  We report first-principles microscopic calculations of the formation energy,
electrical activity, and magnetic moment of Co dopants and a variety of native
defects in TiO2 anatase. Using these results, we use equilibrium thermodynamics
to predict the resulting carrier concentration, the average magnetic moment per
Co, and the dominant oxidation state of Co. The predicted values are in good
agreement with experiment under the assumption of O-poor growth conditions. In
this regime, a substantial fraction of Co dopants occupy interstitial sites as
donors. The incomplete compensation of these donors by substitutional Co
acceptors then leads to n-type behavior, as observed experimentally.
