Tuning Exchange Bias in Core/Shell FeO/Fe₃O₄ Nanoparticles

TLDR

Monodisperse 35‑nm FeO nanoparticles were oxidized in dry air to form core/shell FeO/Fe3O4 particles, with core and shell sizes tuned by reaction temperature and time, and their temperature‑dependent magnetic properties were examined at 60 °C and 100 °C for 30 min. The core/shell FeO/Fe3O4 nanoparticles exhibit a pronounced exchange bias, and their core‑to‑shell size ratio governs coercivity, exchange field, and the dominant reversal mechanism, marking the first demonstration of tunable exchange bias and asymmetric magnetization reversal in such antiferromagnetic core/ferrimagnetic shell systems.

Abstract

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.