Backward Magnetostatic Surface Spin Waves in Coupled Co/FeNi Bilayers

Spin waves (SWs) in coupled Co/FeNi bilayers are probed by the Brillouin light-scattering technique. Two types of SWs, magnetostatic surface spin waves (MSSWs) and perpendicular standing SWs, are identified. The dispersion relations of MSSWs obtained from the Stokes and anti-Stokes measurements display, respectively, positive and negative group velocities. The anti-Stokes branch with positive phase velocities and negative group velocities is called the backward magnetostatic surface mode that originates from the magnetostatic interaction in the bilayer. Experimental data agree well with our theoretical calculations. Our results are used to understand SWs propagation.