Surface spin disorder in ferrite nanoparticles (invited)

Abstract

Anomalous magnetic properties of organic coated ${\mathrm{NiFe}}_2{\mathrm{O}}_{\mathrm{4}}$ 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 Mössbauer spectra indicated that all of the material was magnetically ordered. In the present study, we find that the lack of saturation in high fields is accompanied by irreversibility (i.e., hysteresis loops are open) up to 160 kOe. In addition, the particles exhibit time dependent magnetization in 70 kOe applied field. The high field irreversibility and the loop shift both vanish above 50 K. We propose a model of the magnetization within these particles consisting of ferrimagnetically aligned core spins and a spin- glass-like surface layer. We find that qualitative features of this model are reproduced by a numerical calculation of the spin distribution. The implications of this model for possible macroscopic quantum tunneling in these materials are discussed.