It was noted that the nature of the defect responsible for room-temperature ferromagnetism in Co-doped anatase remained elusive. Here atomic-resolution Z-contrast images of epitaxially grown Co0.03:TiO2 films were reported which showed that Co atoms were dispersed and occupied preferred interstitial sites. Density-functional calculations showed that these sites were the lowest-energy configuration of Co in anatase and that O vacancies (VO), introduced by annealing, anchored the Co atoms into Co-Ti+3-VO complexes. The calculated magnetic moment was in agreement with measured values, as was the calculated Co X-ray absorption spectrum.

Defect-Mediated Ferromagnetism in Insulating Co-Doped Anatase TiO2 Thin Films. Roberts, K.G., Varela, M., Rashkeev, S., Pantelides, S.T., Pennycook, S.J., Krishnan, K.M.: Physical Review B, 2008, 78[1], 014409