Magnetic couplings in CoFe2O4/FeCo-FeO core-shell nanoparticles

CoFe2O4/FeCo-FeO core-shell nanoparticles with an average particle size of about 4.3 nm were prepared by reducing nanoparticles of superparamagnetic cobalt-ferrite under hydrogen and oxygen atmospheres. The low temperature magnetic hysteresis showed an interesting behavior which has only been observed in thin film multilayer systems. At low magnetic fields H(<0.2kOe) the behavior is dominated by the magnetic structure of the magnetically soft shell while for higher values of H the exchange-spring core-shell coupling plays a major role. A particle model which is coherent with the observed magnetic behavior has been also proposed.