MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

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28 Chapter 3 3.1.2.5 Apparatus Transport measurements for this work were made in DC allowing a few seconds for the current and voltages to stabilize. A constant current source (Keithley model 220 or 224) was used to supply the current and a separate voltmeter (Keithley model 181, 182, 196 or 197) used to measure the voltages. Current is not usually measured independently but can be with the addition of an electrometer into the circuit. A computer controlled the current source, read the voltages and temperatures and recorded the data. Typically, 5 pairs of R(I) = (V(I) - V(-I))/2 were measured at slightly different currents (e.g. 20% variation) to estimate the precision. For resistances greater than 10 6 Ω (often T < 100K) two point resistance measurements were made using an electrometer. Most of the low temperature and all the magnetoresistance and Hall effect measurements were made in the cryostat of the Quantum Design MPMS 2 SQUID magnetometer described in section 3.2.1.1. The computer that controls the SQUID magnetometer can also operate subroutines that can control GPIB compatible devices. These EDC (External Device Control) subroutines can be quite general, but have a limited number of available commands. It is necessary to further process the data recorded from the EDC subroutine in order to use it in a graphing or spreadsheet program. The standard SQUID magnetometer software controls the temperature and magnetic field. The thermometer is not very near the sample, so reliable data must be taken after the temperature has stabilized. This usually takes about 5 minutes for routine measurements or 30 minutes if any temperature drift is to be avoided. Since the measurement can run for several days unattended, time is usually not much of a problem. A 10 wire transport probe prefabricated at Quantum Design was used for the measurements. A special tip was made out of 8 bore alumina rod to provide a surface perpendicular to the magnetic field. Films can then be
Electronic and Magnetic Measurements 29 attached to this surface with double-sided sticky tape to measure the Hall effect and transverse magnetoresistance. A simple copper plate attached to the tip of the transport probe provides a surface parallel to the magnetic field for longitudinal and transverse magnetoresistance measurements. One limitation of the SQUID magnetometer is its relatively small sample chamber (8mm diameter). High temperature (300K < T < 1200K) measurements were performed in a furnace (in air) with a slowly varying temperature (≈ 1 K/min.). The temperature was recorded using a Type-K thermocouple positioned a few millimeters from the sample. The sample holder was made from an 8 bore alumina rod. The 2 thermocouple wires and 4 platinum wires which contact the sample were fed through the bores of the alumina rod. The wires are positioned to facilitate changing the sample and attaching new contacts. The furnace was programmed to heat and cool using a standard Eurotherm (model 818) power regulator and temperature controller. Data were continuously recorded by a computer using LabView software as the furnace heated and cooled. Lower temperature measurements can be similarly made by dipping the probe into a dewar of liquid nitrogen or liquid helium. 3.1.3 Analysis Careful transport measurements can help identify transport mechanisms. Knowledge of transport mechanisms not only helps us to understand the microscopic nature of the material but also allows us to predict other properties using previous theoretical or empirical knowledge of other materials with the same transport mechanism. For example a few percent impurity due to processing may not affect the room temperature resistivity of a metal while greatly decreasing that of a band semiconductor. Some of the common transport mechanisms and their predicted transport properties are given below. Since there is not much overlap of mechanisms or properties between metals and insulators, it is convenient to categorize
Page 1 and 2: MAGNETISM AND ELECTRON TRANSPORT IN
Page 3: I certify that I have read this dis
Page 7 and 8: Preface Looking back at the many ye
Page 9 and 10: Table of Contents Abstract v Prefac
Page 11 and 12: 4.2 Electronic Transport 8 5 4.2.1
Page 13 and 14: List of Figures xiii Figure 1-1 The
Page 15 and 16: Figure 5-7 Low temperature resistiv
Page 17: xvii = γT + βT 3 . The triangles
Page 20 and 21: 2 Chapter 1 in themselves, but the
Page 22 and 23: 4 Chapter 1 ferromagnetic metals up
Page 24 and 25: 6 Chapter 1 e g t 2g e g t 2g CaMnO
Page 26 and 27: 8 Chapter 1 near T C (magnetic susc
Page 28 and 29: 10 Chapter 2 reactions, the rate li
Page 30 and 31: 12 Chapter 2 Otherwise one has to s
Page 32 and 33: 14 Chapter 2 atmosphere or vacuum.
