Magnetic properties of binary mixtures of metal oxide nanoparticles

The magnetic properties of mixtures of nanoparticles with similar size distributions but distinct superparamagnetic characteristics were investigated by SQUID magnetometry. Metal oxide nanoparticles were synthesized by the thermal decomposition of metal acetylacetonates. The nanoparticles were characterized by transmission electron microscopy, Fe-57 Mossbauer spectroscopy, and SQUID magnetometry. Cobalt ferrite and maghemite nanoparticles with average sizes of 5.1 and 5.0 nm were obtained. Zero field cooled magnetization measurements indicate that the nanoparticles are superparamagnetic, with blocking temperatures of 150 and 22 K, respectively. The zero field cooled measurements for mixtures of the nanoparticles behaved as the sum of the respective superparamagnetic behaviours of the two individual components. This suggests the energy barrier to spin fluctuation is insensitive to the presence of nanoparticles with distinct superparamagnetic characteristics. However, the magnetization versus field measurements provide evidence for interparticle interactions, even at room temperature. (c) 2006 Published by Elsevier Ltd.