1. Complex Functional Materials
J.A.Krumhansl ............................................. 1
2. Spin, Charge, and Lattice Coupling
in Multiferroic Materials
S.R.Shenoy, T.Bookman, and A.Saxena ....................... 3
2.1. Introduction ............................................... 3
2.2. Order Parameters and Multiferroics ......................... 5
2.3. Strain Tensor and Elastic Compatibility .................... 6
2.3.1. Levels of Description ............................... 6
2.3.2. Definition of Unit-Cell Strain Tensor ............... 7
2.3.3. Strain Free Energy .................................. 8
2.3.4. St Venant Compatibility Constraints ................. 9
2.3.5. Compatibility Potentials ........................... 10
2.3.6. Other Approaches ................................... 12
2.4. Inhomogeneities in Ferroelastics .......................... 13
2.5. Inhomogeneities in Multiferroic Oxides .................... 16
2.6. Charge and Spin as Local Stresses and Transition
Temperatures .............................................. 18
2.6.1. Coupling of Variables .............................. 18
2.6.2. Simulations ........................................ 19
2.7. Summary and Further Work .................................. 21
References ..................................................... 22
3. Disorder in Magnetic and Structural Transitions:
Pretransitional Phenomena and Kinetics
T.Castdn, E.Vives, L.Manosa, A.Planes,
and A.Saxena ............................................. 27
3.1. Introduction .............................................. 27
3.2. Disorder Distribution ..................................... 28
3.2.1. Pretransitional Phenomena .......................... 29
3.2.2. Premonitory Behaviour: Temperature Softening ....... 29
3.2.3. Structural Precursors .............................. 31
3.2.4. Coupling to Magnetism .............................. 33
3.2.5. Ferroic Precursors: Magnetic and Others ............ 35
3.3. Transition Kinetics ....................................... 37
3.3.1. Athermal Transitions ............................... 37
3.3.2. Modelling Athermal Transitions ..................... 40
3.3.3. Avalanche Dynamics ................................. 40
3.3.4. Modelling Avalanches ............................... 43
3.4. Conclusion ................................................ 45
References ..................................................... 46
4. Huge Magnetoresistance in Association
with Strong Magnetoelastic Effects
L.Morellon and M.R.Ibarra ................................. 49
4.1. Introduction .............................................. 49
4.2. Magnetic-Crystallographic Transformations ................. 50
4.2.1. Gd5(SixGe1-x)4 ....................................... 50
4.2.2. MnAs ............................................... 51
4.3. Moment Instabilities and Spin Fluctuations ................ 53
4.3.1. FeRh ............................................... 53
4.3.2. Hf1-x_ТаxFе2......................................... 55
4.3.3. La(FexAl1-x)13 ...................................... 56
4.4. Metal-Insulator Transitions ............................... 57
4.4.1. Colossal Magnetoresistance (CMR)
and Huge Volume Effects ............................ 59
4.4.2. Magnetostriction and Magnetoresistance
in the Paramagnetic Phase ................................. 61
4.4.3. Magnetostriction in Inhomogeneous Electronic
Systems: Static Phase Segregation .................. 63
4.5. Charge/Orbital Instabilities .............................. 65
4.6. Conclusion ................................................ 70
References ................................................ 71
5. Interplay of Spin, Charge, and Lattice
in CMR Manganites and HTSC Cuprates
T.Egami .................................................. 75
5.1. Introduction .............................................. 75
5.2. Stability of Polaronic Phase in the CMR Manganites ........ 76
5.2.1. Colossal Magnetoresistivity ........................ 76
5.2.2. Pulsed Neutron PDF Method .......................... 77
5.2.3. Polaron Stability .................................. 79
5.3. Stability of Spin-Charge Stripes in the Cuprates .......... 83
5.4. Electron-Phonon Coupling and Mechanism
of Superconductivity in the Cuprates ...................... 84
5.4.1. Local Lattice Distortion
and Inhomogeneous Electronic States ................ 84
5.4.2. Electron-Phonon Coupling in the Cuprates ........... 84
5.4.3. Vibronic Mechanism of High Temperature
Superconductivity .................................. 87
5.5. Conclusions ............................................... 89
References ..................................................... 90
6. Neutron Scattering Studies of Anomalous Phonon Behavior
in Functional Materials
S.M.Shapiro .............................................. 93
6.1. Introduction .............................................. 93
6.2. Neutron Scattering ........................................ 94
6.3. Phonon Anomalies .......................................... 95
6.3.1. Kohn Anomalies ..................................... 96
6.3.2. Soft Modes ......................................... 97
6.4. Phonon Anomalies in the Manganites ........................ 99
6.5. Phonon Anomalies in High Temperature Superconductors ..... 100
6.6. Ferromagnetic Shape Memory Alloys ........................ 102
6.6.1. Iron-Based Alloys ................................. 103
6.6.2. Heusler-Based Alloys .............................. 104
6.7. Summary .................................................. 110
References .................................................... 1ll
7. The Structures and Transformation
Mechanism in the Ferromagnetic Shape
Memory Alloy Ni2MnGa
P.J.Brown, T.Kanomata, M.Matsumoto, K.-U.Neumann,
and K.R.A.Ziebeck ....................................... 113
7.1. Introduction ............................................. 113
