  Recent experiments observe the spin-wave-Meissner-current modes in
ferromagnetic insulator-superconductor heterostructures, in which the
coherently excited spin waves seemingly do not decay as usual beneath the
superconductor strip [Borst et al., Science 382, 430 (2023)]. We interpret this
phenomenon by demonstrating that the stray magnetic field emitted by the
magnetization dynamics is reflected, focused, and enhanced inside the
ferromagnet by the supercurrent induced in the superconductor, such that the
group velocity of spin waves is strongly enhanced. Analytical and numerical
calculations based on this model predict that the coherent transport of magnons
is enhanced by close to 500% for yttrium iron garnet capped by superconducting
NbN with a decay length exceeding millimeters. Our finding may augment the
performance of magnons in quantum information and quantum transport processing.
