Defects in Co-doped and (Co, Nb)-doped TiO2 ferromagnetic thin films

Abstract

We have investigated the defect structure in pure Co-doped and (Co, Nb)-doped TiO2 (anatase) thin films grown by pulsed laser deposition on (001) single crystal LaAlO3. Although both films exhibit room temperature ferromagnetism, dilute Nb doping significantly improves the conductivity and microstructure of the TiO2 (anatase) thin film at the cost of a lower saturation magnetization. Z-contrast imaging and electron-energy-loss-spectroscopy study in the scanning transmission electron microscope show cluster-free microstructure in the (Co, Nb)-doped thin film although Co is enriched into the surface and forms a CoxTi1−x−yNbyO2−δ phase. In contrast, metallic Co, rutile TiO2, and cobalt oxide nanoparticles combined with some crystallographic shear defect structures are observed in the pure Co-doped thin film. The magnetic behavior in each film can be explained with the presence of these defect states or second phases.