We investigate the spin wave modes in asymmetric trilayer [Ni80Fe20 (10 nm)/Cu(tCu)/Ni80Fe20 (30 nm)] nanowire structures as a function of the Cu thickness (t(Cu)) in the range from 0 to 20 nm using perpendicular ferromagnetic resonance (pFMR) spectroscopy. For t(Cu) = 0 nm, corresponding to the 40 nm thick single layer Ni80Fe20 nanowires, both the fundamental and first order modes are observed in the saturation region. However, for the trilayer structures, two additional modes, which are the fundamental and first order optical modes, are observed. We also found that the resonance fields of these modes are markedly sensitive to the Cu thickness due to the competing effects of interlayer exchange coupling and magnetostatic dipolar coupling. When the t(Cu) >= 10 nm, the fundamental optical mode is more pronounced. Our experimental results are in quantitative agreement with the dynamic micromagnetic simulations.
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