PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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Introduction 4 Figure 1.1: Temperature dependence of the resistivity of La 1−x Sr x MnO 3 (x = 0.175) under various magnetic fields [2]. % near 77 K. Which corresponds to thousand-fold magnetoresistance. The temperature dependent resistivity of La 1−x Sr x MnO 3 (x = 0.175) single crystal under several magnetic fields are shown in the Fig. 1.1. 1.3 Crystal structure The CMR samples used in this thesis work are of perovskite type (ABO 3 ) structure. The general formula for perovskite type manganites is R 1−x A x MnO 3 , where R is the trivalent rare earth element such as La, Pr, Nd, and Sm etc. and A is the divalent alkaline earth ion such as Sr, Ca, and Ba etc. An ideal cubic perovskite structure is shown in the Fig. 1.2, which contains a MnO 6 octahedron, with one Mn ion at the center and oxygen ions at each of the six corners. The trivalent or divalent ions (R or A) are situated at the corners of the cube. The stability of the cubic perovskite structure depends strongly on the size of the cations (R, A or Mn). If there is a size mismatch occurs between the cations, then the cubic perovskite structure gets
Introduction 5 Figure 1.2: Schematic view of the cubic perovskite structure. distorted. This size mismatch is governed by a factor called tolerance factor (t), and is defined by Goldschmidt [11] as, t = (r A + r O ) √ 2(rMn + r O ) , (1.2) where r A , r Mn and r O are the ionic radii of the A-site(R or A), Mn- and O- atoms respectively. For cubic perovskite structure t = 1, where the Mn-O-Mn bond angles are relatively straight. Due to the ionic size mismatch the Mn-O-Mn bond angles deviate from 180 o and lead to distorted perovskite structure. When t decreases (0.96 < t < 1), the cubic structure transforms to rhombohedral and then to the orthorhombic structure (t < 0.96). The properties of manganites strongly depend on the tolerance factor. In 1995 Hwang et. al. [12] has reported the structure-property relationship as a function of tolerance factor for 30 % dopant concentration with a variety of rare earth and alkaline earth ions, shown in the Fig. 1.3. It shows the presence of three regimes: a paramagnetic insulator at high temperature, a low-temperature ferromagnetic metal at large tolerance factor and a low-temperature ferromagnetic insulator at small tolerance factor. 1.4 The basic electronic structure In manganites the electronically active orbitals are the Mn 3d orbitals. In the cubic lattice, the five-fold degenerate 3d-orbitals of an isolated Mn atom or ion are split into
Page 1 and 2: SPECTROSCOPIC INVESTIGATIONS OF THE
Page 3 and 4: iii To My Parents
Page 5 and 6: Maharana, P. Khare, D. K. Basa, K.
Page 7 and 8: List of Publications/Preprints [1]
Page 9 and 10: Synopsis The perovskite type mangan
Page 11 and 12: to the t 2g and e g spin-up states
Page 13 and 14: Bibliography [1] R. von Helmolt, J.
Page 15 and 16: Contents CERTIFICATE Acknowledgemen
Page 17 and 18: List of Figures 1.1 Temperature dep
Page 19 and 20: 2.2 Schematic view of the energetic
Page 21 and 22: 3.4 O K edge x-ray absorption spect
Page 23 and 24: Chapter 1 Introduction 1
Page 25: Introduction 3 The temperature and
Page 29 and 30: Introduction 7 Figure 1.4: A schema
Page 31 and 32: Introduction 9 Figure 1.6: Schemati
Page 33 and 34: Introduction 11 Figure 1.8: Differe
Page 35 and 36: Introduction 13 Figure 1.10: A sche
Page 37 and 38: Introduction 15 Figure 1.12: Schema
Page 39 and 40: Introduction 17 Figure 1.13: Phase
Page 41 and 42: Bibliography [1] R. von Helmolt, J.
Page 43 and 44: Introduction 21 [31] Damien Saurel,
Page 45 and 46: Chapter 2 Experimental Techniques 2
Page 47 and 48: Experimental Techniques 25 Figure 2
Page 49 and 50: Experimental Techniques 27 Let us n
Page 51 and 52: Experimental Techniques 29 and G(k,
Page 55 and 56: Experimental Techniques 33 point wi
Page 57 and 58: Experimental Techniques 35 negative
Page 59 and 60: Experimental Techniques 37 ∆E = E
Page 63 and 64: Experimental Techniques 41 ( ) dσ
Page 65 and 66: Experimental Techniques 43 The expe
Page 67 and 68: Experimental Techniques 45 are weld
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Bibliography [1] H. Hertz, Ann. Phy
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Experimental Techniques 57 [35] K.
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Chapter 3 Electronic Structure of P
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Electronic Structure of Pr 0.67 Ca
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Bibliography [1] I. G. Deac, J. F.
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Chapter 4 Electronic Structure of P
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Electronic Structure of Pr 1−x Ca
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Electronic Structure of Ca 0.86 Pr
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Bibliography [1] C. Zener, Phys. Re
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Summary and Conclusions 109 In this
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