use of the systems and new concepts discussed in this volume. In this regard, we have in hand as well unique effects such as giant tunneling magnetoresistance (TMR), the phenomenon of the spin blockade, and other fascinating new effects that operate only in nanoscale magnetic systems. New methodologies developed by both theorists and experimentalists drive the field. Examples are the use of spin sensitive atomic force microscopy, and the development of large-scale computer simulations of real structures. The material in this volume is directed toward a broad audience of readers with backgrounds in condensed matter science who may not be experts in the field of nanomagnetism. It is our hope as well that the pedagogical nature of the discussions will also provide some experts with deeper insights in to the fundamental physics associated with areas of the field that have not been the focus of their own research. We comment next on the material discussed in the various chapters.

The first chapter, written by the undersigned volume editors, contains a broad overview. We discuss the unique and special aspects of the magnetism of ultrathin ferromagnetic films. Spin ordering is a long-ranged phenomenon, and the excitations which control both the response characteristics and thermodynamics of ultrathin films are influenced by long ranged couplings as well. Thus, as we shrink magnetic structures down to nanometer length scales, we find fundamental differences in all aspects of their physics. In addition, a large fraction of the moment bearing ions sit in interface or surface sites, with qualitative consequences for both their magnetic and chemical properties. Ultrathin films are the “building blocks” of the magnetic multilayers, spin valves and related structures that have been explored and discussed intensively in recent years. We also introduce materials with lateral structure (nanodot, nanodisc and nanowire arrays), and then turn our attention to the experimental methods, which have proved central to the elucidation of the properties of very small magnetic structures.

The nature of the magnetic moments found in ultrasmall structures can differ dramatically from their bulk counterparts, by virtue of the fact that a large fraction reside on surfaces, at interfaces as noted above. These also can be affected by the chemisorption of selected molecules. It follows that one realizes magnetic anisotropies one or two orders larger than that found in the bulk, and their strength and character are subject to design. Thus, we have spin engineering. R. Wu provides us with a broad survey of the ground state properties of diverse nanoscale magnetic structures. It is impressive to see the success of modern density functional theory in its ability to provide reliable quantitative accounts of the properties of these often-complex systems with low symmetry.

When ultrathin ferromagnetic films are assembled into multilayers or superlattices, weak interactions of exchange character act between the constituent films. These are mediated by the spin polarization induced in nonferromagnetic spacer layers inserted between them. These weak exchange couplings, tunable both in sign and magnitude by varying structural details, lead us to magnetic entities whose underlying structure can be manipulated by very modest or weak applied fields, in contrast to bulk materials whose magnetic ions are tightly coupled by very strong interatomic exchange. This key property allows us to fabricate new, artificial materials whose magnetic properties differ qualitatively from those of bulk materials, and which can be varied over a wide range by design. M.D. Stiles provides us with discussions of the physical origin and nature of these interfilm exchange couplings. Arrays of ultrathin ferromagnetic films coupled by the exchange interactions just discussed display a rich variety of spin structures, where the total (macroscopic) magnetic moment of each ultrathin film plays the role of a large, and consequently fully classical “spin.” R.E. Camley discusses examples of these structures, and the collective excitations they support. We have here the opportunity to develop new materials with microwave response tunable by the application of very modest magnetic fields.

When one wishes to exploit magnetic multilayers in devices, the signal detected has its origin in the rotation of the magnetization vector in one selected film relative to that in a neighboring film, and the resulting influence on properties of the structure such as its electrical resistance, as discussed in the next paragraph. A commonly used structure is the “spin valve,” which consists of two ferromagnetic films separated by a nonmagnetic spacer which gives rise to the weak interfilm interactions described in the previous paragraph. A question is then how one may use an applied magnetic field to rotate the magnetization of one of the two films, while the second remains pinned in place. A phenomenon referred to as “exchange bias,” discovered many decades ago, allows one to selectively “pin” the magnetization of one layer. A.E. Berkowitz and R.H. Kodama introduce us to this central topic, whose origin is only very recently appreciated.

Transport properties of magnetic multilayers have excited many researchers, since A. Fert and his colleagues reported the discovery of the astonishing phenomenon of GMR in 1988. In parallel with their work, P. Grunberg and collaborators observed this phenomenon as well at very close to the same time. The origin of GMR has stimulated efforts by many experimentalists and theorists for some years now, since it is not only a spectacular physical phenomenon, but has provided us with the basis for ultra high-density magnetic storage and its enormous impact on computer technology. A. Fert, A. Barthelemy and F. Petroff present us with a discussion of this most important phenomenon in their chapter, and cover tunneling magnetoresistance as well.

Physicists, materials scientists and engineers actively discuss a new field called “spintronics,” wherein it is the spin of the electron rather than its charge that is exploited and manipulated. This has led to the exploration of new physics that must be understood before the field can prosper. As we have known for decades, we can inject electrons from semiconductors into metals. But if the electrons are highly spin polarized, is the spin polarization preserved or destroyed in the injection process, and if it is the latter how do we make...

Produkt-Details

• Erscheint lt. Verlag: 27.3.2006
• Sprache: englisch
• Themenwelt: Schulbuch / Wörterbuch
• Themenwelt: Naturwissenschaften ► Physik / Astronomie ► Elektrodynamik
• Themenwelt: Naturwissenschaften ► Physik / Astronomie ► Festkörperphysik
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• Themenwelt: Technik ► Maschinenbau
• ISBN-10: 0-08-045717-7 / 0080457177
• ISBN-13: 978-0-08-045717-8 / 9780080457178