Magnetism: Molecules to Materials III Edited by J.S Miller and M Drillon Copyright c 2002 Wiley-VCH Verlag GmbH ISBNs: 3-527-30302-2 (Hardback); 3-527-60014-0 (Electronic) Magnetism: Molecules to Materials III Edited by J S Miller and M Drillon Magnetism: Molecules to Materials III Edited by J.S Miller and M Drillon Copyright c 2002 Wiley-VCH Verlag GmbH ISBNs: 3-527-30302-2 (Hardback); 3-527-60014-0 (Electronic) Further Titles of Interest J S Miller and M Drillon (Eds.) Magnetism: Molecules to Materials Models and Experiments 2001 XVI, 437 pages Hardcover ISBN: 3-527-29772-3 J S Miller and M Drillon (Eds.) Magnetism: Molecules to Materials II Molecule-Based Materials 2001 XIV, 489 pages Hardcover ISBN: 3-527-30301-4 J H Fendler (Ed.) Nanoparticles and Nanostructured Films 1998 XX, 468 pages Hardcover ISBN: 3-527-29443-0 P Braunstein, L A Oro, and P R Raithby (Eds.) Metal Clusters in Chemistry 1999 XLVIII, 1798 pages ISBN: 3-527-29549-6 Magnetism: Molecules to Materials III Edited by J.S Miller and M Drillon Copyright c 2002 Wiley-VCH Verlag GmbH ISBNs: 3-527-30302-2 (Hardback); 3-527-60014-0 (Electronic) Magnetism: Molecules to Materials III Nanosized Magnetic Materials Edited by Joel S Miller and Marc Drillon Magnetism: Molecules to Materials III Edited by J.S Miller and M Drillon Copyright c 2002 Wiley-VCH Verlag GmbH ISBNs: 3-527-30302-2 (Hardback); 3-527-60014-0 (Electronic) Prof Dr Joel S Miller University of Utah 315 S 1400 E RM Dock Salt Lake City UT 84112-0850 USA Prof Dr Marc Drillon CNRS Inst de Physique et Chimie des Matériaux de Strasbourg 23 Rue du Loess 67037 Strasbourg Cedex France This book was carefully produced Nevertheless, editors, authors and publisher not warrant the information contained therein to be free of errors Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate Library of Congress Card No.: applied for A catalogue record for this book is available from the British Library Die Deutsche Bibliothek - CIP Cataloguing-in-Publication-Data A catalogue record for this publication is available from Die Deutsche Bibliothek ISBN 3-527-30302-2 © WILEY-VCH Verlag GmbH, Weinheim (Federal Republic of Germany) 2002 Printed on acid-free paper All rights reserved (including those of translation in other languages) No part of this book may be reproduced in any form - by photoprinting, microfilm, or any other means - nor transmitted or translated into machine language without written permission from the publishers Registered names, trademarks, etc used in this book, even when not specifically marked as such, are not to be considered unprotected by law Composition: EDV-Beratung Frank Herweg, Leutershausen Printing: betz-druck GmbH, Darmstadt Bookbinding: Wilh Osswald + Co KG, Neustadt Printed in the Federal Republic of Germany Magnetism: Molecules to Materials III Edited by J.S Miller and M Drillon Copyright c 2002 Wiley-VCH Verlag GmbH ISBNs: 3-527-30302-2 (Hardback); 3-527-60014-0 (Electronic) Preface The development, characterization, and technological exploitation of new materials, particularly as components in ‘smart’ systems, are key challenges for chemistry and physics in the next millennium New substances and composites including nanostructured materials are envisioned for innumerable areas including magnets for the communication and information sectors of our economy Magnets are already an important component of the economy with worldwide sales of approximately $30 billion, twice those of semiconductors