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Nearly monodispersed CoFe[sub 2]O[sub 4] 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 (Co[sub 0.25]Fe[sub 0.75])[Co[sub 0.75]Fe[sub 1.25]]O[sub 4] for powders with different particle diameters. Rotational hysteresis loss (W[sub r]) analysis showed an average switching field (H[sub p]) of 17 kOe and a magnetic anisotropy field (H[sub k]) of 38 kOe for the 40 nm CoFe[sub 2]O[sub 4] particles. The corresponding magnetocrystalline anisotropy energy constant (K) was about 5.1×10[sup 6] erg/cc. The H[sub c] and H[sub p] results suggest that the critical single-domain size of CoFe[sub 2]O[sub 4] is about 40 nm. The room temperature coercivity (H[sub c]) of the 40 nm CoFe[sub 2]O[sub 4] particles is found to be as high as 4.65 kOe. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]