Energy barrier distribution for dispersed mixed oxide magnetic nanoparticles

Mixed Fe/Co oxide nanoparticles, diameter 8 nm, were prepared using the protein ferritin as a template and characterized by transmission electron microscopy (TEM) and Raman spectroscopy. We show that the latter effectively distinguishes between magnetite (Fe3O4), maghemite (gamma-Fe2O3) and Co ferrite (CoxFe3-xO4). Zero-field-cooled susceptibility measurements show isolated magnetite nanoparticles have a blocking temperature T-B = 18 +/- 1 K, but that adding 0.5% Co raises T-B to 30 K. Data for thermal relaxation from saturation obey a T ln(t/tau(0)) scaling, enabling us to determine the energy barrier distributions for the dispersed nanoparticles. For Fe oxide only a single peak was found. However, with the addition of only 0.5% Co a second component is observed that decreases rapidly with increasing energy. (C) 2012 American Institute of Physics