Hence, research groups worldwide are targeting the preparation and study of new magnets especially in combination with other technologically important features, e g., electrical and optical properties In the past few years our understanding of magnetic materials, thought to be mature, has enjoyed a renaissance as it has been expanded by contributions from many diverse areas of science and engineering These include (i) the discovery of bulk ferro- and ferrimagnets based on organic/molecular components with critical temperature exceeding room temperature, (ii) the discovery that clusters in high, but not necessarily the highest, spin states because of a large magnetic anisotropy or zero field splitting have a significant relaxation barrier that traps magnetic flux enabling a single molecule/ion (cluster) to act as a magnet at low temperature; (iii) the discovery of materials with large negative magnetization; (iv) spin-crossover materials with large hysteretic effects above room temperature; (v) photomagnetic and (vi) electrochemical modulation of the magnetic behavior; (vii) the Haldane conjecture and its experimental realization; (viii) quantum tunneling of magnetization in high spin organic molecules; (ix) giant and colossal magnetoresistance effects observed for 3-D network solids; (x) the realization of nanosized materials, such as self-organized metal-based clusters, dots and wires; (xi) the development of metallic multilayers and (xii) spin electronics for the applications This important contribution to magnetism and more importantly to science in general will lead us into the next millennium Documentation of the status of research, ever since William Gilbert’s de Magnete in 1600, has provided the foundation for future discoveries to thrive As one millennium ends and another beckons, the time is appropriate to pool our growing knowledge and assess many aspects of magnetism This series, entitled Magnetism: Molecules to Materials, provides a forum for comprehensive yet critical reviews on many aspects of magnetism which are on the forefront of science today This third volume reviews the current state of the art in the field of “nanosized materials”, including both metallic and organometallic compounds, experimental as well as theoretical points of view Joel S Miller Salt Lake City, USA Marc Drillon Strasbourg, France Magnetism: Molecules to Materials III Edited by J.S Miller and M Drillon Copyright c 2002 Wiley-VCH Verlag GmbH ISBNs: 3-527-30302-2 (Hardback); 3-527-60014-0 (Electronic) Contents Nanosized Magnetic Materials 1.1 Introduction 1.2 Synthesis 1.2.1 Inert Gas Condensation 1.2.2 Water-in-oil Microemulsion Method 1.2.3 Organic/Polymeric Precursor Method 1.2.4 Sonochemical Synthesis 1.2.5 Hydrothermal Synthesis 1.2.6 Pyrolysis 1.2.7 Arc Discharge Technique 1.2.8 Electrodeposition 1.2.9 Mechanical Alloying 1.2.10 Matrix-mediated Synthesis 1.3 Structure-Property Overview 1.3.1 Quantum Tunneling 1.3.2 Anisotropy 1.3.3 Analytical Instrumentation 1.4 Theory and Modeling 1.4.1 Single-domain Particles 1.4.2 Modeling 1.5 Applications 1.5.1 Magneto-optical Recording 1.5.2 Magnetic Sensors and Giant Magnetoresistance 1.5.3 High-density Magnetic Memory 1.5.4 Optically Transparent Materials 1.5.5 Soft Ferrites 1.5.6 Nanocomposite Magnets 1.5.7 Magnetic Refrigerant 1.5.8 High-T C Superconductor 1.5.9 Ferrofluids 1.5.10 Biological Applications References 1 10 11 12 13 15 16 18 19 20 21 21 22 23 23 25 25 27 27 28 28 29 29 30 