  We propose anti-ferromagnets as optimal targets to hunt for sub-MeV dark
matter with spin-dependent interactions. These materials allow for multi-magnon
emission even for very small momentum transfers, and are therefore sensitive to
dark matter particles as light as the keV. We use an effective theory to
compute the event rates in a simple way. Among the materials studied here, we
identify nickel oxide (a well-assessed anti-ferromagnet) as an ideal candidate
target. Indeed, the propagation speed of its gapless magnons is very close to
the typical dark matter velocity, allowing the absorption of all its kinetic
energy, even through the emission of just a single magnon.
