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.

Automated summary

This study investigates the interaction of surface acoustic waves with spin waves in a system composed of two ferromagnetic layers separated by a nonmagnetic layer. It reveals the formation of symmetric and antisymmetric spin wave modes due to interlayer magnetic dipolar coupling, with highly nondegenerate dispersion for oppositely propagating spin waves. The magnetoacoustic wave propagation properties are tuneable and offer promising applications in building efficient acoustic isolators.