Spin Waves in Dirac Semimetal Ca_{0.6}Sr_{0.4}MnSb_{2} Investigated with Neutrons by the Diffraction Method

The tunability of Dirac semimetals with antiferromagnetic Mn layers is of great interest. The observed sign change of interlayer magnetic coupling between CaMnBi_{2} and SrMnBi_{2} suggests ionic substitution as a potential tuning mechanism. If so, novel behavior near the compensation point could be expected. To explore this, we study a mixed-cation analog, Ca_{0.6}Sr_{0.4}MnSb_{2}, where Bi is replaced by Sb. Conventional inelastic neutron scattering is impractical due to the small crystal size (m≈0.28  g) available for compositional studies; however, we find that using a neutron diffractometer with a wide-angle area detector we can obtain a good quality spin-wave signal, which is shaped by energy-momentum conservation and retains spectroscopic information even without direct energy analysis. Spin-wave modeling reveals an interlayer coupling quantitatively similar to SrMnSb_{2}, indicating it is not directly tuned by ionic size and that the sign change in Bi-based compounds likely arises from the observed change in lattice symmetry. Beyond this key insight, our results present an efficient method for parametric and compositional studies of spin dynamics in small crystals.