An atomic scale Monte Carlo study of exchange bias in homogeneous/inhomogeneous core/shell Fe₃O₄/CoO nanoparticles

The present work focuses on the study of exchange-biased magnetic core/shell nanoparticles at the atomic scale. The nanoparticles (NPs) consist of a perfectly epitaxial crystalline cobalt oxide (CoO) shell on a magnetite (Fe₃O₄) core. The numerical core/shell is built by taken into account the spinel structure of the core (Fe₃O₄) and the face-centered cubic structure of the shell (CoO). Two different configurations of core/shell NPs were examined: homogeneous and inhomogeneous core/shell. Our magnetic simulations are based on a 3D classical Heisenberg model. Monte Carlo simulations performing single spin rotation are used to investigate the effect of exchange bias on the spin configurations and hysteresis loops of core/shell nanoparticles. The numerical results reveal, as expected, a significant hysteresis loop shift obtained for a weak interface coupling. In addition, the magnetization reversal is not perfectly uniform in space when the interfacial coupling is different from zero. Finally, the increase in the magnetic interfacial coupling produces an increase in coercive field and a decrease in the exchange bias field.