Abstract
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 (γ-Fe2O3) and Co ferrite (CoxFe3-xO4). Zero-field-cooled susceptibility measurements show isolated magnetite nanoparticles have a blocking temperature TB = 18 ± 1 K, but that adding 0.5% Co raises TB to 30 K. Data for thermal relaxation from saturation obey a T ln(t/τ0) 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.
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