The nonreciprocal transmission of magneto-acoustic waves in Ni81Fe19/Cu/Co40Fe40B20 magnetic bilayer

Transmission nonreciprocity is of great significance as it is the defining characteristic of microwave isolators. Combining reciprocal Surface Acoustic Waves (SAWs) with nonreciprocal Spin Waves is an effective way to achieve nonreciprocal transmission. In this work, we investigate the characteristics of SAW–SW coupling in a Ni81Fe19/Cu/Co40Fe40B20 system composed of two ferromagnetic layers separated by a nonmagnetic Cu spacer layer. We use the Vector Network Analyzer to measure the transmission parameters S21 and S12 of the SAW devices. Due to the interlayer dipolar coupling (IDC) between the two ferromagnetic layers, two typical resonance modes are excited: the in-phase acoustic mode (AM) and the out-of-phase optical mode (OM). Because of the helicity mismatch effect and nonreciprocal SW dispersion, the SAW transmissions of both modes exhibit nonreciprocal behavior. Additionally, the nonreciprocity of AM is enhanced due to the spin-rotation coupling (SRC) effect derived from the Cu layer. The magnitude of the transmission nonreciprocity can be regulated by adjusting the Cu layer thickness, which affects the strength of IDC and SRC. We believe that this finding can further the development of magneto-acoustic devices.