Structural, electronic, and magnetic properties of a ferromegnetic semiconductor: Co-doped TiO$_2$ rutile

TL;DR

This study uses ab initio density-functional theory to explore the properties of Co-doped TiO2 rutile, revealing clustering tendencies, site preferences, and effects on magnetic moments relevant for understanding room-temperature ferromagnetism.

Contribution

It provides new insights into the local environment and magnetic interactions of Co ions in TiO2 rutile, highlighting the importance of interstitial Co in magnetic behavior.

Findings

Co ions tend to cluster and prefer interstitial sites.

Interstitial Co in low spin state reduces overall magnetic moments.

Substitutional Co alone is insufficient to explain ferromagnetism.

Abstract

Room-temperature ferromagnetism has been recently discovered in Co-doped TiO$_2$ rutile. Our $ab$ $initio$ density-functional theory investigations show that the substitutional Co ions incorporated into 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 is not an appropriate reference bulk system in derterming the local environment of Co in polycrystalline (Ti,Co)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, hence reduces the average magnetic moment of impurity atoms.