MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

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136 Chapter 7 1/χ (10kOe/µ B ) 5 4 3 2 1 0 260 270 280 290 300 inflection point has been observed in the double layer perovskite manganite La 1.4 Sr 1.6 Mn 2 O 7 above T C [169]. 1/χ ∝ (T-T C ) γ γ = 0.7 Temperature (K) A positive χ 3 can be observed in a ferromagnet or spin glass during irreversible magnetization processes; for example, while increasing the magnetic field from a demagnetized state. However, once the material is magnetized at a high field, the curve of M vs. H for decreasing field almost always has a negative χ 3 . The data in the inset of Figure 7-1 includes data for both increasing and decreasing fields, showing no signs of hysteresis [170]. ln(1/χ) 11 10 9 1 2 3 ln(263K-T) Figure 7-4. Inverse magnetic susceptibility, 1/χ 0 as a function of temperature for La 0.67 Ca 0.33 MnO 3 crystal. At each temperature, the value shown is H/M extrapolated to H = 0 as given by the intercept in Figure 7-1. Solid line is fit to 1/χ 0 ∝ (T/T C - 1) γ with γ = 0.7 and T C = 263K.
Critical Transport and Magnetization of La 0.67 Ca 0.33 MnO 3 Thus a positive χ 3 due to the irreversibility of a spin glass seems highly unlikely. More plausible explanations will be discussed in section 7.1.4. 7. 1. 4 Additional magnetic interaction The unusual γ < 1 may be related to χ3 > 0. Both effects suggest an additional mechanism. There could, for instance, be additional magnetic interactions, such as an antiferromagnetic component to the coupling. A canted-antiferromagnet ground-state is expected theoretically from the double-exchange mechanism in conjunction with super-exchange [8], and has been found by neutron diffraction [139, 143] in the related compound Pr 0.67 Ca 0.33 MnO 3 . If there is a competition between antiferromagnetic and ferromagnetic fluctuations in the paramagnetic state, the application of a magnetic field will favor ferromagnetism and thus induce a larger magnetic moment. This would explain the observed positive χ 3 . The corresponding transition in the ordered state is called a metamagnetic transition. Competing magnetic interactions can also cause the downward curvature in 1/χ vs. T seen in Figure 7-4. This is commonly observed in a ferrimagnet (e.g. Gd 0.67 Ca 0.33 MnO 3 described in section 5.1.2) where some strong interactions are antiferromagnetic, but near T C a ferromagnetic susceptibility is observed. The canting of the moments by as much as 45° would still give 92% of the expected net moment, which is about that observed (section 4.1) at low temperatures for La 0.67 Ca 0.33 MnO 3 . A different but more general explanation for the unusual magnetic behavior observed in La 0.67 Ca 0.33 MnO 3 requires some additional mechanism which provides the stronger than mean field behavior, i.e. the ferromagnetic interaction strength appears to increase, beyond that predicted by the mean field approximation, as the magnetization increases. For instance, as T approaches T C , the strength of the interaction increases, effectively increasing T C . This causes the susceptibility to diverge faster than expected, providing 137
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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
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66 Chapter 3 3.2.2.2.9 Irreversibil
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68 Chapter 3 ferromagnet will also
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70 Chapter 3 Magnetization (µ B )
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72 Chapter 3 with temperature and f
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74 Chapter 3 by Mn +4 2 (S = 3/2).
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76 Chapter 3 ferromagnet. Instead,
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78 Chapter 3 3.3.1.1 Apparatus The
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4. Intrinsic Electrical Transport a
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82 Chapter 4 M/M 0 1.0 0.8 0.6 0.4
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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 134 and 135: 116 Chapter 5 5. 3. 1 Relationship
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 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
Page 184 and 185: 166 Appendix A T 3/2 Coefficient (1
Page 186 and 187: 168 Appendix A magnetization with t
Page 188 and 189: 170 Appendix A excitations for T <
Page 190 and 191: 172 Appendix A β changes from Heis
Page 192 and 193: 174 Appendix B c P,H /T (mJ/mol·K
Page 194 and 195: 176 Appendix B Figure 3 at the 90%
Page 196 and 197: 178 Appendix B are vectors. Thus fo
Page 198 and 199: 180 Appendix B corrections are incl
Page 200 and 201: References [1] G. H. Jonker and H.
Page 202 and 203: 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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