Probing origin of room temperature ferromagnetism in Ni ion implanted ZnO films with x-ray absorption spectroscopy

We report x-ray absorption at Zn and Ni K-edges in 200 keV Ni2þ ion implanted ZnO/sapphire films. The implantation fluences are 6×1015 and 2×1016 ions/cm2, corresponding to 2% and 7% Ni in a ZnO matrix. The measurements reveal a marginal substitution of Ni in ZnO in both the films and also rule out the presence of ferromagnetic Ni metal clusters. The M-H and field cooled-zero field cooled measurements performed via SQUID magnetometry show that the films are ferromagnetic at room temperature, and the saturation magnetization of 2% Ni film is appreciably higher than that of 7% Ni film. The origin of ferromagnetism is understood on the basis of the oxygen vacancy mediated bound magnetic polaron model.