Interfacial Exchange Coupling and the Magnetization of Iron Oxide/Nickel Oxide Superlattices

Abstract

The role of interfacial exchange coupling in the magnetic behavior of metal oxide materials has been investigated through the study of Fe₃O₄/NiO superlattices. We report results on a series of superlattices grown where one bilayer constituent was held to a fixed thickness while varying the other from single unit cell dimensions upward. High crystalline quality was confirmed by XRD, RHEED and neutron diffraction. Magnetization profiles show substantial deviations from bulklike iron oxide results, with an increase in domain rotation energies observed in the superlattices over that of bulk iron oxide (increasing with NiO layer thickness) indicating the strong nature of Fe₃O₄/NiO interfacial linkage. Neutron scattering at elevated temperatures shows that the NiO remains ordered above the 523 K bulk Néel temperature. This suggests that at least a portion of the NiO within a layer remains ordered well above the Néel temperature, with an increase in effective Néel transition temperature that approaches the Fe₃O₄ Curie temperature in the limit of very thin NiO layers. Although the exchange coupling dominates these effects, strain also plays an important role.