Magnetic nanoparticles with diameters in the range of a few nanometers are today at the cutting edge of the modern technology and innovation due to their use in numerous applications ranging from engineering to biomedicine. Their unique magnetic properties emerge because their size becomes comparable to various characteristic physical lengths (correlation length, domain wall width) andalso the number of the surface spins with reduced coordinationbecomescomparabletothatofthe corespinsinfluencing theoverallmagneticbehavior.In the case of composite nanoparticles, core/shell interface acts as an additional source of diverse magnetic effects which can be studied and exploited for specific applications. Nanoparticles in the materials exist in assemblies. They are either dispersed or inserted in host materials or even combined with them resulting in random or self-organized nanostructures. The process of synthesis and dispersion of the magnetic nanoparticles in a host is a crucial point in nanotechnology, as the performance of final products is profoundly affected by the state of dispersion of embedded nanoparticles.Above all these, the understanding of the interparticle interactions is necessary to clarify the physics of these assemblies and their use in the development of high-performance magnetic materials.
This book reviews prominent research studies on the static and dynamic magnetic properties of nanoparticle assemblies gathering together experimental and computational techniques in an effort to reveal their optimimum magnetic properties for biomedical use,nanoelectronics,catalysis and as ultra-high magnetic recording media.The selected collection of articles includesstudies on: biogenicand biomimetic magnetic nanoparticle formation and their self-assembly for nanoelectronics, biosensors, and heterogeneous catalysis, nanostructured magnetic materials produced by Gas-Phase nanoparticles, spinel ferrite nanoparticles, FePt films with graded anisotropy forultra-high magnetic recording media, patterned nanoparticle assemblies via Lithography, Monte Carlo simulations for the study of dynamic magnetic behavior of nanoparticles and their assemblies and static magnetic behavior of core/shell nanoparticles and their assemblies, combined NMR and Mössbauer techniques as probes for the microscopic investigation of the electronic fluctuations of magnetic nanoparticles.
In this book, PhD students and researchers in materials science can find current detailed computational and experimental investigations regarding magnetic nanoparticle assemblies and their intraparticle and interparticle interactions in order to understand the underlying physics and to visualize the future applications of them.I would like to thank all the authors for their efforts which made it possible to provide this book to the scientific community.Kalliopi N. TrohidouAthens, Greece April 2014

Preface xi

1.Biogenic and Biomimetic Magnetic Nanoparticles and Their Assemblies Georgia C. Papaefthymiou 1

1.1Introduction 1

1.2Biomineralization of Iron 3

1.3Bacterial Magnetor 4

      1.3.1 Synthetic vs. Biogenic Nanomagnetite 9

      1.3.2 Microarraying of Magnetosomes 12

1.4Ferritin 13

      1.4.1 Nature of the Ferrihydrite Core 15

      1.4.2 Magnetic Properties of Ferritin Biomimetics 18

1.5Biomimetics 23

      1.5.1 Magnetoferritin 25

      1.5.2 Beyond Iron Oxides 29

      1.5.3 Metal and Metal Alloy Nanoparticles 31

1.6Nanoparticle Superstructu 34

      1.6.1 Magnetoferritin Arrays 35

      1.6.1.1 3D arrays 35

      1.6.1.2 2D arrays 39

1.7Conclusion 41

2.Controlling the Structure and Properties of Nanostructured Magnetic Materials Produced by Depositing Gas-Phase Nanoparticles Chris Binns 45

2.1 Introduction 45

2.2 Pure Magnetic Nanoparticle Films 47

      2.2.1 Morphology of Pure Deposited Nanoparticle Films 48

      2.2.2 Magnetic Behavior of Pure Deposited Nanoparticle Films 49

2.3Magnetic Nanoparticles in Matrices 57

      2.3.1 Controlling the Atomic Structure of Nanoparticles in Matrices 57

      2.3.2 Controlling the Magnetic Properties of lsolated Nanoparticles in Matrices 62

      2.3.3Controlling the Magnetic Properties by Nanoparticle Volume Fraction 70

      2.3.4Producing Nanoparticle Hydrosols by Deposition of Gas-Phase Particles into Liquid Matrices 81

3. Time-Dependent Phenomena in Nanoparticle Assemblies Oscar Iglesias 91

3.1 Magnetic Relaxation in Noninteracting Nanoparticle Ensembles 95

3.2Models of Interacting 1D Chains of Nanoparticles 100

3.3Computational Details 103

      3.3.1 Calculationof Dipolar Energies 103

      3.3.2 The Monte Carlo Algorithm 105

      3.3.3 Dipolar Fields in 1D 106

3.4Effective Energy Barrier Distributions 107

3.5Relaxation Curves: TIn〔t/toScaling with Interactions 109

      3.5.1 Simulations of the Time Dependence of Magnetization110

      3.5.2 TIn(t/roScaling in the Presence of Interactions 112

3.6 Evolution offer(Eband of Dipolar Fields 114

3.7Effective Energy Barrier Distributions from TIn[t/ro) Scaling 117

3.8Hysteresis Loops 122

3.9Conclusions 124

4.Elementary Excitations in Magnetic Nanoparticles Probed with 57Fe Nuclear Magnetic Resonance and Mössbauer Spectroscopy 129

