Room-Temperature Ferromagnetism in Co-Doped TiO₂ Anatase: Role of Interstitial Co

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TiO₂ anatase doped with Co has been recently reported to exhibit room-temperature ferromagnetism. Ab initio study on substitutional Co doping, however, yielded much larger magnetic moment for Co than experiment. Our calculations based on density-functional theory show that the substitutional Co ions incorporated into TiO₂ anatase tend to cluster and then the neighboring interstitial tetrahedral sites become energetically favorable for Co to reside, yielding a local environment more like Co₃O₄ than CoTiO₃. The interstitial Co destroys the spin-polarization of the surrounding substitutional Co but enhances the stability of the ferromagnetism significantly. In the absence of carriers, this room-temperature ferromagnetism can only be accounted for by superexchange interaction.