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

Abstract

Nearly monodispersed CoFe 2 O 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 spectroscopyanalyses confirmed the spinel phase and a stoichiometric composition of ( Co 0.25 Fe 0.75 )[ Co 0.75 Fe 1.25 ] 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 CoFe 2 O 4 particles. The corresponding magnetocrystalline anisotropy energy constant (K) was about 5.1×10 6 erg/cc . The H c and H p results suggest that the critical single-domain size of CoFe 2 O 4 is about 40 nm. The room temperature coercivity (H c ) of the 40 nm CoFe 2 O 4 particles is found to be as high as 4.65 kOe.