Temperature dependent coercivity and magnetization of nickel ferrite nanoparticles

Magnetic nanoparticles of nickel ferrite (size: 24 +/- 4 nm) have been synthesized by chemical coprecipitation method using stable ferric and nickel salts. Coercivity of nanoparticles has been found to increase with decrease in temperature of the samples. It has been observed that the coercivity follows a simple model of thermal activation of particle's moment over the anisotropy barrier in the temperaturerange (10-300K), in accordance with Kneller's law for ferromagnetic materials. Saturation magnetization follows the modified Bloch's law in the temperature range from 300 to 50 K. However, below 50K, an abrupt increase in magnetization of nanoparticles was observed. This increase in magnetization at lower temperatures was explained with reference to the presence of freezed surface-spins and some paramagnetic impurities at the shell of nanoparticles that a reactivated at lower temperatures in core-shell nickel ferrite nanoparticles. (C) 2010 Elsevier B.V. All rights reserved.