Carrier induced ferromagnetism in Nb doped Co:TiO₂ and Fe:TiO₂ epitaxial thin film

We have investigated the carrier induced magnetic properties of anatase Ti1-x-y Nbx My O2 (M=Co,Fe) epitaxial films grown by the pulsed laser deposition technique. For Ti0.95-x Nbx Co0.05 O2, the n -type carrier density could be controlled in a wide range (4.9× 1017 cm-3 to 2.7× 1021 cm-3) by Nb doping (x=0-0.2). The temperature dependence of the resistivity showed metallic behavior, suggesting that Ti0.95-x Nbx Co0.05 O2 undergoes a semiconductor to metal transition along with a slight carrier doping less than x<0.03. In both Co-doped and Fe-doped films, we have confirmed hysteresis in M-H curves, and the anomalous Hall effect at room temperature. This strongly suggests that the charge carriers are spin polarized and mediate ferromagnetic interaction between local spins on transition metal ions. In the case of Ti0.94-x Nbx Fe0.06 O2, ferromagnetism is sensitive to carrier concentration. That is, the x=0.002 film is nonmagnetic even at 3 K, while room-temperature ferromagnetism appears at x=0.01.