Room-temperature ferromagnetism was observed in the bulk CoxTi1-xO2-δ (0.06 ≤ x ≤ 0.1) samples synthesized by the solid-state reaction method for the mixed powder of Ti and Co oxides, followed by a 500C furnace annealing process in a 10% hydrogen-argon mixture of ambient gases. The X-ray diffraction, X-ray photoelectron spectroscopy, and measurements of magnetic susceptibility, χ, vs temperature, T, indicate polycrystalline Co-doped TiO2 anatase without Co clusters was fabricated. A phase transformation from CoTiO3 to CoxTi1-xO2-δ occurred after the hydrogenation process. These results were strong indications for the formation of oxygen vacancies near to the high spin Co2+ sites, and the formed oxygen vacancies were essential for the generation of room-temperature ferromagnetism in CoxTi1-xO2-δ. Based on these observations, ferromagnetism in bulk CoxTi1-xO2-δ anatase could be attributed to the exchange interaction between Co2+ mediated by oxygen vacancies near Co2+ sites, not being caused by Co clusters.

Oxygen-Vacancies-Related Room-Temperature Ferromagnetism in Polycrystalline Bulk Co-Doped TiO2. Kong, L.G., Kang, J.F., Wang, Y., Sun, L., Liu, L.F., Liu, X.Y., Zhang, X., Han, R.Q.: Electrochemical and Solid-State Letters, 2006, 9[1], G1-3