Magneto-structural distortions in the noncollinear hexagonal antiferromagnet UPdSn

Abstract

The intermetallic compound UPdSn has been studied by means of high-resolution neutron powder diffraction at low temperature, and both orthorhombic and monoclinic structural distortions have been observed. In addition to confirming the previously published noncollinear antiferromagnetic structures with these symmetries, full Rietveld refinements including the magnetic cross sections have been made on the new data, jointly with older low-resolution data. The structure is monoclinic below 25 K with space group P2 1 and magnetic symmetry P c2 1, orthorhombic between 25 and 40 K with structural space group Cmc2 1 and magnetic space group P Cm′c2 1, and paramagnetic above 40 K with space group P6 3mc. The temperature variations of the orthorhombicity ( b/√3− a) and monoclinicity (γ−90) parameters have been extracted and the monoclinicity is linearly coupled to the x-component of the uranium magnetic moment μ x , the ‘magnetic monoclinicity’ order parameter. In contrast, the orthorhombicity seems to be coupled to the total uranium magnetic moment. The sign of the coupling between μ x and γ−90 has also been determined. It is positive, that is the projected moments prefer to point across the shorter diagonal of the monoclinic basal plane, rather that the longer diagonal.