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.