Role of oxygen vacancies in tuning magnetic properties of Co-doped SnO2 insulating films

A series of films deposited under different oxygen partial pressures and annealed under different atmospheres were prepared to investigate the role of oxygen vacancies in tuning magnetic properties of Co-doped SnO2 films. The inclusive Co in SnO2 is in the 2+ state and substitutes for the Sn4+ site. Intrinsic room temperature ferromagnetism is observed in all films, which is not carrier mediated, but coexists with the dielectric behavior. A maximum magnetic moment of 2.37 mu(B)/Co is achieved by vacuum annealing due to the increase and diffusion of oxygen vacancies rather than the improvement of crystallinity of the film, and the magnetic moment decreases considerably after air annealing or increasing the oxygen partial pressure during deposition. The changes of oxygen vacancy concentration and distribution are indirectly demonstrated by the relative shifts of Co 2p(3/2) peaks in XPS spectra. The band gap of Co-doped SnO2 film is larger than that of pure SnO2 film, suggesting the influence of inclusive Co on the electronic states, and further blueshift after annealing is also visible. The F-center model is modified to explain the ferromagnetism in insulating Co-doped SnO2 films. (c) 2007 American Institute of Physics.