31 VIII Contents Magnetism and Magnetotransport Properties of Transition Metal Zintl Isotypes 2.1 Introduction 2.2 Structure 2.3 Magnetism 2.3.1 Alkaline Earth Compounds 2.3.2 High-temperature Paramagnetic Susceptibility 2.3.3 Ytterbium Compounds 2.3.4 Europium Compounds 2.4 Heat Capacity 2.5 Magnetotransport 2.5.1 Alkaline Earth and Ytterbium Compounds 2.5.2 Resistivity and Magnetoresistance of the Europium Compounds 2.5.3 Comparison with other Magnetoresistive Materials 2.6 Summary and Outlook References 37 37 38 41 43 43 48 49 53 54 54 57 60 61 61 63 63 Magnetic Properties of Large Clusters 3.1 Introduction 3.2 Calculation of the Energy Levels and Experimental Confirmations 3.2.1 Calculations 3.2.2 Inelastic Neutron Scattering 3.2.3 Polarized Neutron Scattering 3.2.4 High-field Magnetization 3.3 Magnetic Measurements 3.3.1 Introduction 3.3.2 AC Susceptibility Measurements 3.3.3 Cantilever Magnetometry 3.3.4 MicroSQUID Arrays 3.4 Magnetic Resonance Techniques 3.4.1 Introduction 3.4.2 HF-EPR 3.4.3 Zero-field EPR 3.4.4 Low-frequency EPR 3.4.5 NMR 3.4.6 µSR 3.5 Control of the Nature of the Ground State and of the Anisotropy 3.6 Fe8 – A Case History 3.7 Conclusions and Outlook References 65 65 68 70 72 76 76 77 79 83 85 85 85 87 88 89 94 97 99 103 104 IX Contents Quantum Tunneling of Magnetization in Molecular Complexes with Large Spins – Effect of the Environment 4.1 Introduction 4.2 Mn12 -acetate 4.2.1 Experimental Results 4.2.2 Basic Model 4.3 Fe8 Octanuclear Iron(III) Complexes 4.3.1 Experimental Results 4.3.2 Basic Model 4.4 Environmental Effects 4.4.1 Experimental Picture 4.4.2 Thermally Assisted Tunneling Regime 4.4.3 Ground-state Tunneling References Studies of Quantum Relaxation and Quantum Coherence in Molecular Magnets by Means of Specific Heat Measurements 5.1 Introduction 5.2 Experimental Techniques 5.3 Theoretical Background 5.3.1 Spin-Hamiltonian for Molecular Magnets – Field-dependent Quantum Tunneling 5.3.2 Resonant Tunneling via Thermally Activated States 5.3.3 Master Equation – Calculation of 5.3.4 Calculation of Time-dependent Specific Heat and Susceptibility 5.4 Experimental Results and Discussion 5.4.1 Superparamagnetic Blocking in Zero Applied Field 5.4.2 Phonon-assisted Quantum Tunneling in Parallel Fields 5.4.3 Phonon-assisted Quantum Tunneling in Perpendicular Fields 5.4.4 Time-dependent Nuclear Specific Heat 5.4.5 Detection of the Tunnel Splitting for High Transverse Fields 5.5 Effect of Decoherence 5.6 Incoherent Tunneling and QC in Molecules with Half-integer Spin 5.7 Conclusions References Self-organized Clusters and Nanosize Islands on Metal Surfaces 6.1 Introduction 6.2 First Stage of Growth Kinetics 6.2.1 Island Density 6.2.2 Island Shapes 6.3 Growth Modes 109 109 110 110 116 126 126 130 137 138 145 154 165 169 169 172 174 174 178 182 185 186 187 190 193 197 199 202 202 206 208 211 211 212 212 214 216 X Contents 6.3.1 Thermodynamic Growth Criterion 6.3.2 Microscopic Model 6.3.3 Elastic and Structural Considerations 6.4 Organized Growth 6.4.1 Incommensurate Modulated Layers 6.4.2 Atomic-scale Template 6.4.3 Self Organization 6.4.4 Periodic Patterning by Stress Relaxation 6.4.5 Organization on Vicinal Surfaces 6.4.6 Low-temperature Growth 6.5 Magnetic Properties 6.5.1 Magnetism in Low-dimensional Systems 6.5.2 Anisotropy in Ferromagnetic Nanostructures 6.5.3 Magnetic Domains 6.5.4 Superparamagnetism 6.5.5 Dimensionality and Critical Phenomena 6.6 Magnetic Nanostructures – Experimental Results 