Observation of room temperature ferromagnetism in spray pyrolyzed polycrystalline Ti1-xCoxO2 thin films

摘要

We report here the room temperature ferromagnetic ordering in spray-pyrolized polycrystalline thin films of Ti1-xCoxO2 (x = 0, 0.025, 0.05 and 0.10). The saturation magnetic moment in these films with x = 0.05 and 0.10 is estimated to be of 0.28 and 0.38 mu(B)/Co, respectively. The ambient air employed in the present case of the spray pyrolysis process resulted in ferromagnetic films possessing very high resistivity values (similar to 10(7) Omega cm). This is in contrast to Ti1-xCoxO2 films deposited by vacuum based techniques, which are relatively conducting. While pristine TiO2 film exhibited the anatase phase, the rutile phase of TiO2 also evolved with an increase in Co concentration in these films. X-ray diffractograms could not reveal segregation of Co atoms in the TiO2 matrix; also, no formation of any oxide of cobalt was detected. X-ray photoelectron spectroscopic investigation established the Co cations to be bivalent, and suggests that Co+2 is substituting Ti+4 in the TiO2 matrix. Due to the highly resistive nature of these films, the observed room temperature ferromagnetism could be attributed either to the formation of bound magnetic polarons or to defect mediated ordering.