Page 34 and 35: 16 Chapter 2 a particular diffracti
Page 36 and 37: 18 Chapter 2 Thus XAFS probes the l
Page 38 and 39: 20 Chapter 3 relationships resistiv
Page 40 and 41: 22 Chapter 3 inadequate. l ≈ a is
Page 42 and 43: 24 Chapter 3 Hall effect measuremen
Page 44 and 45: 26 Chapter 3 3.1.2.3 Contacts Bad c
Page 48 and 49: 30 Chapter 3 materials as metals or
Page 50 and 51: 32 Chapter 3 overcome the barrier a
Page 52 and 53: 34 Chapter 3 The dichotomy between
Page 54 and 55: 36 Chapter 3 position; however, thi
Page 56 and 57: 38 Chapter 3 Complex materials ofte
Page 58 and 59: 40 Chapter 3 3.2.1.1 Apparatus The
Page 60 and 61: 42 Chapter 3 The second-derivative
Page 62 and 63: 44 Chapter 3 substances. Otherwise,
Page 64 and 65: 46 Chapter 3 3.2.2.1.2 Conduction e
Page 66 and 67: 48 Chapter 3 Susceptibility (emu/G
Page 68 and 69: 50 Chapter 3 H/M (T/μ B ) 80 70 60
Page 70 and 71: 52 Chapter 3 Magnetization (μ B )
Page 72 and 73: 54 Chapter 3 is called the Stoner c
Page 74 and 75: 56 Chapter 3 Energy Magnetic Excita
Page 76 and 77: 58 Chapter 3 moments [84]. Thus, a
Page 78 and 79: 60 Chapter 3 Arrott plot), will the
Page 80 and 81: 62 Chapter 3 Curie temperature, the
Page 82 and 83: 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,
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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
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86 Chapter 4 Resistivity (10 -3 Ω
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88 Chapter 4 dependence of R 2 is s
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90 Chapter 4 magnetoresistance obse
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92 Chapter 4 4. 2. 2 High Temperatu
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94 Chapter 4 measurements (Figure 4
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96 Chapter 4 somewhat too large com
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98 Chapter 4 Magnetization 8 6 4 2
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100 Chapter 4 above room temperatur
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102 Chapter 5 5.1 Ferrimagnetism Th
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104 Chapter 5 calculation predicts
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106 Chapter 5 χ mol (µ B /mol/T)
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108 Chapter 5 of the nonlinear M 2
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110 Chapter 5 rather than a simple
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112 Chapter 5 ln(ρ /Ω cm) 24 22
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114 Chapter 5 A high, temperature i
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116 Chapter 5 5. 3. 1 Relationship
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6. Magnetoconductivity in La 0.67Ca
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120 Chapter 6 resistivity ρ ∝ M
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122 Chapter 6 The Hall effects are
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124 Chapter 6 ∆R/R (%) 1 0 -1 -2
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126 Chapter 6 R(H)-R(-H) (µΩ cm)
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7. Critical Transport and Magnetiza
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130 Chapter 7 2 ) M 2 (µ B The foc
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132 Chapter 7 for T > T C . The reg
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134 Chapter 7 spin-relaxation measu
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136 Chapter 7 1/χ (10kOe/µ B ) 5
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138 Chapter 7 γ < 1. Also, at a co
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140 Chapter 7 ∆R/R (%) 10 0 -10 -
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142 Chapter 7 (Figure 7-6), in so f
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144 Chapter 7 The related form ρ
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146 Chapter 7 σ H (10 -5 mΩcm -1
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148 Chapter 7 σ E exp[M 0 (T)/M E
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150 Chapter 7 σ = σ E exp[M(H,T)/
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Appendix A. Critical Behavior and A
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154 Appendix A samples. Magnetizati
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156 Appendix A while the pellet dis
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158 Appendix A M 0 (µ B ) 1.0 0.8
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160 Appendix A (T/T C - 1) γ . Fro
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162 Appendix A provides highly reve
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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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