  It is shown that dilute niobium doping has significant effect on the
ferromagnetism and microstructure of dilutely cobalt-doped anatase TiO2 films.
Epitaxial films of anatase TiO2 with 3% Co, without and with 1% niobium doping
were grown by pulsed-laser deposition at 875 C at different oxygen pressures.
For growth at 10^{-5} Torr niobium doping suppresses the ferromagnetism, while
it enhances the same in films grown at 10^{-4} Torr. High-resolution Z-contrast
Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy
show uniform surface segregation of cobalt-rich Ti_{1-x-y}Co_{x}Nb_{y}O_{2-d}
phase, but without cobalt metal clusters.
