Magnetic response of core-shell cobalt ferrite nanoparticles at low temperature

Cobalt ferrite nanoparticles (size: 26 +/- 4 nm) have been synthesized by coprecipitation route. The coercivity of nanoparticles follows a simple model of thermal activation of particle moments over the anisotropy barrier in the temperature range of 30-300 K in accordance with Kneller's law; however, at low temperatures (< 30 K), the coercivity shows some deviation from this law. The saturation magnetization follows the modified Bloch's law in the temperature range of 10-300 K. Exchange bias (H-ex) studies of the samples show that H-ex increases with decreasing temperature of the sample. A strong increase in the H-ex values is found below 30 K for the low applied field (+/- 20 kOe), while a smaller increase is found for the high applied field (+/- 90 kOe). The slow increase in the exchange bias at high applied field has been attributed to the high field effects on the surface (shell) spins. These shell spins align along the field direction that weakens the core-shell interface interactions leading to the reduction in the exchange bias at lower temperatures. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3139293]