Anomalous magnetic properties of organic coated NiFe2O4 nanoparticles
have been reported previously (Berkowitz et al.).(5) These properties
included low magnetization with a large differential susceptibility at
high fields and shifted hysteresis loops after field cooling, while M
ossbauer spectra indicated that all of the material was magnetically o
rdered. In the present study, we find that the lack of saturation in h
igh fields is accompanied by irreversibility (i.e., hysteresis loops a
re open) up to 160 kOe. In addition, the particles exhibit time depend
ent magnetization in 70 kOe applied field. The high field irreversibil
ity and the loop shift both vanish above 50 K. We propose a model of t
he magnetization within these particles consisting of ferrimagneticall
y aligned core spins and a spin-glass-like surface layer. We find that
qualitative features of this model are reproduced by a numerical calc
ulation of the spin distribution. The implications of this model for p
ossible macroscopic quantum tunneling in these materials are discussed
. (C) 1997 American Institute of Physics.