Negligible magnetism in excellent structural quality CrxTi1-xO2 anatase: Contrast with high-TC ferromagnetism in structurally defective CrxTi1-xO2

We reexamine the mechanism of ferromagnetism in doped TiO2 anatase, using epitaxial Cr: TiO2 with excellent structural quality as a model system. In contrast to highly oriented but defective Cr: TiO2 (∼0.5μB/Cr), these structurally superior single crystal films exhibit negligible ferromagnetism. Similar results were obtained for Co: TiO2. We show for the first time that charge-compensating oxygen vacancies alone, as predicted by F-center mediated exchange, are not sufficient to activate ferromagnetism. Instead, the onset of ferromagnetism correlates with the presence of structural defects. © 2005 The American Physical Society.