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Cavity Ferromagnetic Resonance Study of Acoustic and Optic excitations in Ru/Cr/Co and Ru/Co multilayers

Two series of [Ru10/Cox)]12 and [Ru6/Cr3/Cox)]12 x=16-60 easy-plane anisotropy multilayers (all thicknesses in Å), prepared by sputter deposition, are studied by cavity FMR. The acoustic modes are excited by setting the RF field perpendicular to the saturating in-plane field. Their resonance fields are in good agreement with the values predicted for acoustic modes by macrospin models, using the interlayer RKKY exchange and anisotropy fields derived by the magnetic measurements. The resonance fields of the modes excited by setting the RF field parallel to the saturating field, are lower than those expected for optical modes. This and could attributed to the existence of hybridized mixed modes, according to results of micromagnetic simulations which also show that the mode mixing is related to the inhomogeneous magnetization profile along the multilayer thickness.