Structural properties and magnetic structure of the heavy-fermion compound UCu5Sn

The heavy-fermion compound UCu5Sn has been studied by means of neutron powder diffraction. This compound exhibits the hexagonal CeNi5Sn-type structure with space group P6(3)/mmc. We have determined the lattice parameters and atomic positions at room temperature, using a high-resolution diffractometer The neutron diffraction diagrams were also measured at various temperatures between 1.4 and 61 K, using an 800-cell position-sensitive diffractometer. The observed minimum in the unit-cell volume at temperature of 30 K, bring rather far below the magnetic ordering one (T-C = 53 K), may suggest a change in the strength of the Sf-conduction electron hybridization which probably takes place at low temperatures. It was found that the magnetic phase transition at Te corresponds to the onset of a ferrimagnetic order. The magnetic structure is characterized by forming the ferromagnetic planes (001) with the (+ - + -) stacking sequence, where the U moments are oriented parallel to the hexagonal c-axis. However, the size of the U moments arranged in the z = 0 plane (M-U1), and formed by the sites 2(a), is substantially larger than these arranged in the adjacent z = (1)/(4) plane (M-U2), and formed by the sites 2(c). At 1.4 K, they are M-U1 = 2. 14(2) mu(B)/atom U and M-U2 = 0.18(4) mu(B)/atom U. We discuss several mechanisms leading to a reduction of the magnetic moment at the one site compared to the other one.