Ferromagnetism at the interfaces of antiferromagnetic FeRh epilayers

The nanoscale magnetic structure of FeRh epilayers has been studied by polarized neutron reflectometry. Epitaxial films with a nominal thickness of 500 angstrom were grown on MgO (001) substrates via molecular-beam epitaxy and capped with 20 angstrom of MgO. The FeRh films show a clear transition from the antiferromagnetic (AF) state to the ferromagnetic (FM) state with increasing temperature. Surprisingly the films possess a FM moment even at a temperature 80 K below the AF-FM transition temperature of the film. We have quantified the magnitude and spatial extent of this FM moment, which is confined to within similar to 60-80 angstrom of the FeRh near the top and bottom interfaces. These interfacial FM layers account for the unusual effects previously observed in films with thickness <100 angstrom. Given the delicate energy balance between the AF and FM ground states we suggest a metastable FM state resides near to the interface within an AF matrix. The length scale over which the FM region resides is consistent with the strained regions of the film.