Exchange coupling in Fe/NiO/Co film studied by soft x-ray resonant magnetic reflectivity

Abstract

A 10 nm Fe/5 nm NiO/10 nm Co polycrystalline trilayer prepared by magnetron sputtering has been investigated by resonant magnetic reflectivity of linearly polarized soft x rays (transversal magneto-optical Kerr effect geometry) at the L 3 edges of Fe, Ni, and Co. The exchange coupling in the trilayer, introduced during synthesis, has been altered by cooling through the Neel temperature T N of NiO at three different cooling fields H F C . We show that the chemical structure at the interfaces, the grain size and the magnetic properties of the trilayer are governed by factors such as successive heat treatment, magnetic field applied during synthesis and cooling through T N of NiO. Clear evidence for oxidation of Fe and Co states has been obtained, suggestive for the formation of mixed oxides at the Fe/NiO and NiO/Co interfaces. Oxidized Fe and Co are found to carry a net magnetic moment, which couples to the moments of the metallic states. We find that the coupling between oxidized and metallic Co spins is ferromagnetic, whereas the coupling between oxidized and metallic Fe spins is antiferromagnetic. We observe the presence of two types of excess Ni spins. A rigid excess Ni magnetic moment is induced at the interface during synthesis as follows from the strong asymmetry of the x-ray Kerr loops of the oxidized Co spins at H F C = 0 Oe and metallic Co spins at H F C =240 Oe. Heat treatment leads to appearence of a soft excess Ni magnetic moment, which is ferromagnetically coupled to the metallic Fe and Co spins and exhibits a conventional hysteresis loop. Competing interactions lead to frustrated magnetization at the NiO/Fe interface and cause a tilt of the Fe spins away from the direction of applied magnetic field. This manifests itself in the presence of characteristic humps in the Fe Kerr loops.