7.2. The Crystal Structure of the Cubic Austenite Phase ....... 113
7.3. Bulk Magnetic Properties ................................. 116
7.4. Spin Dynamics ............................................ 118
7.5. Paramagnetic Response .................................... 118
7.6. Inelastic Neutron Scattering ............................. 119
7.7. Neutron Diffraction ...................................... 121
7.8. Pre-Martensitic Phase .................................... 122
7.8.1. Field Dependence .................................. 125
7.9. The Martensitic Phase .................................... 126
7.10.Structural and Magnetic Phase Diagram .................... 127
7.11.Mechanism ................................................ 129
7.12.Martensitic Twinning in Ni2МnСа .......................... 130
7.13.Non-Stoichiometric Samples ............................... 133
7.14.Electron Concentration ................................... 135
7.15.Polarised Neutron Scattering ............................. 136
7.16.Conclusion ............................................... 137
References .................................................... 138
8. Imaging Techniques in Magnetoelastic Materials
S.P.Venkateswaran and M.De Graef ........................ 141
8.1. Introduction ............................................. 141
8.2. Lorentz Image Formation Theory ........................... 141
8.2.1. Classical Description ............................. 141
8.2.2. Quantum Mechanical Description .................... 143
8.2.3. Phase Reconstruction .............................. 145
8.3. Applications of LTEM to Ferromagnetic Shape Memory
Alloys ................................................... 148
8.3.1. Ni2MnGa, Austenitic State ......................... 149
8.3.2. Ni2MnGa, Martensitic State ........................ 152
8.3.3. Co2NiGa, Austenitic State ......................... 153
8.3.4. Co2NiGa, Martensitic State ........................ 154
8.4. Summary .................................................. 156
References .................................................... 156
9. A Way to Search for Multiferroic Materials
with "Unlikely" Combinations of Physical Properties
R.D.James and Z.Zhang ................................... 159
9.1. Introduction ............................................. 159
9.2. Single Phase Multiferroics ............................... 160
9.3. Basic Idea ............................................... 160
9.4. Lattice Parameter Sensitivity ............................ 161
9.5. What Makes Big First Order Phase
Transformations Reversible? .............................. 162
9.6. Specific Relationships Among Lattice Parameters
for a High Degree of Reversibility ....................... 169
9.6.1. Cubic to Orthorhombic Transformations ............. 170
9.6.2. Cubic to Monoclinic Transformations ............... 170
9.6.3. Relationships for Martensite/Martensite
Transitions ....................................... 171
9.7. Tuning Lattice Parameters
to Satisfy Two of the Proposed Conditions
in the NiTiCuPd System ................................... 171
9.8. Further Comparisons with Experiment ...................... 172
9.9. Summary and Outlook: A General Method
for Seeking New Classes of Functional Materials .......... 174
References .................................................... 174
10. Invar and Anti-Invar:
Magnetovolume Effects in Fe-Based Alloys Revisited
E.F.Wassermann and M.Acet ................................ 177
10.1.Introduction ............................................. 177
10.2.Invar .................................................... 177
10.3.From Invar to Anti-Invar ................................. 181
10.4.Allotropy of Pure Fe ..................................... 184
10.5.Ground State Properties of Invar and Anti-Invar .......... 187
10.6.Pressure Experiments: Evidence for High Spin
to Low Spin State Transitions ............................ 189
10.7.HS-LS Transitions in a Microscopic Picture ............... 192
10.8.Questions and Outlook .................................... 195
References .................................................... 196
11.Magnetocaloric Effect Associated with
Magnetostructural Transitions
V.K.Pecharsky and K.A.Gschneidner, Jr. .................. 199
11.1.Introduction ............................................. 199
11.2.Magnetic Cooling or Why Having a Strong
Magnetocaloric Effect in a Weak Magnetic
Field Makes a Difference? ................................ 199
11.3.Gd5(Si4-xGex) System and the
Giant Magnetocaloric Effect .............................. 202
11.4.Altering Crystal Structures with a Magnetic Field ........ 206
11.5.To What Extent a Structural Change
Enhances the Giant Magnetocaloric Effect? ................ 214
11.6.Conclusions .............................................. 219
References .................................................... 220
12.Entropy Change and Magnetocaloric Effect
in Magnetostructural Transformations
F.Casanova, X.Battle, A.Labarta, J.Marcos,
E.Vives, L.Manosa, and A.Planes ......................... 223
12.1.Introduction ............................................. 223
12.2.Multiscale Origin of the MCE in Ni-Mn-Ga Alloys .......... 225
12.3.Direct Determination of the Entropy Change
at a First-Order Transformation .......................... 229
12.4.Magnetostructural Transformation in Gd-Si-Ge Alloys ...... 230
12.5.Conclusions .............................................. 235
References .................................................... 235
13.Functional Magneto-Structural Materials:
Summary and Perspectives
P.-A.Lindgdrd ........................................... 237
References .................................................... 243
Index ......................................................... 247
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