Unusually high coercivity and critical single-domain size of nearly monodispersed CoFe2O4 nanoparticles

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 Mossbauer spectroscopy analyses confirmed the spinel phase and a stoichiometric composition of (Co0.25Fe0.75)[Co0.75Fe1.25]O-4 for powders with different particle diameters. Rotational hysteresis loss (W-r) analysis showed an average switching field (H-p) of 17 kOe and a magnetic anisotropy field (H-k) of 38 kOe for the 40 nm CoFe2O4 particles. The corresponding magnetocrystalline anisotropy energy constant (K) was about 5.1x10(6) erg/cc. The H-c and H-p results suggest that the critical single-domain size of CoFe2O4 is about 40 nm. The room temperature coercivity (H-c) of the 40 nm CoFe2O4 particles is found to be as high as 4.65 kOe. (C) 2003 American Institute of Physics.