The annealing effects on the microstructure and magnetic properties of Zn0.92Co0.08O films annealed at different temperatures (200, 400 and 600 °C) in air and vacuum have been systematically investigated. The as-grown and annealed films are all highly textured with (002) orientation and all show room temperature ferromagnetism. The as-grown film is in a state of compressive stress and the film becomes almost stress-free after an annealing treatment at 400 °C in vacuum, while the film annealed at 600 °C goes into a state of tension. The magnetic moment increases after annealing in vacuum and decreases after annealing in air at the same temperature, compared with that of as-grown film. Moreover, the magnetic moment increases with the annealing temperature up to 400 °C and then shows a rapid decline at an annealing temperature of 600 °C in vacuum. The experimental results indicate that the enhancement of ferromagnetism is strongly correlated with the increase in oxygen vacancies and that the existence of interstitial Zn atoms plays an important role in mediating the ferromagnetism in Co-doped ZnO films.
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Liu et al. (2007) studied this question.