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 °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