Spectroscopic study of transition metal compounds.

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.