X-ray Magnetic Circular Dichroism Study of Room Temperature Ferromagnetism in Co-doped TiO2 Thin Films
Room temperature ferromagnetic semiconductors are necessary for semiconductor spintronics. Some wide gap semiconductors such as oxide doped with transition metal ions have been reported to show ferromagnetism at room temperature. However, their magnetic as well as electronic properties have not been established in comparison with the well characterized (Ga,Mn)As. Among the room temperature ferromagnetic semiconductors reported so far, (Ti,Co)O2  has been most extensively studied. There has been a long debate about the origin of the room temperature ferromagnetism in Co-doped TiO2. Magnetization measurements as a function of carrier density can give direct information about the origin of carrier-induced ferromagnetism at room-temperature.
In this study, we have evaluated the magnetization in Co-doped TiO2 quantitatively for various carrier densities by means of XMCD, which can extract an atom-selective magnetization. We have demonstrated that the magnetization obtained by XMCD shows a monotonical increase with carrier density, consistent with a carrier-mediated mechanism . In addition, we found that the surface magnetization is significantly suppressed in comparison with the bulk magnetization by using the surface-sensitive total electron yield (TEY) mode and the bulk-sensitive total fluorescence yield (TFY) mode [2,3]. This result points to the presence of magnetically dead layer of ~5 nm thickness at the surface. Also, random crystal fields implied by a previous resonant x-ray absorption study and our XMCD results will provide important information for understanding the exchange coupling in real space.