  Anatase Ti0.96Co0.04O2 films were grown epitaxially on SrTiO3 (001)
substrates by using plused laser deposition with in-situ reflection high-energy
electron diffraction. The oxygen partial pressure, PO2, during the growth was
systematically varied. As PO2 decreased, the growth behavior was changed from a
2-dimensional layer-by-layer-like growth to a 3-dimensional island-like one,
which resulted in an increase in the saturation magnetization. These structural
and magnetic changes were explained in terms of the formation of cobalt
clusters whose existence was proved by transmission-electron-microscopie
studies. Our work clearly indicates that the cobalt clustering will cause
room-temperature ferromagnetism in the Co-doped TiO2 films.
