  Despite theoretical predictions for a Cherenkov-type radiation of spin waves
(magnons) by various propagating magnetic perturbations, fast-enough moving
magnetic field stimuli have not been available so far. Here, we experimentally
realize the Cherenkov radiation of spin waves in a Co-Fe magnonic conduit by
fast-moving (>1 km/s) magnetic flux quanta (Abrikosov vortices) in an adjacent
Nb-C superconducting strip. The radiation is evidenced by the microwave
detection of spin waves propagating a distance of 2 micrometers from the
superconductor and it is accompanied by a magnon Shapiro step in its
current-voltage curve. The spin-wave excitation is unidirectional and
monochromatic, with sub-40 nm wavelengths determined by the period of the
vortex lattice. The phase-locking of the vortex lattice with the excited spin
wave limits the vortex velocity and reduces the dissipation in the
superconductor.
