Copyright
IIT Kharagpur
Contents
Chapter 1
Brief Review and Objective of the Thesis 1.1 Introduction
1.1.1 Perovskite Compounds
1.2 Mixed valence Manganite perovskites 1.2.1 Crystal structure
1.2.2 Double exchange
1.2.3 Electronic structure and Jahn-Teller distortion 1.2.4 Magnetic structure
1.2.5 Charge ordering 1.2.6 Orbital ordering 1.2.7 Electronic transport
1.2.7.1 Transport in paramagnetic phase (T>TC) 1.2.7.2 Transport in ferromagnetic phase (T<TC) 1.2.7.3 Two phase behavior
1.2.8 Colossal magnetoresistance (CMR) 1.2.9 Magnetocaloric effect (MCE) 1.2.10 Applications
1.3 Motivation behind CMR manganites
1.4 Review on A-site monovalent doped manganites 1.5 Objective of the thesis
Bibliography Chapter 2
Experimental Techniques and Measurements 2.1 Introduction
2.2 Electrical Resistivity measurements 2.2.1 Introduction
2.2.2 Cryostat for Resistivity measurement 2.2.3 Electrical Schematic
2.2.4 Estimation of error for Resistivity measurement 2.3 AC susceptibility
2.3.1 Introduction 2.3.2 Cryostat
2.3.3 Electrical Schematic 2.3.4 Measurement procedure 2.4 Thermoelectric power
2.4.1 Introduction 2.4.2 Cryostat
2.4.3 Electrical Schematic and Measurement Procedure
2.4.4 Estimation of Error for thermoelectric power measurement 2.5 Measurement of Magnetocaloric effect (MCE)
2.5.1 Introduction
Page 1 1 2 3 3 5 6 8 9 9 10 12 14 15 18 21 22 25 29 31
36 36 36 36 37 40 40 40 41 42 42 44 44 46 47 49 49 49 iv
Copyright
IIT Kharagpur
2.5.2 Vibrating Sample Magnetometer (VSM) 2.5.3 Operation of VSM
2.5.4 Theory of VSM
2.5.5 Design and construction details of VSM 2.5.5.1 Systems used:
2.5.5.2 Sample Insert 2.5.5.3 Vibrating Unit
2.5.5.4 Detection coil Design 2.5.6 Noise Reduction of VSM
2.5.7 Saddle Point Detection (Optimum Sample Placement) 2.5.8 Calibration of VSM
2.5.9 Specifications of the developed VSM
2.5.10 Measurement of magnetocaloric effect from Magnetization measurements in the developed VSM
Bibliography Chapter 3
Sample preparation and characterization 3.1 Introduction
3.2 Sample preparation 3.2.1 Introduction
3.2.2 Material Synthesis by ‘Pyrophoric method’
3.3 Sample Characterization
3.3.1 Structural Characterization by X-Ray diffraction (XRD) 3.3.1.1 Rietveld analysis
3.3.1.2 Structure analysis 3.3.2 Grain size and morphology 3.3.3 Oxygen Nonstoichiometry
3.3.3.1 Principle of iodometric titration
3.3.3.2 Steps followed for iodometric titration and results 3.3.4 Density measurement
3.3.5 Magnetic characterization (AC susceptibility) 3.4 Summary and Conclusions
Bibliography Chapter 4
Electrical Resistivity of polycrystalline La1-xKxMnO3 system 4.1 Introduction
4.2 Experimental results 4.3 Discussions
4.3.1 Resistivity temperature behavior above TMI (TMI to 300K) 4.3.1.1 Temperature dependent DC conduction mechanism 4.3.1.2 Analysis of resistivity behavior at T>TMI
4.3.2 Resistivity temperature behavior below TMI (50K to TMI)
4.3.2.1 Low temperature resistivity by scattering mechanisms 4.3.2.2 Low temperature resistivity by polaron tunneling
51 52 54 55 56 56 60 60 61 61 62 65 67 69
70 71 71 71 74 74 75 76 78 80 81 82 82 83 85 87
89 90 93 94 94 97 102 104 107 v
Copyright
IIT Kharagpur
4.3.2.3 Analysis of resistivity behavior below TMI
4.3.3 Resistivity behavior below 50K
4.3.3.1 Resistivity minimum due to electron-electron interaction
4.3.3.2 Resistivity minimum due to Coulomb blockade 4.3.3.3 Resistivity minimum due to intergrannular tunneling 4.3.3.4 Analysis of resistivity minimum below 50K
4.3.4 Two phase behavior 4.3.4.1 Two phase model
4.3.4.2 Analysis of two phase resistivity behavior 4.4 Conclusions
Bibliography Chapter 5
Thermoelectric Power of polycrystalline La1-xKxMnO3 system 5.1 Introduction 5.2 Experimental Results
5.3 Discussions
5.3.1 Thermopower above TC (TC to 300K) 5.3.1.1 Theoretical Background
5.3.1.2 Analysis of thermopower above TC
5.3.2 Thermopower below TC (50K to TC)
5.3.2.1 Scattering mechanisms of thermopower at low temperature
5.3.2.1.1 Electron-phonon scattering: Phonon drag effect 5.3.2.1.2 Electron-magnon scattering: Magnon drag effect 5.3.2.1.3 Spin wave fluctuation
5.3.2.2 Analysis of thermopower below TC
5.3.3 Two phase behavior
5.3.3.1 Phase separation model
5.3.3.2 Analysis of two phase behavior 5.4 Conclusions
Bibliography Chapter 6
Magnetoresistance of polycrystalline La1-xKxMnO3 system 6.1 Introduction
6.2 Experimental Results 6.3 Discussions
6.3.1 Magnetoresistance behavior in light of spin dependent hopping (magnetic cluster) model
6.3.1.1 Theoretical Background
6.3.1.2 Analysis of magnetoresistance
6.3.2 Magnetoresistance in terms of spin polarized intergrain tnneling model
6.3.2.1 Theoretical Background
109 113 115 116 117 119 123 124 125 127 129
133 134 136 136 137 139 142 143 143 144 145 145 147 148 151 153 154
156 158 161 161 161 163 166 167 vi
Copyright
IIT Kharagpur
6.3.2.2 Analysis of magnetoresistance 6.3.3 Comparative discussions
6.4 Conclusions Bibliography Chapter 7
Magnetocaloric property of polycrystalline La1-xKxMnO3 system 7.1 Introduction
7.2 Basic thermodynamics of magnetocaloric effect 7.3 Experimental Results
7.4 Discussions
7.5 Modeling of Magnetocaloric Effect
7.5.1 MCE modeling by Molecular Mean Field Theory 7.5.2 MCE modeling by Landau Theory of phase transition 7.6 Conclusions
Bibliography Chapter 8
Conclusions and Scope of future studies 8.1 Conclusions
8.2 Scope of future studies
169 172 175 177
179 181 184 187 195 195 198 202 204
206 209
vii
