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

TLDR

Cobalt ferrite nanoparticles (~26 nm) were synthesized by coprecipitation, and their shell spins align with the field, weakening core–shell interactions and reducing exchange bias at low temperatures. Coercivity follows Kneller’s law from 30–300 K but deviates below 30 K, saturation magnetization obeys a modified Bloch’s law from 10–300 K, and exchange bias rises sharply below 30 K—especially at low fields—due to surface spin effects.

Abstract

Cobalt ferrite nanoparticles (size: 26±4nm) 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 30–300K in accordance with Kneller’s law; however, at low temperatures (<30K), the coercivity shows some deviation from this law. The saturation magnetization follows the modified Bloch’s law in the temperature range of 10–300K. Exchange bias (Hex) studies of the samples show that Hex increases with decreasing temperature of the sample. A strong increase in the Hex values is found below 30K for the low applied field (±20kOe), while a smaller increase is found for the high applied field (±90kOe). 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.