Influence of oxygen vacancies on the electronic structure and magnetic properties of NiFe2O4 thin films

We report stabilization of magnetic glassy state in non-stoichiometric nickel ferrite thin films prepared by pulse laser deposition. Details of electronic structure of the films are presented and compared with stoichiometric bulk counterpart. Hard x-ray photoelectron spectroscopy shows significant amount of oxygen vacancies and enhanced cationic inversion for thin films. Films show spin glass (SG) features which is contrary to the usual ferrimagnetic response of the bulk nickel ferrite. Films exhibit spin freezing temperature which is above room temperature in low fields (0.1 T) and shifts to lower temperature (∼250 K) in the presence of a large applied field of 3 T. An exceptionally large exchange bias (EB) of 170 Oe at a significantly higher temperature (∼50 K) is measured in cooling field of 3 T. In comparison, bulk samples do not show exchange bias and magnetic irreversibility vanishes in significantly weaker fields (i.e., few kOe). Role of oxygen vacancies is to induce spin canting by destabilizing indirect super exchange interaction. Consequently, the spin-glass like behavior occurs that is coupled with huge suppression in saturation magnetization in the thin films. Observation of exchange bias is explained to be due to oxygen vacancies (hence non-stoichiometry) which generates random anisotropy in exchange coupled grains.