It had recently been shown that magnetism in this material often originated in surface nanoparticles or in Co-rich regions that had a much-enhanced substitutional Co content up to 40% of Ti sites. The Co-substituted material was not a true dilute magnetic semiconductor, but rather a high-density spin system. A computational study of the Co-rich material was performed by using the generalized gradient approximation to density functional theory within a super-cell model. The total-energy calculations revealed a strong tendency to Co-atom clustering or segregation at Ti sites. There was also a strong tendency for the O vacancies to form complexes with Co atoms. It was found that O stoichiometry played an essential role in determining the magnetic order of the system. The largest ordered moments required at least enough O vacancies to put all of the Co atoms into the +2 charge state; as they appeared to be experimentally. A small, but non-negligible, spin density was found which was associated with Ti atoms near to vacancy sites; suggesting that there was an F-center mediated interaction between the much larger Co moments.

Oxygen Vacancies and Ferromagnetism in CoxTi1-xO2-x-y. J.E.Jaffe, T.C.Droubay, S.A.Chambers: Journal of Applied Physics, 2005, 97[7], 073908 (6pp)