High resolution magnetic neutron diffraction study of the chiral lattice antiferromagnet CoNb₃S₆

CoNb₃S₆ is a chiral lattice antiferromagnet which belongs to a class of intercalated transition metal dichalcogenides known for complex magnetic textures, nontrivial band topology, and superconductivity. Previous neutron diffraction measurements below the Néel temperature (T_N ~ 26 K) pointed to a collinear spin arrangement in this material. However, a more recent discovery of a large anomalous Hall effect (AHE), three orders of magnitude large than that explained by its weak field-induced magnetism, raises questions about the true nature of its magnetic ordering. Here, we present results on high resolution magnetic neutron diffraction and magnetic circular dichroism (MCD) measurements to unravel the details of its magnetic structure. We identify a long wavelength incommensurate magnetic structure with moments lying in the ab plane. Higher harmonic peaks are absent down to 2 K, consistent with a helical ground state. The results are complemented by MCD data suggesting changes in the underlying magnetic chirality in an applied field, which may help explain the origin of the large AHE.