Recently, orthorhombic CuMnAs has been proposed to be a magnetic material where topological fermions exist around the Fermi level. Here we report the magnetic structure of the orthorhombic ${\mathrm{Cu}}_{0.95}\mathrm{MnAs}$ and ${\mathrm{Cu}}_{0.98}{\mathrm{Mn}}_{0.96}\mathrm{As}$ single crystals. While ${\mathrm{Cu}}_{0.95}\mathrm{MnAs}$ is a commensurate antiferromagnet below 360 K with a propagation vector of $\mathbf{k}=0,{\mathrm{Cu}}_{0.98}{\mathrm{Mn}}_{0.96}\mathrm{As}$ undergoes a second-order paramagnetic to incommensurate antiferromagnetic phase transition at 320 K with $\mathbf{k}=(0.1,0,0)$, followed by a second-order incommensurate to commensurate antiferromagnetic phase transition at 230 K. In the commensurate antiferromagnetic state, the Mn spins order parallel to the crystallographic $b$ axis but antiparallel to their nearest neighbors, with the spin orientation along the $b$ axis. This magnetic order breaks ${\mathrm{S}}_{2z}$, the two-fold rotational symmetry around the $c$ axis, resulting in finite band gaps at the crossing point and the disappearance of the massless topological fermions. However, our first-principles calculations suggest that orthorhombic CuMnAs can still host spin-polarized surface states and signature induced by nontrivial topology, which makes it a promising candidate for antiferromagnetic spintronics.

• Received 23 August 2017

DOI:https://doi.org/10.1103/PhysRevB.96.224405