Magnetic order induces symmetry breaking in the single-crystalline orthorhombic CuMnAs semimetal

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 Cu0.95MnAs and Cu0.98Mn0.96As single crystals. While Cu0.95MnAs is a commensurate antiferromagnet below 360 K with a propagation vector of k = 0, Cu0.98Mn0.96As undergoes a second-order paramagnetic to incommensurate antiferromagnetic phase transition at 320 K with 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 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.