Room-Temperature Ferromagnetism in Co-Doped TiO$_2$ Anatase: Role of Interstitial Co

TL;DR

This study uses density-functional theory to show that interstitial Co in Co-doped TiO$_2$ anatase stabilizes room-temperature ferromagnetism through superexchange interactions, explaining experimental observations.

Contribution

It reveals the crucial role of interstitial Co in stabilizing ferromagnetism, which was not captured by previous substitutional-only models.

Findings

Interstitial Co favors clustering and local environment similar to Co$_3$O$_4$.

Interstitial Co destroys spin-polarization of neighboring substitutional Co.

Room-temperature ferromagnetism is explained by superexchange interaction without carriers.

Abstract

TiO$_2$ anatase doped with Co has been recently reported to exhibit room-temperature ferromagnetism. $Ab$ $initio$ study on substitutional Co doping, however, yielded much larger magnetic moment for Co than experiment. Our calculations based on density-functional theory show that the substitutional Co ions incorporated into TiO$_2$ anatase tend to cluster and then the neighboring interstitial tetrahedral sites become energetically favorable for Co to reside, yielding a local environment more like Co$_3$O$_4$ than CoTiO$_3$. The interstitial Co destroys the spin-polarization of the surrounding substitutional Co but enhances the stability of the ferromagnetism significantly. In the absence of carriers, this room-temperature ferromagnetism can only be accounted for by superexchange interaction.