Enhanced ferromagnetism in Co-doped TiO2 powders

We studied the structural and magnetic properties of T1-xCoxO2 powder samples synthesized by the sol-gel method. In order to study the post annealing effect on the magnetic properties, the same precursor was annealed at 770 K for 5 h in air and in the pressure of 10(-6) Torr. In the case of samples annealed in vacuum, enhanced ferromagnetism was observed at room temperature, while only paramagnetic behavior was observed in samples annealed in air. Moreover, the magnetization of the vacuum-annealed sample sharply decreased around 650 K while measuring in air, but such decrease in the magnetization was almost absent while measuring in He gas up to 750 K. The structural studies showed dominant anatase and minor rutile phases in vacuum-annealed samples, while an additional nonmagnetic CoTiO3 phase was observed in air-annealed samples. We attribute the enhanced magnetization in the vacuum-annealed sample to the oxygen deficiency which prevents the formation of non-magnetic CoTiO3 complexes. It is also possible that the reduced oxygen environment can promote the clustering of Co-rich clusters as evidenced by transmission electron microscopy in samples of x = 0.1, thus giving rise to an enhanced ferromagnetic behaviour. However, no direct observation of Co clusters in samples for x less than or equal to 0.05 was made. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.