Coexistence of intrinsic and extrinsic origins of room temperature ferromagnetism in as implanted and thermally annealed ZnO films probed by x-ray absorption spectroscopy

The present work reports the structural and ferromagnetic properties of 200 KeV Ni2+ ion implanted ZnO films before and after annealing at 650 °C in air and correlates these properties with their electronic structures. X-ray absorption spectroscopy (XAS) at Zn and Ni K-edges reveal that Zn metal cluster is absent in ZnO matrix, but there is an indication of clustering of Ni metal as a secondary phase. The XAS analysis of O K-edge shows (i) non-stoichiometric NiO segregation in as-implanted film and its absence/reduction in air annealed film, (ii) partial substitution of Ni2+ ions at tetrahedral Zn sites, and (iii) presence of lattice defects such as oxygen vacancies in both films. The observed ferromagnetic behavior of as-implanted and air annealed films has both intrinsic as well as extrinsic origins. The intrinsic and extrinsic origins are discussed in terms of bound magnetic polaron model and presence of antiferromagnetic NiO phase/ferromagnetic Ni metal clusters, respectively.