Surface effects on the exchange bias and magnetic irreversibility in hollow polycrystalline NiFe2O4 nanoparticles

We studied the effect of the presence of inner surface on polycrystalline hollow single oxide nanoparticles of NiFe2O4 via extensive field and temperature dependent magnetic. Hollow morphology was obtained by post-synthesis annealing of Ni33Fe67/NiFe2O4 core/shell particles, prepared in an inert gas condensation system. High resolution electron microscopy studies revealed that the hollow nanoparticles were polycrystalline. Particles, as well as voids, exhibited size distribution. High values of up to 2522 Oe coercivity (HC) and 444 Oe exchange bias (EB) fields were measured at low temperatures. Through observation of low saturation magnetization, larger HC and the presence of EB show drastically different magnetic response when compared to the bulk counterpart and solid single oxide nanoparticles. Results showed presence of strong coupling between the ferrimagnetic (FiM) and the disordered surface spins that arises due to enhancement in the surface to volume ratio associated with the morphology of the particles. Coercivity and exchange bias were found to decrease as a function of temperature and persisted up to 80 K, where their magnitudes were 1128 Oe and 16 Oe, respectively.