  We report neutron diffraction measurements of Ca$_{0.6}$Sr$_{0.4}$MnSb$_2$, a
low-carrier-density Dirac semimetal in which the antiferromagnetic Mn layers
are interleaved with Sb layers that host Dirac fermions. We have discovered
that we can detect a good quality inelastic spin wave signal from a small (m ~
0.28 g) single crystal sample by the diffraction method, without energy
analysis, using a neutron diffractometer with a position-sensitive area
detector; the spin-waves appear as diffuse scattering that is shaped by
energy-momentum conservation. By fitting this characteristic magnetic
scattering to a spin-wave model, we refine all parameters of the model spin
Hamiltonian, including the inter-plane interaction, through use of a
three-dimensional measurement in reciprocal space. We also measure the
temperature dependence of the spin waves, including the softening of the spin
gap on approaching the Neel temperature, $T_N$. Not only do our results provide
important new insights into an interplay of magnetism and Dirac electrons, they
also establish a new, high-throughput approach to characterizing magnetic
excitations on a modern diffractometer without direct energy analysis. Our work
opens exciting new opportunities for the follow-up parametric and compositional
studies on small, ~0.1 g crystals.
