The structure and magnetic moment distribution in the antiferromagnetic phase of

The antiferromagnetic structure of the intermetallic compound has been determined from neutron polarimetric measurements and refined by combining these data with integrated intensity measurements. The structure was found to be non-collinear with the U moments confined to the a - b plane. The moments of U atoms in each of the two sets of sixfold sites are arranged hexagonally with rotations of between them and the two sets are rotated with respect to one another by . The third (twofold) set of U atoms has no ordered moment. These conclusions are in disagreement with a previous determination of the structure from powder data which gave a collinear structure with moments parallel to the c axis. Magnetization measurements made on single crystals in the temperature range 300 - 2 K can be understood in terms of a transition to a non-collinear easy plane antiferromagnetic structure stable below 22 K. Polarized neutron measurements have been used to determine the contribution of each of the U sites to the susceptibility between 22 and 2 K. These show that of the two sixfold U sites, that with the smaller ordered moment contributes more than half of the total susceptibility. The twofold site, which is characterized by a small U - U separation, makes the smallest contribution.