MAGNETISM IN UPDSI

We report on bulk electronic properties of UPdSi in conjunction with r esults of X-ray-and neutron-diffraction studies. Both, diffraction exp eriments clearly show that UPdSi crystallises in the TiNiSi-type (spac e group Pnma) structure. Magnetic susceptibility, specific heat and el ectrical resistivity anomalies around 33 and 27 K indicate that UPdSi orders magnetically below 33 K and undergoes an additional magnetic ph ase transition at 27 K. The antiferromagnetic ground state is decoment ed by purely magnetic reflections in the neutron-powder-diffraction pa tterns collected at low temperatures and by metamagnetic transitions a t 4 and 7 T observed in the magnetisation curve at 4.2 K. The magnetis ation measured on a sample, consisting of powder particles that are fr ee to be oriented by the applied field does not saturate even in field s up to 35 T, where it reaches 1.3 mu(B)/f.u. The magnetic structure d etermined at low temperatures (T = 3.5 K) is collinear antiferromagnet ic with the propagation vector q = (0.25, 0, 0.25). This structure is built-up of eight-layer + + + + - - - - blocks of ferromagnetic planes of U moments (mu(U) = 1.401 (7)mu(B)) piled up along the [101] direct ion. Between T-N(= 33 K) and 27 K, we observe an incommensurate magnet ic structure with the propagation vector q = (0.217, 0, 0.217). It is characterized by a sine-wave profile along [1,0, 1] of b-axis oriented U-moments with an amplitude of 0.65(18)rho(B) at 31 K.