An investigation was made of the role of transition metal impurities and O vacancies, in the formation of ferromagnetism in Co-doped TiO2, by using the LSDA+U approach which took account of strong on-site Coulomb correlations for electronic structure calculations. Several model super-cells of rutile TiO2, with a Co2+ ion in a high-spin state substituted at a Ti site and with O vacancies, were considered. It was found that the exchange interaction of Co magnetic ions was ferromagnetic, but very weak due to the large average impurity–impurity distance. However, it became 3 times stronger when there was a nearby magnetic vacancy. The magnetic moment values were 3μB for Co2+ and -1μB for a vacancy. The investigation showed that the exchange interaction energy of Co and vacancy moments varied from 330 to 40meV; depending upon the distance between them in the super-cell. It was assumed that the strong interaction between Co and vacancy moments should be taken into account in order to explain the high Curie temperature in Co-doped TiO2.

The Role of Transition Metal Impurities and Oxygen Vacancies in the Formation of Ferromagnetism in Co-Doped TiO2. Anisimov, V.I., Korotin, M.A., Nekrasov, I.A., Mylnikova, A.S., Lukoyanov, A.V., Wang, J.L., Zeng, Z.: Journal of Physics - Condensed Matter, 2006, 18, 1695-704