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      Spectroscopic study of transition metal compounds.

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      Final_Thesis.pdf (1018.Kb)
      Date
      2010-06
      Author
      Choudhury, Sanjukta
      Type
      Thesis
      Degree Level
      Masters
      Metadata
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      Abstract
      The electronic structure of some transition metal compounds, specifically, Ca-doped LaMnO₃, fundamental Mn oxides (MnO, Mn₂O₃, Mn₃O₄, and MnO₂), and Fe-doped ZnO is studied using a combination of soft X-ray spectroscopy and atomic multiplet calculations. X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) are used as experimental tools to probe the unoccupied and occupied partial density of electronic states, respectively. Ca-doped LaMnO₃ perovskites have attracted great attention due to their colossal magnetoresistance and a wide range of magnetic and structural transitions. The magnetic and charge transport properties of these perovskites are directly related with Mn 3d-occupancy or Mn-valency and therefore, an investigation of the Mn-valence at Ca-doped LaMnO₃ system is important. In this system, the Mn-valency is generally considered as a mixture of Mn³⁺ and Mn⁴⁺. But my research suggests the presence of Mn2+ at the surface of Ca-doped LaMnO₃ samples. It is observed that increasing Ca-doping decreases Mn2+ concentration, and conversely, increases Mn³⁺ concentration. High temperature annealing at 1000 °C in air leads to the full reduction of surface Mn2+. Mechanisms for these observations are proposed in this study. Mn oxides (MnO, Mn₂O₃, Mn₃O₄, and MnO₂) are often used as reference standards for determining the Mn-valency in Mn-related complex systems and therefore a detailed understanding of their electronic structure is necessary. The Mn L₂,₃ XAS and O K XAS are measured for the four Mn oxides consisting of three common Mn oxidation states (Mn2+ in MnO, Mn³⁺ in Mn₂O₃, mixture of Mn2+ and Mn³⁺ in Mn₃O₄, and Mn⁴⁺ in MnO₂). A significant energy shift with a systematic trend is observed in measured Mn L₂,₃ and O K absorption edges. These energy shifts are identified as a characteristic shift for different Mn oxidation states. Mn L₂,₃ Resonant Inelastic X-ray Scattering (RIXS) spectroscopy is demonstrated as a powerful tool in describing low energy excitations, e.g. d-d excitations and charge-transfer excited states in Mn oxides. For the first time, a RIXS study of Mn₂O₃, Mn₃O₄, and MnO₂ is accomplished. Atomic multiplet calculations are used to successfully reproduce the energy positions and intensity variations of d-d excitation peaks observed in the experiment, and thus to describe the experimental RIXS spectra. Finally, the local electronic structure of Fe implanted ZnO samples, a useful diluted magnetic semiconductor for spintronics, is investigated to shed light on the existing debate about the origin of ferromagnetism in these materials. Fe L₂,₃ XAS reveals that doped Fe ions are present in both Fe²⁺ and Fe³⁺ valence states. A combined theoretical and experimental study shows that doped ions are incorporated into Zn-sites of ZnO in tetrahedral symmetry. Fe L₃-RIXS measurements demonstrate that a high Fe-ion dose of 8 × 10⁷ cm-2 causes formation of FeO clusters, while low dose samples exhibit more free carriers.
      Degree
      Master of Science (M.Sc.)
      Department
      Physics and Engineering Physics
      Program
      Physics and Engineering Physics
      Supervisor
      Moewes, A.; Chang, Gap Soo
      Committee
      Koustov, S.; Mueller, J.; McWilliams, K.; Tse, J.
      Copyright Date
      June 2010
      URI
      http://hdl.handle.net/10388/etd-08212010-132458
      Subject
      Mn oxides
      Ca-doped LaMnO3
      Atomic multiplet theory
      X-ray absorption spectroscopy
      Fe-implanted ZnO
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