The role of transition metal impurities and oxygen vacancies in the formation of ferromagnetism in Co-doped TiO2

We have investigated the role of transition metal impurities and oxygen vacancies in the formation of ferromagnetism in Co-doped TiO2 using the LSDA + U approach which takes into account strong on-site Coulomb correlations for electronic structure calculations. Several model supercells of rutile TiO2, with a Co2+ ion in high-spin state substituted into the titanium site and with oxygen vacancies, were considered. We found that the exchange interaction of Co magnetic ions is ferromagnetic, but very weak due to the large average impurity-impurity distance. However, it becomes three times stronger when there is a magnetic vacancy nearby. The magnetic moment values are 3 mu(B) for Co2+ and -1 mu(B) for vacancy. Our investigation showed that the exchange interaction energy of Co and vacancy moments varies from 330 to 40 meV depending on the distance between them in the supercell. We assume that the strong interaction between Co and vacancy moments should be taken into account to explain the high Curie temperature value in Co-doped TiO2.