Neutron diffraction study of magnetic ordering in UPd2Si2, UPd2Ge2, URh2Si2and URh2Ge2

Abstract

Neutron diffraction measurements on polycrystalline samples show that UPd₂Si₂, UPd₂Ge₂ and URh₂Si₂ become antiferromagnetic below temperatures of 150K, 140K and 137K 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+or-0.010)c* in UPd₂Si₂ and q=(0, 0, 0.748+or-0.010)c* in UPd₂Ge₂. At 40K, 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 4.2K are found to be 1.96 mu _B, 1.97 mu _B and 1.96 mu _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 4.2K.