Magnetic properties of epitaxial Co-doped anatase TiO2 thin films with excellent structural quality

Abstract

The heteroepitaxy of Co-doped anatase TiO2 on LaAlO3(001) has been refined with the goal of determining the relationship between structural quality and magnetic ordering. By significantly reducing the deposition rate and substrate temperature, well-ordered Co:TiO2 films with high crystalline quality were obtained by oxygen-plasma-assisted molecular beam epitaxy, as characterized by x-ray diffraction. These films exhibit uniform Co doping, with no evidence of Co segregation or secondary phases throughout the film depth or on the surface. Despite the improvement in crystalline quality and Co distribution, the films exhibit negligible ferromagnetism, with saturation moments of only ∼0.1μB∕Co. This loss of ferromagnetism is in stark contrast to faster-grown Co:TiO2 films, where higher growth rate and substrate temperature typically result in lower crystalline quality, a highly nonuniform Co distribution, and average saturation moments of ∼1.2μB∕Co. The presence of ferromagnetism in faster-grown Co:TiO2 does not appear to arise from intrinsic point defects present in the bulk material, such as charge-compensating oxygen vacancies, but is instead attributed to the presence of extended structural defects.