Ferromagnetism in Co doped anatase TiO$_{2}$ thin films mediated by Co-Ti$^{+3}$-V$_{O}$ complexes

The correction of spherical aberration in the STEM has enabled sub-Angstrom imaging and spectroscopy, and, in favorable cases, direct imaging of light atoms and interstitials. We identify the origin of ferromagnetism in Co$_{0,03}$:TiO$_{2}$ anatase thin films by combining STEM, EELS and DFT calculations. The films are insulating and ferromagnetic at room temperature. Ferromagnetism is enhanced by a post growth vacuum annealing suggesting a defect-mediated mechanism in these films. DFT finds interstitial Co to be energetically preferred over substitutional Co. STEM imaging reveals the interstitials in the predicted sites, and EELS finds reduced Ti in adjacent columns, also predicted by DFT. The combination of STEM-EELS-DFT therefore identifies the defect responsible for the magnetism: an O vacancy binds to the interstitial Co to form a Co-Ti$^{+ 3}$-V$_{O}$ complex, with a magnetic moment in good agreement with the observed value. Research sponsored by Div. of Materials Sciences and Engineering US DOE, and NSF/ECS 0224138.