Antiferromagnetism and domain effects in UPdSn

Neutron-diffraction experiments have been performed on a single crystal of the hexagonal noncollinear antiferromagnetic compound UPdSn as a function of temperature and magnetic field. The use of a special horizontal-field magnet (with very wide horizontal access to the neutron beams) has allowed the study of the principal magnetic Bragg reflections in all three antiferromagnetic domain pairs throughout the magnetic phase diagram for $Bl3\mathrm{T}$ and $Tg6\mathrm{K}.$ The data confirm a picture in which one domain pair (1) grows at the expense of the other two domain pairs (2 and 3), for fields along the [100] axis for domain 1. On the other hand, if the field is applied along the perpendicular axis, [010] for domain 1, the other two domains are preferred. These results are consistent with the picture given in a previous vertical-field study of only one magnetic reflection from one domain, in which the 3-T field-induced transition is viewed as a spin-flop transition. There is, however, a small amount of irreversible moment rotation (from $\ensuremath{\theta}=43\ifmmode^\circ\else\textdegree\fi{}$ to 48\ifmmode^\circ\else\textdegree\fi{}, where \ensuremath{\theta} is the moment canting angle within the hexagonal basal plane), on passing through the spin-flop transition. This seems to be connected with whether the sample is single or multidomain. In addition, the field independence of the N\'eel temperature ${(T}_{N}=37\mathrm{K})$ has been measured up to 3 T, and data on the domain kinetics are presented.