Tuning Exchange Bias in Core/Shell FeO/Fe3O4 Nanoparticles

Monodisperse 35 nm FeO nanoparticles (NPs) were synthesized and oxidized in a dry air atmosphere into core/shell FeO/Fe3O4 NPs with both FeO core and Fe3O4 shell dimensions controlled by reaction temperature and time. Temperature-dependent magnetic properties were studied on FeO/Fe3O4 NPs obtained from the FeO NPs oxidized at 60 and 100 degrees C for 30 min. A large exchange bias (shift in the hysteresis loop) was observed in these core/shell NPs. The relative dimensions of the core and shell determine not only the coercivity and exchange field but also the dominant reversal mechanism of the ferrimagnetic Fe3O4 component. This is the first time demonstration of tuning exchange bias and of controlling asymmetric magnetization reversal in FeO/Fe3O4 NPs with antiferromagnetic core and ferrimagnetic shell.