Electronic States and Magnetic Coupling in Fe/Fe 3 O 4 Junctions

Exchange coupling observed recently in Fe/ Fe 3 O 4 (001) junctions shows comparable intensity to that in Co/Ru/Co trilayers, and has potential applicability to spintronics devices. To clarify the mechanism of the exchange coupling, electronic and magnetic states of Fe/ Fe 3 O 4 junctions are calculated in the first principles method by assuming four junction structures of bcc Fe and Fe 3 O 4 layers. It is shown that the local moments of bcc Fe atoms at the interface increase, but those of Fe ions at the interface of Fe 3 O 4 layer decrease. The total energy of the junctions is plotted as a function of distance between Fe and Fe 3 O 4 layers. Calculated results of the coupling energy between Fe and Fe 3 O 4 layers, however, are larger than experimental ones by two orders of magnitude, and they correspond to inter-atomic exchange interactions at the interface. In order to explain the experimental results, we propose a mechanism of exchange coupling mediated by impurity-like states of interfacial Fe atoms which possess reversed magnetic moments in bcc Fe layer. Frustration effects in exchange coupling between Fe and Fe 3 O 4 layers are also discussed.