Magnon-Cherenkov effect in superconductor-ferromagnet heterostructures

When a particle moves through a medium at a speed greater than the speed of light in that medium, it emits photons. This phenomenon, known as the Cherenkov effect, theoretically allows for the generation of excitations by moving sources in relativistic and non-relativistic systems. Magnons, the quasi-particles of spin waves, are collective excitations in magnetically ordered media used for wave-based computing and vow to be a building block for hybrid quantum systems. So far, while attractive for nanoscale magnonic circuits, short-wavelength magnons have not been accessible due to the lack of fast-moving sources. Here, we demonstrate the generation of magnons in a Co-Fe magnonic conduit by fast-moving (>1 km/s) magnetic flux quanta (Abrikosov vortices) in an adjacent Nb-C superconducting strip. Our findings demonstrate the generation of sub-40nm wavelength magnons, their unidirectional excitation, and self-locked magnon-fluxon dynamics at the Cherenkov resonance condition. Beyond these initial demonstrations, the magnon-Cherenkov effect sustains superconductivity and enables on-chip magnon generation at high speeds. Our illustration of the magnon-Cherenkov effect could be expanded to encompass other wave types, such as surface acoustic waves.