  We report inelastic neutron scattering measurements of magnetic excitations
in YbMnSb2, a low-carrier-density Dirac semimetal in which the
antiferromagnetic Mn layers are interleaved with Sb layers that host Dirac
fermions. We observe a considerable broadening of spin waves, which is
consistent with substantial spin fermion coupling. The spin wave damping,
$\gamma$, in YbMnSb2 is roughly twice larger compared to that in a sister
material, YbMnBi2, where an indication of a small damping consistent with
theoretical analysis of the spin-fermion coupling was reported. The inter-plane
interaction between the Mn layers in YbMnSb2 is also much stronger, suggesting
that the interaction mechanism is rooted in the same spin-fermion coupling. Our
results establish the systematics of spin-fermion interactions in layered
magnetic Dirac materials.
