Nonreciprocal Magnetoacoustic Waves in Dipolar-Coupled Ferromagnetic Bilayers

We study the interaction of surface acoustic waves (SAWs) with spin waves (SWs) in a Co₄₀Fe₄₀B₂₀/Au/Ni₈₁Fe₁₉ system composed of two ferromagnetic layers separated by a nonmagnetic Au spacer layer. Because of interlayer magnetic dipolar coupling between the two ferromagnetic layers, a symmetric and an antisymmetric SW mode form, which both show a highly nondegenerate dispersion relation for oppositely propagating SWs. Due to magnetoacoustic SAW-SW interaction, we observe highly nonreciprocal SAW transmission in the piezoelectric-ferromagnetic hybrid device. We experimentally and theoretically characterize the magnetoacoustic wave propagation as a function of frequency, wave vector, and external magnetic field magnitude and orientation. Additionally, we demonstrate that the nonreciprocal SW dispersion of a coupled magnetic bilayer is highly tuneable and not limited to ultrathin magnetic films, in contrast to the nonreciprocity induced by the interfacial Dzyaloshinskii-Moriya interaction. Therefore, magnetoacoustic coupling in ferromagnetic multilayers provides a promising route towards building efficient acoustic isolators.