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Spin-density waves in Fe/Cr trilayers and multilayers

This paper reviews the behaviour of spin-density waves (SDWs) in Fe/Cr trilayers and multilayers. After providing an overview of SDWs in bulk Cr, we discuss the predicted effects of SDWs on the exchange coupling of an Fe/Cr trilayer with perfect interfaces, which is nearly realized experimentally by a Cr wedge grown atop an Fe whisker and covered with a thin Fe film. While the observed phase slips in the exchange coupling are produced by transitions between collinear SDW phases with different numbers of nodes, the growth in the distance between phase slips with increasing temperature is attributed to a transition between incommensurate and commensurate SDWs. The theoretical consequences of Fe-Cr interdiffusion and interfacial steps at imperfect interfaces are then described. Whereas Fe-Cr interdiffusion causes the observed sign change in the exchange coupling, the effects of interfacial steps depend on which of the interfacial, magnetostatic, or bulk energies are dominant. For rough interfaces, the non-collinear coupling between the Fe moments can be explained by two competing models, one of which requires the formation of a non-collinear, helical SDW that is unstable in bulk Cr. Although both collinear and non-collinear SDWs have been directly observed in Fe/Cr multilayers with neutron scattering, the role of SDWs in the exchange coupling remains uncertain. This review focuses on the following open questions. Can a SDW be detected from the exchange coupling alone? What are the requirements for a non-collinear SDW in an Fe/Cr trilayer or multilayer? Can the proximity to a SDW instability be observed? Finally, what is the origin of the discrepancy between the observed exchange coupling and the much larger coupling predicted by phenomenological, tight-binding, and first-principles calculations?