Nearly monodispersed CoFe2O4 nanoparticles with average sizes between 8 and 100 nm were synthesized by using seed-mediated growth dominant coprecipitation and modified oxidation methods. X-ray diffraction and Mössbauer spectroscopy analyses confirmed the spinel phase and a stoichiometric composition of (Co0.25Fe0.75)[Co0.75Fe1.25]O4 for powders with different particle diameters. Rotational hysteresis loss (Wr) analysis showed an average switching field (Hp) of 17 kOe and a magnetic anisotropy field (Hk) of 38 kOe for the 40 nm CoFe2O4 particles. The corresponding magnetocrystalline anisotropy energy constant (K) was about 5.1×106 erg/cc. The Hc and Hp results suggest that the critical single-domain size of CoFe2O4 is about 40 nm. The room temperature coercivity (Hc) of the 40 nm CoFe2O4 particles is found to be as high as 4.65 kOe.

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A modified polyol process for the synthesis of cubic‐shaped FeCo particles at 403 K is demonstrated. Control of the particle composition and size was achieved by varying the experimental conditions. The Fe content in the FeCo particles can be varied from 90 to 20 mol %, and the size of the cubes can be controlled to obtain particles with sizes between 300 and 35 nm (see figure). The magnetic properties of the particles closely resemble those of bulk FeCo.