Nanoparticles of Co-doped TiO2 were synthesized using a simple sol–gel route taking 7.5, 9.5 and 10.5 mol% of cobalt concentration. Formation of nanoparticles was confirmed by X-ray diffraction and transmission electron diffraction. An increase in d-spacing occurred for (004) and (200) peak, with an increase in impurity content. Valence states of Co and its presence in the doped material was confirmed by XPS and EDX. The entire vacuum annealed samples showed weak ferromagnetism. Increased magnetization was found for 9.5mol% but this value again decreased for 10.5mol% due to antiferromagnetic interactions. A blocking temperature of 37.9K was obtained, which revealed a shift to high temperature as the dopant concentration was increased. The air annealed sample showed only paramagnetic behavior. Temperature dependent magnetic measurements for the air annealed sample showed antiferromagnetic behavior with a Curie–Weiss temperature of −16K. It was reported that oxygen vacancy and cobalt aggregates were a key factor for inducing ferromagnetism–superparamagnetism in the vacuum annealed sample. Appearance of negative Curie–Weiss temperature revealed the presence of antiferromagnetic Co3O4, which was the oxidation result of metallic Co or cobalt clusters present on the host TiO2.

Effect of Oxygen Vacancy and Dopant Concentration on the Magnetic Properties of High Spin Co2+ Doped TiO2 Nanoparticles. B.Choudhury, A.Choudhury, A.K.M.Maidul Islam, P.Alagarsamy, M.Mukherjee: Journal of Magnetism and Magnetic Materials, 2011, 323[5], 440-6