PHYS07200604007 Manas Kumar Dala - Homi Bhabha National ...

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Introduction 2 1.1 An overview The perovskite type manganites having general formula R 1−x A x MnO 3 (where R is a trivalent rare earth element like La, Pr, Nd. and A is a divalent alkaline earth element like Sr, Ca, Ba.) have been one of the interesting compounds among the transition metal oxide systems due to their intriguing physical property such as colossal magnetoresistance (CMR) [1, 2, 3]. CMR is defined as the colossal decrease of resistance by the application of magnetic field. These materials are insulators at high temperatures and poor metals at low temperatures. Accompanying this insulator-metal transition is a magnetic transition from a high temperature paramagnetic phase to a low temperature ferromagnetic phase. These transitions occurs around the same transition temperature T C . Applying a magnetic field around T C will greatly reduce the resistivity, i.e negative magnetoresistance effect. These materials were first described in 1950 by Jonker and van Santen [4] in perovskite type manganites. One year later (1951), Zener [5, 6] explained this unusual correlation between magnetism and transport properties by introducing a novel concept, so-called ”Double exchange” mechanism (DE). The pioneering work of Zener was followed by Anderson and Hasegawa [7] in 1955 and de Gennes [8] in 1960. Double exchange theory argues that the electron hopping is related to the relative spin orientations of the neighbouring sites. Although it qualitatively explains the connection between the insulator metal and the magnetic transitions, it was noticed latter by A. J. Millis et. al. [9] that it fails to explain the quantitative change in resistivity and the very low T C . Thus it is believed that double-exchange mechanism is not enough to explain the CMR effect. Several mechanisms, such as electron-phonon interactions (the polaronic effect), electronic phase separation, charge and/or orbital ordering etc. have been proposed to account for the discrepancy. Although the detailed picture is still unclear, there seems to be a growing consensus that one or more of these additional effects probably exists, with the various phenomena both competing and cooperating with each other. This can lead to very rich and exotic behaviour, as observed in the manganites. This mixed valence manganites have been studied for more than five decades but are still considered modern materials because of their wide potential for technological applications. Potential applications of the CMR effect in these manganites include magnetic sensors, magnetoresistive read heads, magnetoresistive random access memory (MRAM) etc.
Introduction 3 The temperature and doping dependent metal-insulator transitions in these materials are found to be closely related to the unique electronic structure derived from the Mn 3d and O 2p hybridized orbitals of MnO 6 octahedra. The 3d orbitals of Mn in the MnO 6 octahedra, which are split by the crystal field into t 2g and e g states, are further split by the Jahn-Teller distortion. With charge carrier doping, many of them show well defined charge ordering (CO) and/or orbital ordering (OO) at low temperatures, especially when x equals a commensurate value (e.g x = 0.5). The CO state where the Mn 3+ (t 3 2ge 1 g) and Mn 4+ (t 3 2g) ions arranged like in a checkerboard was found to bear a strong influence on the one electron band width (W) of the e g band and the transfer interaction of the e g holes (electrons). The work presented in this thesis consists of the investigations of the electronic structure of Pr 1−x Ca x MnO 3 using different spectroscopic techniques as photoemission spectroscopy, inverse photoemision spectroscopy and x-ray absorption spectroscopy etc. The thesis is structured as follows : Following this overview, the Chapter 1 gives a general idea about the physical properties (structural, electronic and magnetic) and the mechanisms associated with the CMR manganites. Chapter 2 describes the principles and operations of the experimental techniques used for this thesis work. Chapter 3 presents the electronic structure of Pr 0.67 Ca 0.33 MnO 3 at different temperatures using ultra-violet photoelectron and x-ray absorption spectroscopy. The electronic structure of the Pr 1−x Ca x MnO 3 series at different temperatures using ultra-violet photoemission and inverse photoemission spectroscopy are discussed in Chapter 4. In Chapter 5, the electronic structure of Ca 0.86 Pr 0.14 MnO 3 at different temperatures using high resolution photoemission and x-ray absorption spectroscopy are discussed. Chapter 6 gives the summary and conclusions of this thesis. 1.2 Colossal magnetoresistance Colossal magnetoresistance is a property of some materials, which is the gigantic decrease of resistance by the application of magnetic field. The magnetoresistance can be defined as, MR = R(H) − R(0) , (1.1) R(0) where R(H) and R(0) are the resistances with and without magnetic field H respectively. Expressing the results as a percentage (i.e multiplying by an additional factor 100), it has been shown by Jin et. al. [10] that the MR value as large as -100,000
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: Chapter 1 Introduction 1
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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
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Experimental Techniques 53 Figure 2
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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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