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Title:X-ray scattering studies of strongly correlated materials: orbital ordering in KCuF3 and stripe-like domain structures in BiFeO3 epitaxial thin films
Author(s):Lee, James
Director of Research:Abbamonte, Peter M.
Doctoral Committee Chair(s):Cooper, S. Lance
Doctoral Committee Member(s):Abbamonte, Peter M.; Phillips, Philip W.; Yang, Liang
Department / Program:Physics
Discipline:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):X-ray scattering
Soft x-ray scattering
Resonant x-ray scattering
Orbital ordering
KCuF3
Kugel-Khomskii
Ferroelectrics
Multiferroics
Bismuth ferrite (BiFeO3)
Abstract:The results of x-ray scattering experiments on orbitally ordered KCuF3 and periodic arrays of stripe-like ferroelectric domains in epitaxial thin films of multiferroic BiFeO3 are presented in this dissertation. Both resonant soft x-ray elastic scattering and non-resonant hard x-ray elastic scattering probes were used. Our experiments on KCuF3 have revealed a previously unidentified structural phase transition at T = 50 K, involving GdFeO3 like rotations of the CuF6 octahedra. These rotations are quasi-ordered and exhibit glassy hysteresis, but serve to stabilize the Neel state at T = 39 K. Based on these observations, we have formulated an orbital ordering model based on the archetypical Kugel-Khomskii model. Our modified Kugel-Khomskii model takes into account direct orbital exchange interactions due to a combination of electron-electron interactions and charge transfer effects. The effect of this term is to create a near degeneracy that dynamically frustrates the spin order that is lifted at low temperature by subdominant orbital-lattice interactions. A strong optical effect seen in the Cu L3 resonant scattering is also discussed. We also describe on-going studies of BiFeO3 epitaxial thin films. Based on non-resonant diffraction data, we have created simple charge density models that describe the structure of stripe-like ferroelectric domains with majority 109 degree and majority 71 degree domain walls. Resonant soft x-ray scattering near the Fe L3 transition edge has revealed a spatial modulation of predominantly Fe 3d valence states with a period equal to that of the ferroelectric domain structure. The possibility of a magnetic origin to the Fe L3 edge resonant scattering is discussed. No O K edge resonant scattering was observed, indicating that O 2p states are not significantly influenced by the domain structure. The absence of O K edge resonant scattering is also discussed in terms of proposed mechanisms for the anomalously high electrical conductivity of 109 degree domain walls. We also briefly describe the observation of scattering near the substrate peak due to the thin film domain structure, which is causing a modulation of the substrate strain.
Issue Date:2013-05-24
URI:http://hdl.handle.net/2142/44314
Rights Information:Copyright 2013 by James Lee.
Date Available in IDEALS:2013-05-24
Date Deposited:2013-05


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