Low-temperature magnetic structure of UNiGe

We report on the magnetic structure of single-crystalline UNiGe in its low-temperature commensurate antiferromagnetic phase. UNiGe crystallizes in the orthorhombic TiNiSi structure type with space group Pnma and undergoes two magnetic transitions at 51 and 42 K. Our data, taken at 20 K, rule out both of the previously reported magnetic structures for the low-temperature phase, though we find an ordering vector q=(0,1/2,1/2) in agreement with the previous powder study of Murasik et al. [J. Phys. Condens. Matter 3, 1841 (1991)]. We observe both domains of the single-q structure, and show that this is preferred over a 2-q structure. We extract a moment of 0.91 mu(B) per U atom, and it is likely that the moment lies within the b-c plane in a collinear moment-density-wave configuration. However, out-of-plane moment components are allowed by symmetry and our best refinement gives noncollinear canting out of the plane by approximately 20 degrees. This structure belongs to one of the two possible irreducible representations, and we discuss its relationship to a more conventional Shubnikov-group analysis.