Magnon dispersion in Ni/Co multilayers grown on Cu(100)

Motivated by recent studies on current-driven domain-wall motion we have explored the dispersion of spin waves (magnons) in ultrathin nickel/cobalt multilayers. The layers are grown epitaxially on Cu(100) surfaces and consist of n nickel single-atom layers, each topped by a cobalt bilayer with n=2, 3, and 4(n×Ni1Co2). Layers of the type 2×Ni2Co1 are also studied. While parallel to the film plane the magnon dispersion is nearly equal to that of pure cobalt films, magnons with wave vectors perpendicular to the film plane (standing waves) are considerably softened. The softening is attributed to a reduction of the effective interlayer exchange coupling between the two layers next to the interface with the Cu(100) substrate.