X-Ray Magnetic Circular Dichroism and Structural Model for Co-Doped TiO₂ (Anatase) Thin Film

Abstract

We have succeeded in measuring fluorescence X-ray magnetic circular dichroism (XMCD) and absorption spectra at the Co K absorption edge for a small amount of doped Co ions in a Co_xTi_1−xO₂ anatase film (x=0.07; 700-Å-thickness) on a LaAlO₃ (001) substrate. X-ray fluorescence intensity was accumulated for 3 d at the pseudo-total-reflection geometry in a synchrotron undulator beamline, BL-39XU at SPring-8. Negative-to-positive dispersion-type XMCD spectra were clearly observed at E=7.725 keV, suggesting the ferromagnetism at room temperature and unlikelihood on the existence of Co-metal clusters. XANES spectra support the Co²⁺ oxidation state and participation of a partial structure of ilmenite-type CoTiO₃. We have proposed a crystal structure of Co-doped anatase films and the ferromagnetic order scheme by Co–Ti–O polysynthetic twin lamellae. A (012) CoTiO₃ layer interstitially distributes parallel to the (001) plane of anatase and replaces a quarter of the anatase cell at intervals of three and half times of the cell along the c axis.