Mixing of acoustic and optical excitations in Ru/Co based multilayers

Two series of [Ru-10/Co-x)](12) and [Ru-6/Cr-3/Co-x)](12)x = 16-60 easy-plane anisotropy multilayers (all thicknesses in angstrom), prepared by sputter deposition, are studied by cavity FMR. The resonance fields of the modes excited by setting the rf field perpendicular to the saturating (in-plane) field are in good agreement with the values predicted for pure acoustic modes by a simple two-macrospin model. In contrast, the resonance fields of the modes excited by setting the rf field parallel to the saturating field, are lower than those expected for pure optical modes. This is attributed to the existence of hybridized mixed modes. Micromagnetic simulations show that (a) the inhomogeneous magnetization profile along the multilayer thickness is sufficient to give rise to mode mixing and (b) that mode mixing is not limited to the region where the frequencies of the acoustic and optical modes coincide.