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

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 Co_xTi_1-x-yNb_yO_2-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. [ABSTRACT FROM AUTHOR]