SURFACE SPIN DISORDER IN FERRITE NANOPARTICLES (INVITED)

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.