Twisted magnetization state at the interface of an antiferromagnetically coupled Fe/Si multilayer as probed by specular and off-specular polarized neutron scattering

We have investigated strongly coupled Fe/Si multilayers by polarized neutron scattering in specular and off-specular modes. By using the specular reflectivity patterns as a function of the applied field, we have extracted the magnetization angles for layers buried in the stack. In order to explore the magnetization depth profile across the interfaces the neutron data have been combined with micromagnetic simulations. A rigid and/or uniform approximation of the layer magnetization is found to describe the data over a considerable range of the applied field but reveals significant shortcomings around an intermediate field between remanence and saturation. For these intermediate field values, we found (i) close agreement of the magnetization state with a twisted state model, which indicates the formation of exchange-spring-type spin structure; and (ii) depending on the magnetic history of the specimen, an enhanced off-specular magnetic scattering of neutrons at the antiferromagnetic Bragg-peak positions, which indicates the presence of buried domains. However, an extraction of the state of magnetic chirality within a single ferromagnetic layer is in itself challenging and is further complicated as the length scale of these domains becomes smaller than the neutron coherence length.