The antiferromagnetic (AFM) semimetal ${\mathrm{YbMnSb}}_2$ has recently been identified as a candidate topological material, driven by time-reversal symmetry breaking. Depending on the ordered arrangement of Mn spins below the Néel temperature, $T_{\mathrm{N}}=345\mathrm{K}$, the electronic bands near the Fermi energy can either have a Dirac node, a Weyl node, or a nodal line. We have investigated the ground state magnetic structure of ${\mathrm{YbMnSb}}_2$ using unpolarized and polarized single crystal neutron diffraction. We find that the Mn moments lie along the $c$ axis of the $P4/nmm$ space group and are arranged in a $C$-type AFM structure, which implies the existence of gapped Dirac nodes near the Fermi level. The results highlight how different magnetic structures can critically affect the topological nature of fermions in semimetals.

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• First-principles calculations
• Magnetic order
• Topological materials
• Neutron scattering