Non-Reciprocal Spin-Wave Emission from Topological Spin Textures

Investigations of spin waves are of great interest for both fundamental science and applications. For the excitation of spin waves with short wavelengths, it was typically necessary to either use patterned transducers with sizes on the order of the desired wavelengths or to generate such spin waves parametrically.
Here, we will show a combined experimental and theoretical study of spin waves in a stacked vortex pair system formed in a NiFe/Ru/CoFeB trilayer. The magnetization dynamics was imaged by means of time-resolved scanning transmission x-ray microscopy (STXM). Thereby, two different spin wave regimes were identified. For excitation frequencies above 500 MHz, mainly 2D plane waves within the magnetic domains were observed. However, a transition from 2D to 1D wave transport occurs for excitation frequencies below 500 MHz. In this case almost no spin waves were detected within the domains but high amplitudes were found within the 180° domain walls. An analytic and numerical analysis was done for both regimes, resulting in both a qualitative and quantitative understanding of the finite frequency gap in the spin wave dispersion relation for the ferromagnetic domains. Moreover, the dispersion relation was found to exhibit a strong non-reciprocity.