Direct Detection of Spin-Dependent Sub-GeV Dark Matter via Migdal Effect
  Motivated by the current strong constraints on the spin-independent dark
matter (DM)-nucleus scattering, we investigate the spin-dependent (SD)
interactions of the light Majorana DM with the nucleus mediated by an
axial-vector boson. Due to the small nucleus recoil energy, the ionization
signals have now been used to probe the light dark matter particles in direct
detection experiments. With the existing ionization data, we derive the
exclusion limits on the SD DM-nucleus scattering through Migdal effect in the
MeV-GeV DM mass range. It is found that the lower limit of the DM mass can
reach about several MeVs. Due to the momentum transfer correction induced by
the light mediator, the bounds on the SD DM-nucleus scattering cross sections
can be weakened in comparison with the heavy mediator.
