Interface magnetism and magnetic structure of GdN/Fe multilayers studied by x-ray magnetic circular dichroism

摘要

We have used the element specificity of x-ray magnetic circular dichroism to separate the contributions of the component layers to the magnetization and local magnetic structure of artificial nanoscale ferrimagnetic multilayers GdN/Fe. The Fe layers, by strong antiferromagnetic interlayer exchange coupling, not only magnetize a narrow interface region in paramagnetic GdN but induce long-range magnetic order in the volume of the GdN layers at temperatures considerably above the Curie temperature of the bare layers (T_C~(GdN) ≈ 60 K), in support of a theoretical mean-field prediction. We propose that the effect may be related to the special electronic band structure of GdN, which shows a transition from narrow-gap semiconducting to metallic conduction at T_C~(GdN). In an elevated external magnetic field the GdN- and Fe-sublayer magnetic moments adopt a canted configuration. In this state the local magnetization in each GdN layer is highly nonuniform at low temperature. The interior turns its moment into the field direction almost abruptly from one atomic layer to the next in a single block. This indicates weak magnetic coupling between the interfacial and volume magnetizations in GdN. The results are at variance with the much studied "model" multilayer system Gd/Fe, where magnetic order in the Gd-layer volume appears intrinsically only significantly below the Curie temperature of bulk Gd. Furthermore, magnetization reversal in the Gd layers in a magnetic field occurs gradually on a considerably larger length scale, only if they are sufficiently thick.