The solubility of Co in TiO2 anatase and rutile and its effect on the magnetic properties

Co-doped anatase and rutile bulk-samples prepared by the sol–gel technique are found to be paramagnetic at room-temperature. Only further annealing in Ar/H2 gas results in a ferromagnetic behavior. X-ray diffraction and electron-microscope studies reveal for low doping levels <4% the formation of Co-doped rutile samples and the formation of CoTiO3 as a new phase. Co3O4 can be detected in anatase samples with Co doping levels ⩾4%. The observed Co oxides are reduced by Ar/H2 to Co metal. The room-temperature ferromagnetism can therefore be traced back to a segregation of metallic Co.