  We analyze the decay of spin waves into Stoner excitations in magnetic Weyl
semimetals. The lifetime of a mode is found to have a universal dependence on
its frequency and momentum, and on a few parameters that characterize the
relativistic Weyl spectrum. At the same time, Gilbert damping by Weyl electrons
is absent. The decay rate of spin waves is calculated perturbatively using the
s-d model of itinerant Weyl or Dirac electrons coupled to local moments. We
show that many details of the Weyl spectrum, such as the momentum-space
locations, dispersions and sizes of the Weyl Fermi pockets, can be deduced
indirectly by probing the spin waves of local moments using inelastic neutron
scattering.
