Structural, electronic, and magnetic properties of a ferromagnetic semiconductor: Co-dopedTiO2rutile

The local environment around magnetic impurity atoms was reported to be unchanged in a structural phase transition of nanocrystalline Co-doped ferromagnetic ${\mathrm{TiO}}_2.$ Our ab initio density functional theory investigations show that the substitutional Co ions incorporated into ${\mathrm{TiO}}_2$ 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 may not be an appropriate reference bulk system in determining the local environment of Co in polycrystalline $(\mathrm{T}\mathrm{i},\mathrm{C}\mathrm{o}){\mathrm{O}}_2$ rutile. We also find that the interstitial Co is in the low spin state and destroys the spin polarization of the surrounding substitutional Co, and hence reduces the average magnetic moment of impurity atoms.