Surface spin freezing of ferrite nanoparticles evidenced by magnetization measurements

Magnetization and in-field Mossbauer measurements were performed on copper ferrite nanoparticles with average sizes ranging from 3.5 to 10.4 nm. Our results show that the nanoparticles are well-crystallized single domains with a magnetically disordered surface shell. A sharp increase in the saturation magnetization at low temperatures, in addition to the usual modified Bloch behavior, was observed for the smallest particles. This jump in magnetization curves seems to be related to the freezing of the surface spins below a temperature of about 45 K. (C) 2006 American Institute of Physics.