Neutron diffraction study of magnetic ordering in UPd2Si2, UPd2Ge2, URh2Si2 and URh2Ge2

Neutron diffraction measurements on polycrystalline samples show that UPd2Si2, UPd2Ge2 and URh2Si2 become antiferromagnetic below temperatures of 150 K, 140 K and 137 K respectively. The magnetic moment ordering in UPd2Si2 and UPd2Ge2 is represented by a static longitudinal wave propagating along the c axis of the tetragonal body-centred crystal lattice. The magnetisation wave is polarised in the direction of the c axis: q = (0,0,0.662 +- 0.010)c* in UPd2Si2 and q = (0,0,0.748 +- 0.010)c* in UPd2Ge2. At 40 K, UPd2si2 exhibits a magnetic phase transition: a commensurate magnetisation wave with q = (0,0,1)c* represents the growth of a simple antiferromagnetic ordering. URh2Si2 is a collinear antiferromagnet with coinciding magnetic and chemical unit cells. The sequence of ferromagnetic sheets piled up along the tetragonal axis is given by +-. The values of the root mean square of the magnetic moment on the U ion at 4.2 K are found to be 1.96 μsub(B), 1.97 μsub(B) and 1.96 μsub(B) for UPd2Si2, UPd2Ge2 and URh2Si2 respectively. The observed magnetic ordering schemes are discussed in terms of the RKKY exchange interaction. Neutron diffraction measurements performed at zero magnetic field show that URh2Ge2 is paramagnetic at 4.2 K. (author)