LOW-TEMPERATURE MAGNETIC-STRUCTURES OF UPDSN

The intermetallic compound UPdSn has been studied by means of dc magnetic susceptibility and neutron diffraction. While it has been assumed in previous work that the space group is P6(3)/mmc and there is disorder between the Pd and Sn atoms, we find a significantly better fit to our data with the related space group P6(3)mc which allows Pd/Sn ordering. There are two magnetic phase transitions at approximately 25 and 40 K. Above 40 K, there is no long-range magnetic order, but between 25 and 40 K, a canted antiferromagnetic structure forms, in which the moments lie in, or close to, the y-z-plane of a double-sized orthorhombic cell, with an angle of approximately 54-degrees away from the z-axis. Below 25 K, the moments rotate out of the y-z-plane by approximately 45-degrees to form a second related non-collinear structure. At the lowest temperature measured, the uranium moment is (2.05 +/- 0.13)-mu-B.