Incommensurate magnetic order in the heavy fermion superconductor UNi2Al3

Neutron scattering measurements show UNi2Al3 to be unique among heavy fermion superconductors in that below TN∼5.2 K it displays long range magnetic order which is incommensurate with its chemical lattice. The ordering wave vector is (1/2±δ, 0, 1/2) where δ=0.110±0.003. The measured superlattice intensities are well described by the presence of a longitudinal spin density wave within the hexagonal basal plane, polarized along a*. The magnitude of the maximum ordered moment is found to be very small; μord=(0.24±0.10)μB.