XPS depth profiles were used to investigate the effects of rapid thermal annealing under varying conditions on the structural, magnetic and optical properties of Ni-doped ZnO thin films. Oxidization of metallic Ni from its metallic state to two-valence oxidation state occurred in the film annealed in air at 600 degrees C, while reduction of Ni2+ from its two-valence oxidation state to metallic state occurred in the film annealed in Ar at 600 and 800 degrees C. In addition, there appeared to be significant diffusion of Ni from the bottom to the top surface of the film during annealing in Ar at 800 degrees C. Both as-deposited and annealed thin films displayed obvious room temperature ferromagnetism (RTFM) which was from metallic Ni, Ni2+ or both with two distinct mechanisms. Furthermore, a significant improvement in saturation magnetization (M-s) in the films was observed after annealing in air (M-s - 0.036 mu B/Ni) or Ar (M-s - 0.033 mu B/Ni) at 600 degrees C compared to that in as-deposited film (M-s = 0.017 mu B/Ni). An even higher Ms value was observed in the film annealed in Ar at 800 degrees C (M-s = 0.055 mu B/Ni) compared to that at 600 degrees C mainly due to the diffusion of Ni. The ultraviolet emission of the Ni-doped ZnO thin film was restored during annealing in Ar at 800 degrees C, which was also attributed to the diffusion of Ni. (C) 2011 Elsevier B.V. All rights reserved.