The stabilization of magnetic glassy state in non-stoichiometric nickel ferrite thin films prepared by pulse laser deposition was studied. Details of electronic structure of the films were 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 was contrary to the usual ferrimagnetic response of the bulk nickel ferrite. Films exhibit spin freezing temperature which was above room temperature in low fields (0.1T) and shifts to lower temperature (∼250K) in the presence of a large applied field of 3T. An exceptionally large exchange bias (EB) of 170Oe at a significantly higher temperature (∼50K) was measured in cooling field of 3T. 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 was to induce spin canting by destabilizing indirect super exchange interaction. Consequently, the spin-glass like behavior occurs that was coupled with huge suppression in saturation magnetization in the thin films. Observation of exchange bias was explained to be due to oxygen vacancies (hence non-stoichiometry) which generates random anisotropy in exchange coupled grains.

Influence of Oxygen Vacancies on the Electronic Structure and Magnetic Properties of NiFe2O4 Thin Films. G.H.Jaffari, A.K.Rumaiz, J.C.Woicik, S.I.Shah: Journal of Applied Physics, 2012, 111[9], 093906