SURFACE SPIN DISORDER IN NIFE2O4 NANOPARTICLES

Nickel ferrite nanoparticles exhibit anomalous magnetic properties at low temperatures: low magnetization with a large differential suscepti bility at high fields, hysteresis loops which are open up to 160 kOe, time-dependent magnetization in 70 kOe applied field, and shifted hyst eresis loops after field cooling. We propose a model of the magnetizat ion within these particles consisting offerrimagnetically aligned core spins and a spin-glass-like surface layer. We find that qualitative f eatures of this model are reproduced by a numerical calculation of the spin distribution. Implications of this model for possible macroscopi c quantum tunneling in these materials are discussed.