LOW-TEMPERATURE MAGNETIC-STRUCTURE OF UNIGE

We report on the magnetic structure of single-crystalline UNiGe in its low-temperature commensurate antiferromagnetic phase. UNiGe crystalli zes in the orthorhombic TiNiSi structure type with space group Pnma an d undergoes two magnetic transitions at 51 and 42 K. Our data, taken a t 20 K, rule out both of the previously reported magnetic structures f or 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 str ucture. 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-densi ty-wave configuration. However, out-of-plane moment components are all owed by symmetry and our best refinement gives noncollinear canting ou t of the plane by approximately 20 degrees. This structure belongs to one of the two possible irreducible representations, and we discuss it s relationship to a more conventional Shubnikov-group analysis.