The origin of oxygen vacancy induced ferromagnetism in undoped TiO(2).

Using the full-potential linearized augmented plane wave method, we have investigated the oxygen vacancy defect induced ferromagnetism in both rutile and anatase TiO(2). It has been found that the oxygen vacancy induces lattice distortion in rutile TiO(2), whereas there is no such meaningful change in the anatase structure. Interestingly, the lattice distorted rutile TiO(2) shows an oxygen vacancy induced ferromagnetic state with a magnetic moment of 0.22 µ(B) in the Ti atom neighboring the vacancy site, while only 0.06 µ(B) is observed in the Ti atom in anatase TiO(2). We attribute the sizable magnetic moment due to the oxygen vacancy in rutile TiO(2) to the charge redistribution owing to lattice distortion. Experimentally measured magnetic hysteresis curves for undoped rutile and anatase TiO(2) films clearly display ferromagnetic behavior at room temperature. The observed magnetic strength of the rutile sample turns out to be larger than that of the anatase sample, in accordance with the theoretical calculations.