MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

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116 Chapter 5 5. 3. 1 Relationship of structure to CMR Unlike the colossal magnetoresistance manganites, Gd 0.67 Ca 0.33 MnO 3 does not have a metal-insulator transition or a large magnetoresistance near the Curie temperature. The negative magnetoresistance is only of order 1% in large fields and is proportional to H 2 (or M 2 since it scales with χ 2 ) like that observed in La 0.67 Ca 0.33 MnO 3 in the paramagnetic state (section 7.2.1) [150]. Thus, Gd 0.67 Ca 0.33 MnO 3 is clearly in the class of 1/3 doped manganites which, due to the small size of Gd 3+ [100], change from paramagnetic insulators to ferromagnetic insulators at T C . There is much interest in finding a structural change at T C in the giant magnetoresistive manganites, since it is predicted to have a large effect on the resistive transition [11]. There is now evidence [151- 154] for a decrease in structural disorder as the GMR manganites become ferromagnetic metals. Gd 0.67 Ca 0.33 MnO 3 shows the contrapositive: there is no discontinuity in the atomic disorder for a material which does not become metallic at T C . This has also been noted in manganites with different dopant concentrations: La 0.88 Ca 0.12 MnO 3 [151] and La 0.5 Ca 0.5 MnO 3 [154]. 5.4 Conclusion Gd 0.67 Ca 0.33 MnO 3 is ferrimagnetic with a compensation temperature of about 15K due to the interaction and ordering of the Gd 3+ ions. The qualitative features of the magnetic properties can be accounted for with a simple two sublattice (Gd and Mn) molecular field model. The large high field susceptibility at low temperatures may indicate a canting of the magnetic sublattices. Contrary to the conclusion in [136] that there does not exist long- range magnetic order (only spin-glass magnetism) in the ferromagnetic- insulator region of the tolerance-factor phase-diagram, Gd0.67Ca0.33MnO3 shows definite long-range order and a compensation point which can not be explained by spin-glass phenomena alone. The resistivity is consistent with small polaron conductivity over a broad temperature range, with a possible crossover to a different mechanism such as variable-range hopping at low
Local Structure and Magnetism in Gd 0.67 Ca 0.33 MnO 3 temperatures. There is no noticeable change in the structure, as determined by XAFS, or conductivity at the ferrimagnetic transition. Recent theoretical and experimental work conclude that structural effects at T C are associated with the metal-insulator transition and large magnetoresistance. Since Gd 0.67 Ca 0.33 MnO 3 is in the doping regime where a metal-insulator transition and large magnetoresistance are not observed at T C , this work supports that conclusion, i. e. that a significant structural change is associated not with the ferromagnetic transition, but the metal-insulator transition. 117
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MAGNETISM AND ELECTRON TRANSPORT IN
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I certify that I have read this dis
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Preface Looking back at the many ye
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Table of Contents Abstract v Prefac
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4.2 Electronic Transport 8 5 4.2.1
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List of Figures xiii Figure 1-1 The
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Figure 5-7 Low temperature resistiv
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xvii = γT + βT 3 . The triangles
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2 Chapter 1 in themselves, but the
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4 Chapter 1 ferromagnetic metals up
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6 Chapter 1 e g t 2g e g t 2g CaMnO
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8 Chapter 1 near T C (magnetic susc
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10 Chapter 2 reactions, the rate li
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12 Chapter 2 Otherwise one has to s
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14 Chapter 2 atmosphere or vacuum.
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16 Chapter 2 a particular diffracti
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18 Chapter 2 Thus XAFS probes the l
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20 Chapter 3 relationships resistiv
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22 Chapter 3 inadequate. l ≈ a is
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24 Chapter 3 Hall effect measuremen
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26 Chapter 3 3.1.2.3 Contacts Bad c
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28 Chapter 3 3.1.2.5 Apparatus Tran
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30 Chapter 3 materials as metals or
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32 Chapter 3 overcome the barrier a
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34 Chapter 3 The dichotomy between
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36 Chapter 3 position; however, thi
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38 Chapter 3 Complex materials ofte
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40 Chapter 3 3.2.1.1 Apparatus The
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42 Chapter 3 The second-derivative
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44 Chapter 3 substances. Otherwise,
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46 Chapter 3 3.2.2.1.2 Conduction e
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48 Chapter 3 Susceptibility (emu/G
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50 Chapter 3 H/M (T/μ B ) 80 70 60
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52 Chapter 3 Magnetization (μ B )
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54 Chapter 3 is called the Stoner c
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56 Chapter 3 Energy Magnetic Excita
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58 Chapter 3 moments [84]. Thus, a
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60 Chapter 3 Arrott plot), will the
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62 Chapter 3 Curie temperature, the
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64 Chapter 3 The energy ω of a spi
Page 84 and 85: 66 Chapter 3 3.2.2.2.9 Irreversibil
Page 86 and 87: 68 Chapter 3 ferromagnet will also
Page 88 and 89: 70 Chapter 3 Magnetization (µ B )
Page 90 and 91: 72 Chapter 3 with temperature and f
Page 92 and 93: 74 Chapter 3 by Mn +4 2 (S = 3/2).
