Spin-density waves in Fe/Cr trilayers and multilayers

Abstract

This paper reviews the behaviour of spin-density waves (SDWs) inFe/Cr trilayers and multilayers. After providing an overview ofSDWs in bulk Cr, we discuss the predicted effects of SDWs on theexchange coupling of an Fe/Cr trilayer with perfect interfaces,which is nearly realized experimentally by a Cr wedge grown atop anFe whisker and covered with a thin Fe film. While the observedphase slips in the exchange coupling are produced by transitionsbetween collinear SDW phases with different numbers of nodes, thegrowth in the distance between phase slips with increasingtemperature is attributed to a transition between incommensurate andcommensurate SDWs. The theoretical consequences of Fe-Crinterdiffusion and interfacial steps at imperfect interfaces arethen described. Whereas Fe-Cr interdiffusion causes the observedsign change in the exchange coupling, the effects of interfacialsteps depend on which of the interfacial, magnetostatic, or bulkenergies are dominant. For rough interfaces, the non-collinearcoupling between the Fe moments can be explained by two competingmodels, one of which requires the formation of a non-collinear,helical SDW that is unstable in bulk Cr. Although both collinearand non-collinear SDWs have been directly observed in Fe/Crmultilayers with neutron scattering, the role of SDWs in theexchange coupling remains uncertain. This review focuses on thefollowing open questions. Can a SDW be detected from the exchangecoupling alone? What are the requirements for a non-collinear SDWin an Fe/Cr trilayer or multilayer? Can the proximity to a SDWinstability be observed? Finally, what is the origin of thediscrepancy between the observed exchange coupling and the muchlarger coupling predicted by phenomenological, tight-binding, andfirst-principles calculations?