Structural, electronic, and magnetic properties of a ferromegnetic semiconductor: Co-doped TiO₂ rutile

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Room-temperature ferromagnetism has been recently discovered in Co-doped TiO₂ rutile. Our ab initio density-functional theory investigations show that the substitutional Co ions incorporated into TiO₂ rutile tend to cluster and then the neighboring interstitial sites become energetically favorable for Co to reside. This suggests that a Co-doped rutile containing only substitutional Co is not an appropriate reference bulk system in derterming the local environment of Co in polycrystalline (Ti,Co)O₂ rutile. We also find that the interstitial Co is in the low spin state and destroys the spin-polarization of the surrounding substitutional Co, hence reduces the average magnetic moment of impurity atoms.