Page 94 and 95: 76 Chapter 3 ferromagnet. Instead,
Page 96 and 97: 78 Chapter 3 3.3.1.1 Apparatus The
Page 98 and 99: 4. Intrinsic Electrical Transport a
Page 100 and 101: 82 Chapter 4 M/M 0 1.0 0.8 0.6 0.4
Page 102 and 103: 84 Chapter 4 M/M 0 1.00 0.96 0.92 0
Page 104 and 105: 86 Chapter 4 Resistivity (10 -3 Ω
Page 106 and 107: 88 Chapter 4 dependence of R 2 is s
Page 108 and 109: 90 Chapter 4 magnetoresistance obse
Page 110 and 111: 92 Chapter 4 4. 2. 2 High Temperatu
Page 112 and 113: 94 Chapter 4 measurements (Figure 4
Page 114 and 115: 96 Chapter 4 somewhat too large com
Page 116 and 117: 98 Chapter 4 Magnetization 8 6 4 2
Page 118 and 119: 100 Chapter 4 above room temperatur
Page 120 and 121: 102 Chapter 5 5.1 Ferrimagnetism Th
Page 122 and 123: 104 Chapter 5 calculation predicts
Page 124 and 125: 106 Chapter 5 χ mol (µ B /mol/T)
Page 126 and 127: 108 Chapter 5 of the nonlinear M 2
Page 128 and 129: 110 Chapter 5 rather than a simple
Page 130 and 131: 112 Chapter 5 ln(ρ /Ω cm) 24 22
Page 132 and 133: 114 Chapter 5 A high, temperature i
Page 136 and 137: 6. Magnetoconductivity in La 0.67Ca
Page 138 and 139: 120 Chapter 6 resistivity ρ ∝ M
Page 140 and 141: 122 Chapter 6 The Hall effects are
Page 142 and 143: 124 Chapter 6 ∆R/R (%) 1 0 -1 -2
Page 144 and 145: 126 Chapter 6 R(H)-R(-H) (µΩ cm)
Page 146 and 147: 7. Critical Transport and Magnetiza
Page 148 and 149: 130 Chapter 7 2 ) M 2 (µ B The foc
Page 150 and 151: 132 Chapter 7 for T > T C . The reg
Page 152 and 153: 134 Chapter 7 spin-relaxation measu
Page 154 and 155: 136 Chapter 7 1/χ (10kOe/µ B ) 5
Page 156 and 157: 138 Chapter 7 γ < 1. Also, at a co
Page 158 and 159: 140 Chapter 7 ∆R/R (%) 10 0 -10 -
Page 160 and 161: 142 Chapter 7 (Figure 7-6), in so f
Page 162 and 163: 144 Chapter 7 The related form ρ
Page 164 and 165: 146 Chapter 7 σ H (10 -5 mΩcm -1
Page 166 and 167: 148 Chapter 7 σ E exp[M 0 (T)/M E
Page 168 and 169: 150 Chapter 7 σ = σ E exp[M(H,T)/
Page 170 and 171: Appendix A. Critical Behavior and A
Page 172 and 173: 154 Appendix A samples. Magnetizati
Page 174 and 175: 156 Appendix A while the pellet dis
Page 176 and 177: 158 Appendix A M 0 (µ B ) 1.0 0.8
Page 178 and 179: 160 Appendix A (T/T C - 1) γ . Fro
Page 180 and 181: 162 Appendix A provides highly reve
Page 182 and 183: 164 Appendix A 1/χ 0 (emu -1 mol G
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166 Appendix A T 3/2 Coefficient (1
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168 Appendix A magnetization with t
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170 Appendix A excitations for T <
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172 Appendix A β changes from Heis
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174 Appendix B c P,H /T (mJ/mol·K
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176 Appendix B Figure 3 at the 90%
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178 Appendix B are vectors. Thus fo
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180 Appendix B corrections are incl
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References [1] G. H. Jonker and H.
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184 References [40] G. J. Snyder an
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186 References [80] E. C. Stoner, P
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188 References [115] M. F. Hundley,
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190 References [151] S. J. L. Billi
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192 References [187] L. Klein, (unp
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MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

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