Nucleation, growth, electronic and magnetic properties of epitaxial Co-doped TiO2 anatase

Co-doped TiO₂ anatase (Co_xTi_1-xO_2-x), as grown by oxygen plasma assisted molecular beam epitaxy, has been shown to be strongly ferromagnetic up to ∼700 K. The robust magnetism is tied to the presence of a secondary phase of Co-enriched anatase, which appears as nano- and mesoscale epitaxial particles that nucleate on the surface of continuous anatase epitaxial films. Co in both secondary (concentrated) and primary (dilute) phases substitutes for Ti, and is in the +2 formal oxidation state. The magnetization is proportional to the n-type conductivity, suggesting electron mediated exchange interaction. This material may thus be useful in the rapidly developing field of spintronics for which ferromagnetic semiconductors with Curie points well above room temperature are critically important.