The annealing effects on structure and magnetism for Co-doped ZnO films under air, Ar, and Ar/ H 2 atmospheres at 250°C have been systematically investigated. Room-temperature ferromagnetism has been observed for the as-deposited and annealed films. However, the saturation magnetization ͑M s ͒ varied drastically for different annealing processes with M s ϳ 0.5, 0.2, 0.9, and 1.5 B / Co for the as-deposited, air-annealed, Ar-annealed, and Ar/ H 2-annealed films, respectively. The x-ray absorption spectra indicate all these samples show good diluted magnetic semiconductor structures. By comparison of the x-ray near edge spectra with the simulation on Zn K edge, an additional preedge peak appears due likely to the formation of oxygen vacancies. The results show that enhancement ͑suppression͒ of ferromagnetism is strongly correlated with the increase ͑decrease͒ of oxygen vacancies in ZnO. The upper limit of the oxygen vacancy density of the Ar/ H 2-annealed film can be estimated by simulation to be about 1 ϫ 10 21 cm −3 .

We herein report on the density-and temperature-dependent decay of the 93m Nb nuclear excited atom and present a simple interpretation of the underlying physics. This anomaly indicates nuclear resonant absorption and delocalisation of the long-lived Mössbauer state in the crystal. A non-linear magnetoelectric response, on low-frequency drive current, appeared in the bulk metal of a high-purity niobium crystal and then disappeared along with the disappearance of delocalised nuclear excitation. Several non-linear resonant peaks, of the order of several hundred Hertz, increased in magnitude with the applied magnetic field, and the central frequencies of these peaks decreased with temperature.

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