6.6.1 Isolated Islands 6.6.2 Interacting Islands and Chains 6.6.3 The 2D Limit 6.7 Conclusion and Outlook References Spin Electronics – An Overview 7.1 Introduction 7.2 The Technical Basis of Spin Electronics – The Two-spin Channel Model 7.2.1 2.1 Spin Asymmetry 7.2.2 Spin Injection Across an Interface 7.2.3 The Role of Impurities in Spin Electronics 7.3 Two Terminal Spin Electronics – Giant Magnetoresistance (GMR) 7.3.1 The Analogy with Polarized Light 7.3.2 CIP and CPP GMR 7.3.3 Comparative Length Scales of CIP and CPP GMR 7.3.4 Inverse GMR 7.3.5 Methods of Achieving Differential Switching of Magnetization – RKKY Coupling Compared with Exchange Pinning 7.3.6 GMR in Nanowires 7.4 Three-terminal Spin Electronics 7.5 Mesomagnetism 7.5.1 Giant Thermal Magnetoresistance 7.5.2 The Domain Wall in Spin Electronics 7.6 Spin Tunneling 7.6.1 Theoretical Description of Spin Tunneling 216 218 219 220 221 222 224 226 227 227 228 229 230 232 233 233 234 234 238 242 246 248 253 253 254 254 255 256 257 258 259 260 260 260 261 261 263 263 264 266 267 XI Contents 7.6.2 Applications of Spin Tunneling Hybrid Spin Electronics 7.7.1 The Monsma Transistor 7.7.2 Spin Transport in Semiconductors 7.7.3 The SPICE Transistor [55, 56] 7.7.4 Measuring Spin Decoherence in Semiconductors 7.7.5 Methods of Increasing Direct Spin-injection Efficiency 7.8 Novel Spin Transistor Geometries – Materials and Construction Challenges 7.9 The Rashba effect and the Spin FET 7.9.1 The Rashba Effect 7.9.2 The Datta–Das Transistor or Spin FET [68] 7.10 Methods for Measuring Spin Asymmetry 7.10.1 Ferromagnetic Single-electron Transistors (FSETs) 7.10.2 Spin Blockade 7.11 Unusual Ventures in Spin Electronics 7.12 The Future of Spin Electronics 7.12.1 Fast Magnetic Switching 7.12.2 Optically Pumped Magnetic Switching 7.12.3 Spin Diode 7.12.4 Spin Split Insulator as a Polarizing Injector – Application to Semiconductor Injection 7.12.5 Novel Fast-switching MRAM Storage Element 7.12.6 Quantum-coherent Spin Electronics 7.12.7 The Tunnel-grid Spin-triode 7.12.8 Multilayer Quantum Interference Spin-stacks 7.12.9 Multilayer Tunnel MRAM 7.12.10 Quantum Information Technology References 7.7 271 272 273 274 274 275 277 278 280 280 280 281 281 284 285 286 286 287 287 288 288 288 290 291 291 292 293 297 297 298 298 299 301 303 303 303 305 307 311 NMR of Nanosized Magnetic Systems, Ultrathin Films, and Granular Systems 8.1 Introduction 8.2 Local Structure 8.2.1 Introduction 8.2.2 Local Atomic Configuration and Resonance Frequency 8.2.3 A Typical Example 8.2.4 Summary 8.3 Magnetization and Magnetic Anisotropy 8.3.1 Principles – Hyperfine Field in Ferromagnets 8.3.2 Local Magnetization 8.3.3 Local Anisotropy 8.4 Magnetic Stiffness – Anisotropy, Coercivity, and Coupling 8.4.1 Principles – NMR in Ferromagnets, Restoring Field, and Enhancement Factor 8.4.2 Local Magnetic Stiffness 311 313 ... 3-527-60014-0 (Electronic) Magnetism: Molecules to Materials III Nanosized Magnetic Materials Edited by Joel S Miller and Marc Drillon Magnetism: Molecules to Materials III Edited by J.S Miller... Drillon (Eds.) Magnetism: Molecules to Materials Models and Experiments 2001 XVI, 437 pages Hardcover ISBN: 3-527-29772-3 J S Miller and M Drillon (Eds.) Magnetism: Molecules to Materials II Molecule-Based... Metal Clusters in Chemistry 1999 XLVIII, 1798 pages ISBN: 3-527-29549-6 Magnetism: Molecules to Materials III Edited by J.S Miller and M Drillon Copyright c 2002 Wiley- VCH Verlag GmbH ISBNs: 3-527-30302-2