  Scattering of light dark matter with sub-eV energy deposition can be detected
with collective excitations in condensed matter systems. When dark matter has
spin-independent couplings to atoms or ions, it has been shown to efficiently
excite phonons. Here we show that, if dark matter couples to the electron spin,
magnon excitations in materials with magnetic dipole order offer a promising
detection path. We derive general formulae for single magnon excitation rates
from dark matter scattering, and demonstrate as a proof of principle the
projected reach of a yttrium iron garnet target for several dark matter models
with spin-dependent interactions. This highlights the complementarity of
various collective excitations in probing different dark matter interactions.
