Exchange-bias and exchange-spring coupling in magnetic core-shell nanoparticles

Nanopowders of CoFe2O4 with an average particle size of 72.7 nm were produced by an oxidation process and they were used to obtain core-shell nanoparticles of CoFe2O4/CoFe2 by reducing them under hydrogen atmospheres. Hysteresis loops were measured from 10 to 300 K by field-cooling the sample under an applied magnetic field of 12 kOe. The exchange coupling among the core-shell structure was found to vary with the thickness of the shell. A phenomenological model which takes into account the thickness of a disordered layer at the core-shell surface was used for determining the exchange-bias anisotropy constant KEB (=0.22 erg/cm2).