MAGNETIC-ORDERING IN UPTSI

We report on bulk electronic properties of polycrystalline UPtSi in co njunction with results of neutron-powder-diffraction studies performed al low temperatures. Specific anomalies in the temperature dependenci es of magnetic susceptibility, specific heat and electrical resistivit y around 51 K indicate that UPtSi orders antiferromagnetically below t his temperature. The antiferromagnetic ground state is documented by p ure magnetic reflections in the neutron-powder-diffraction patterns co llected below 51 K and by metamagnetic transitions in the magnetizatio n at 4.2 K. The magnetization above the metamagnetic transitions measu red on a field-oriented powder sample reaches 0.94 mu(B)/f.u. in 52 T. Higher fields are needed, however, to obtain saturation. Neutron-diff raction results in the paramagnetic range confirm that UPtSi crystalli zes in the orthorhombic CeCu2-type structure (space group Imma). The m agnetic structure determined by means of neutron powder diffraction at low temperatures (T = 1.5 K) is collinear, incommensurate with the cr ystallographic structure (propagation vector k = (0.535(2), 0, 0)): th is structure consists of sine-modulated U moments directed within the a-c plane, with maximum size of 1.30(5)mu(B)/f.u.