Niobium doping effects on the ferromagnetism and microstructure of anatase Co: TiO$_{2}$ films*

Niobium doping is shown to have a significant effect on the ferromagnetism and microstructure of dilutely cobalt-doped anatase TiO$_{2}$ films. Epitaxial films of anatase TiO$_{2}$ with 3{\%} Co, with and without 1{\%} niobium doping were grown by pulsed-laser deposition at 875 $^{0}$C at different oxygen pressures. For growth at 10$^{-5 }$Torr Niobium doping suppresses ferromagnetism, while it enhances the same in films grown at 10$^{-4 }$Torr. The results of High-resolution Z-contrast Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy study reveal uniform surface enrichment of cobalt in the form of Ti$_{1-x-y}$Co$_{x}$Nb$_{y}$O$_{2-\delta }{\rm g}$ phase, without cobalt metal clusters. The transport and Hall effect results will also be presented and discussed.