MAGNETIC-PROPERTIES OF UCUGA AND U2CUGA3 BY AC-MAGNETIC SUSCEPTIBILITY AND NEUTRON-DIFFRACTION

The uranium intermetallics UCuGa and U2CuGa3 have been studied by mean s of neutron diffraction and AC-, DC-magnetic susceptibility measureme nts. UCuGa crystallizes either in the hexagonal LiGaGe-type (space gro up P6(3)mc) or in the CaIn2-type structure (P/6(3)mmc), whereas U2CuGa 3 belongs to the slightly different hexagonal structure of Lu2CoGa3-ty pe (P/6(3)mmc). Below T-N = 30 K, UCuGa shows a collinear antiferromag netic ordering which corresponds to a double-sized orthorhombic cell w ith k = (0, 1/2, 0). The uranium magnetic moments are parallel to the b-axis of the crystal unit cell with a value of (1.27 +/- 0.06)mu(B) a t 1.3 K. Such a magnetic structure is consistent with a two-dimensiona l X-Y magnetic moment system. For U2CuGa3 no magnetic transition can b e detected by neutron diffraction. Indeed, magnetic susceptibility exp eriments show that its magnetic behaviour could be interpreted on the basis of spin-glass properties.