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 he xagonal paramagnetic state to a noncollinear antiferromagnetic state w ith 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 rot ate 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 t he lower 23-K transition. While previous powder studies were rather in conclusive on this point, in this study new single-crystal neutron-dif fraction results are presented that show the second picture to be corr ect. In addition, the structural distortions that accompany the change in symmetry are discussed and show that there is phase-II type magnet ic short-range order between 23 and 35.5 K