Michael Fardis, Alexios P. Douvalis, George Diamantopoulos, loannis Rabias, Thomas Bakas, HaeJin Kim,and Georgios Papavassiliou

4.1Introduction 129

4.2Magnetization Dynamics in Magnetic Nanoparticles 132

      4.2.1 Superparamagnetic and Blocking States 132

      4.2.2 Uniform Mode in Mössbauer and Nuclear Magnetic Resonance Spectroscopies 135

      4.2.2.1 Hyperfine magnetic fieldin Mössbauer spectroscopy 135

      4.2.2.2 Nuclear relaxationinnuclear magnetic resonance spectroscopy 137

4.357Fe Mössbauer Spectroscopy Experiments 140

4.457Fe Nuclear Magnetic Resonance Spectroscopy Experiments 143

      4.4.1 Nuclear Magnetic Resonance Line Shapes 143

      4.4.2 Nuclear T2 Transverse Relaxation 147

4.5Concluding Remarks 154

5.Magnetic Properties of Spinel Ferrite Nanoparticles:Influence of the Magnetic Structure Davide Peddis 159

5.1Introduction 159

5.2Magnetism in Nanoparticles: An Introduction 161

      5.2.1 Magnetism in Condensed Matter 161

      5.2.2 Magnetic Single-Domain Particles 163

      5.2.3 Magnetic Anisotropy 165

      5.2.3.1 Magnetocrystalline anisotropy 166

      5.2.3.2 Magnetostatic anisotropy(shape anisotropy) 166

      5.2.3.3 Surface anisotropy 166

5.3Magnetic Structure of Nanoparticles 168

      5.3.1 Spin Canting 168

      5.3.1.1 Temperature dependence of spin canting 171

      5.3.2 Iron Oxides with a SpinelStructure 172

      5.3.3 Spin Canting and Cationic Distribution:Magnetic Structure of Spinel Ferrite Nanoparticles 174

5.4Magnetic Properties of Spinel Ferrite Nanoparticles: Influence of the Magnetic Structure 181

      5.4.1 Surface Magnetism 181

      5.4.2 Magnetic Anisotropy 186

      5.4.2.1 Influence of the cationic distribution 187

      5.4.3 Saturation Magnetization 188

6.FePt Films with Graded Anisotropy for Magnetic Recording 199

Th. Speliotis and D. Niarchos

6.1 Short History of Magnetic Recording 199

6.2 Perpendicular Recording Media for 1 Tb/in2 and beyond 201

6.3High Ku Materials 204

6.4Fabrication Methods 207

      6.4.1 Sputtering 207

      6.4.2 Thermal Evaporation 208

      6.4.3 Thin-Film Growth 208

6.5Technologies for Future Recording Media 209

6.6FePt Graded Media for Perpendicular Magnetic Recording 211

6.7Fundamental Properties of L10FePt 211

      6.7.1Optimization of FePt Single Layers on MgO 211

      6.7.2L10FePton Amorphous Substrates 214

      6.7.2.1 Texture controland seed layer 214

      6.7.3 L10FePt Based Exchange-Spring Phenomenon 217

      6.7.4Production of PrototypeL10/A1 FePt Nanostructures 217

      6.7.4.1 L1o/A1 FePt semicore-shell nanocomposites 218

      6.7.5 Hard/Graded FePt Granular Layers 219

      6.7.5.1 Growth of L10 FePt/graded FePt nanocomposites prepared using UHV sputtering on MgO(002) substrates 220

7.Fabrication of Patterned Nanoparticle Assemblies via Lithography Gang Chen 227

7.1Introduction 227

7.2Fabrication Techniques 229

      7.2.1 Direct Patterning Assembly 229

      7.2.2 Fabrication of NP Assemblies on Patterned Templates 231

7.3 Summary and Perspective 246

8. Magnetic Behavior of Composite Nanoparticle Assemblies 253

Marianna Vasilakaki, George Margaris, and Kalliopi N. Trohidou

8.1Introduction 253

8.2The Model and Simulation Method 260

      8.2.1 Simulations of the Magnetic Behavior of Noninteracting Core/Shell Nanoparticles in the Atomic Scale 262

      8.2.2 Simulations of the Magnetic Behavior of Interacting Core/Shell Nanoparticles in the Mesoscopic Scale 265

8.3Magnetic Behavior of Noninteracting Core/Shell Nanoparticles: Study of Intraparticle Characteristics269

8.4Magnetic Behavior of Interacting Core/Shell Nanoparticles: Interparticle Interactions Effects 273

      8.4.1 Random Assemblies 273

      8.4.2 Ordered Arrays of Core/Shell Nanoparticles 278

8.5 Concluding Remarks 280

Index 287

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