Electronic structures of doped anatase TiO₂: Ti₁₋ₓMₓO₂ (M=Co, Mn, Fe, Ni)

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We have investigated electronic structures of a room temperature diluted magnetic semiconductor: Co-doped anatase TiO₂. We have obtained the half-metallic ground state in the local-spin-density approximation(LSDA) but the insulating ground state in the LSDA+U+SO incorporating the spin-orbit interaction. In the stoichiometric case, the low spin state of Co is realized with the substantially large orbital moment. However, in the presence of oxygen vacancies near Co, the spin state of Co becomes intermediate. The ferromagnetisms in the metallic and insulating phases are accounted for by the double-exchange-like and the superexchange mechanism, respectively. Further, the magnetic ground states are obtained for Mn and Fe doped TiO₂, while the paramagnetic ground state for Ni-doped TiO₂.