TEMPERATURE-DEPENDENCE OF MAGNETIC ORDER IN SINGLE-CRYSTALLINE UPDSN

The noncollinear hexagonal antiferromagnet UPdSn exhibits two magnetic phase transitions, at 35.5 and 23 K The first transition is from a hexagonal paramagnetic state to a noncollinear antiferromagnetic state with a doubled unit cell (phase 1). The second 23-K transition is to a monoclinic magnetic structure (phase II). Ever since these transitions were discovered, the question has been whether the moments simply rotate at 23 K, or whether the y and z components of the moment order at 35.5 K while the x component orders out of incipient fluctuations at the lower 23-K transition. While previous powder studies were rather inconclusive on this point, in this study new single-crystal neutron-diffraction results are presented that show the second picture to be correct. In addition, the structural distortions that accompany the change in symmetry are discussed and show that there is phase-II type magnetic short-range order between 23 and 35.5 K