Nonreciprocal spin waves driven by left-hand microwaves

It is conventional wisdom that a left-hand microwave cannot efficiently excite the spin wave (SW) in the ferromagnet due to the constraint of angular momentum conservation. In this work, we show that the left-hand microwave can drive nonreciprocal SWs in the presence of a strong ellipticity mismatch between the microwave and precessing magnetization. A compensation frequency is predicted, at which the left-hand microwave cannot excite SWs. Away from it the SW amplitude sensitively depends on the microwave ellipticity, in sharp contrast to the case driven by right-hand microwaves. By tuning the driving frequency, we observe a switchable SW nonreciprocity in the ferromagnetic layer. A mode-dependent mutual demagnetizing factor is proposed to explain this finding. Our work advances the understanding of photon-magnon conversion and paves the way to designing diodelike nanoscale magnonic devices.