Effects of high-temperature postannealing on magnetic properties of Co-doped anatase TiO_2 thin films

摘要

The origin of ferromagnetism in cobalt-doped anatase TiO_2 (Co:TiO_2) has been quite controversial due to the possible formation of Co nanoparticles. Recently, Shinde et al. claimed that high-temperature postannealing (i.e., at 900℃) should incorporate Co ions into the TiO2 matrix even from Co-clustered samples, thus proposing a new method to obtain intrinsic diluted magnetic semiconductors using Co:TiO_2 [Phys. Rev. B 67, 115211 (2003)]. In order to verify this intriguing possibility, we investigated the effects of high-temperature annealing on the magnetic and structural properties of two kinds of Co-doped anatase TiO_2 samples, one prepared by Co ion implantation and another by pulsed laser deposition with a Co-doped polycrystalline target. After postannealing under oxygen pressure of 1.0 X 10~(-6) Torr at 900℃, we observed drastic changes in the magnetic properties of both kinds of films. Although the Co nanoclusters, formed during the implantation, seemed to disappear inside the film, we found the formation of large clusters composed of metallic Co on the surface of the film using transmission electron microscopy. After annealing the samples prepared by pulsed laser deposition, similar large Co clusters were found at the interface. The results from x-ray-absorption near-edge spectroscopy and x-ray magnetic circular dichroism measurements on the postannealed film also demonstrated that the observed ferromagnetism in our films should come from Co metal.