Abstract
The crystal and electronic structure of multiferroic o-YMnO3 polycrystals prepared under high pressure and temperature were investigated as a function of temperature up to 873 K and external pressure up to 35 GPa by synchrotron x-ray diffraction (SXRD), high-resolution x-ray absorption spectra (XAS) at Mn K-edge, 1s3p resonant x-ray emission spectra (RXES) at Mn K-edge, and Mn K-edge XAS with full multiplet-scattering calculations using FDMNES code. No phase transition in metastable o-YMnO3 up to pressure 25 GPa or temperature 873 K was observed. The Jahn-Teller (JT) reduction at high temperature is mainly caused by the increasing Mn-O-Mn bond angle or the reduction of MnO6 octahedra distortion, whereas JT reduction at high pressure is dominated by the shortening of long MnO bond in MnO6 octahedra. The reduction of JT distortion at high temperature or pressure will result in larger oxygen-mediated Mn 4p‒Mnʹ 3d intersite hybridization and thus will have a significant influence on the physical properties of o-YMnO3. Moreover, a temperature- or pressure-induced metallic behavior is enclosed in o-YMnO3. In the last part of the thesis, Fe-substituted o-YMn1-xFexO3(020) (for x=0.125, 0.25, 0.50) epitaxial thin films on substrate YAlO3(010) were investigated by the measurements of single crystal XRD, magnetization as a function of temperature and magnetic field, the polarization-dependent XAS at the O K-edge with LDA+U calculations, and the polarization-dependent XAS at the Mn-L2,3 and Fe-L2,3 edges with the configuration-interaction multiplet calculations. Upon Fe substitution, the b-axis is clearly decreased, while a- and c-axis are slightly increased. It is found that although Mn3+-Δeg is significantly reduced from 0.9 eV in pure o-YMnO3 to 0.6 eV in the half Fe-substituted film, a single eg electron is still strongly constrained to d3y2-r2 orbital. On the other hand, the largest Fe3+-Δeg of 0.5 eV is derived for 12.5% Fe concentration and gradually reduced to 0.15 eV in the half Fe-substituted film. Besides, the local octahedral-site distortion of Fe3+ ion inside YMnO3 lattice exhibits the similar way as that in Mn3+ ion, while JT distortion and GdFeO3-type distortion of Mn3+ ions are reduced by the spherical high spin Fe3+ ions. Moreover, the speculation of G-type magnetic structure is favorable in o-YMn0.5Fe0.5O3 thin film. The comprehensive results provide deeper insights of the variation of JT distortion and orbital anisotropy, as well as intriguing magnetic structures in the studied o-YMn1-xFexO3 thin films.