Interplay of local structure and magnetism in Co-doped TiO2 anatase

We report ab initio density-functional theory investigations on the local structure and magnetization of Co ions doped in TiO2 anatase. The calculated formation energy of the pair of substitutional Co ions indicates that they have a tendency to cluster; but clustering has no noticeable effect on the low-spin state of Co. The interstitial Co, which is energetically unstable in reference to bulk cobalt, is found to be strongly attracted to a substitutional Co, and even more strongly to a substitutional Co pair. Interestingly, in a one-to-one binding, the interstitial Co enhances the magnetization of the two; whereas in a one-to-two binding, it destroys the magnetic moment of the substitutional Co pair and therefore reduces the average magnetic moment of Co ions. Our results could explain the strong sample-to-sample variability of the magnetic moment of Co measured in experiments. The magnetic interaction between substitutional and interstitial Co is discussed with bonding analysis.