Based upon a series of super-cell density functional calculations of Fe-doped TiO2, with or without O vacancies (VO), it was shown that VO played an important role in determining the magnetic properties of dilute magnetic semiconductors. Without VO, two Fe atoms in the rutile lattice were ferromagnetically coupled, except at a separation distance of 3.57Å; where they were antiferromagnetically coupled. The VO introduced 2 electrons into the conduction bands of rutile. These were either captured by the Fe dopants, or formed a shallow impurity state. The ferromagnetic coupling between two Fe atoms was enhanced, through enhancement of the ferromagnetic double exchange, if the VO were sufficiently close.

Vacancy-Enhanced Ferromagnetism in Fe-Doped rutile TiO2. Chen, J., Rulis, P., Ouyang, L., Satpathy, S., Ching, W.Y.: Physical Review B, 2006, 74[23], 235207