Annealing effects on the structure and magnetism of Co-doped ZnO films under air, Ar, and Ar/H2 atmospheres at 250C were investigated. Room-temperature ferromagnetism was observed in as-deposited and annealed films. However, the saturation magnetization varied markedly for differing annealing processes: with Ms ~ 0.5, 0.2, 0.9 and 1.5µB/Co for as-deposited, air-annealed, Ar-annealed and Ar/H2-annealed films, respectively. X-ray absorption spectra indicated that all of the samples exhibited good dilute magnetic semiconductor structures. Upon comparing the X-ray near-edge spectra with the simulated Zn K edge, an additional pre-edge peak appeared which was probably due to the formation of O vacancies. The results showed that enhancement (suppression) of ferromagnetism was strongly related with an increase (decrease) of O vacancies. The upper limit of the O-vacancy density in Ar/H2-annealed films was estimated, by simulation, to be about 1021/cm3.

Evidence of Oxygen Vacancy Enhanced Room-Temperature Ferromagnetism in Co-Doped ZnO. Hsu, H.S., Huang, J.C.A., Huang, Y.H., Liao, Y.F., Lin, M.Z., Lee, C.H., Lee, J.F., Chen, S.F., Lai, L.Y., Liu, C.P.: Applied Physics Letters, 2006, 88[24], 242507