Local structure of amorphous EuO-TiO ₂thin films probed by x-ray absorption fine structure

We have investigated the local structure of amorphous oxides in EuO-TiO ₂system to understand the mechanism that gives rise to curious ferromagnetic properties recently observed in the system. X-ray absorption spectroscopy has been performed on amorphous EuTiO ₃and Eu ₂TiO ₄thin films prepared by the pulsed-laser-deposition method. The Ti K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data analyses reveal that the coordination number of Ti ⁴⁺ in the amorphous thin films is around 4, which is in sharp contrast to their crystalline counterparts, where Ti ⁴⁺ ions are octahedrally coordinated by oxide ions. It is therefore inferred that Ti ⁴⁺ acts as a network-forming cation in the random network structure of the amorphous oxides. From the Eu L ₃-edge EXAFS data analyses, it is found that the coordination number of Eu ²⁺ is much lower and the nearest Eu-O bond length is shorter in the amorphous thin films than in the crystalline counterparts. The amorphization-induced enhancement of ferromagnetic interactions observed in the EuO-TiO ₂system is discussed in terms of the local environment around Eu ²⁺ ions.