We report inelastic neutron scattering measurements of magnetic excitations in ${\mathrm{YbMnSb}}_2$, a low-carrier-density Dirac semimetal in which the antiferromagnetic Mn layers are interleaved with Sb layers that host Dirac fermions. We observe a measurable broadening of spin waves, which is consistent with substantial spin-fermion coupling. The spin-wave damping $\gamma$ in ${\mathrm{YbMnSb}}_2$ is roughly twice larger compared to that in a sister material, ${\mathrm{YbMnBi}}_2$, where an indication of a small damping consistent with a theoretical analysis of the spin-fermion coupling was reported. The interplane interaction between the Mn layers in ${\mathrm{YbMnSb}}_2$ 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.

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• Dirac fermions
• Exchange interaction
• Magnons
• Spin dynamics
• Topological materials