  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 (C-AFM) below 360
K with a propagation vector of k = 0, Cu0.98Mn0.96As undergoes a second-order
paramagnetic to incommensurate antiferromagnetic (IC-AFM) phase transition at
320 K with k = (0.1,0,0), followed by a second-order IC-AFM to C-AFM phase
transition at 230 K. In the C-AFM state, the Mn spins order parallel to the
b-axis but antiparallel to their nearest-neighbors with the easy axis along the
b axis. This magnetic order breaks Ry gliding and S2z rotational symmetries,
the two crucial for symmetry analysis, resulting in finite band gaps at the
crossing point and the disappearance of the massless topological fermions.
However, the spin-polarized surface states and signature induced by non-trivial
topology still can be observed in this system, which makes orthorhombic CuMnAs
promising in antiferromagnetic spintronics.
