  We report neutron scattering measurements on YbMnSb$_2$ which shed new light
on the nature of the magnetic moments and their interaction with Dirac
fermions. Using half-polarized neutron diffraction we measured the
field-induced magnetization distribution in the paramagnetic phase and found
that the magnetic moments are well localised on the Mn atoms. Using triple-axis
neutron scattering we measured the magnon spectrum throughout the Brillouin
zone in the antiferromagnetically ordered phase, and we determined the dominant
exchange interactions from linear spin-wave theory. The analysis shows that the
interlayer exchange is five times larger than in several related compounds
containing Bi instead of Sb. We argue that the coupling between the Mn local
magnetic moments and the topological band states is more important in
YbMnSb$_2$ than in the Bi compounds.
