Incommensurate magnetic order in the heavy fermion superconductorUNi2Al3

Neutron scattering measurements show ${\mathrm{UNi}}_2$ ${\mathrm{Al}}_3$ to be unique among heavy fermion superconductors in that below ${\mathit{T}}_{\mathit{N}}$∼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 ${\mathit{a}}^{\mathrm{*}}$. The magnitude of the maximum ordered moment is found to be very small; ${\mathrm{\mu }}_{\mathrm{ord}}$=(0.24±0.10)${\mathrm{\mu }}_{\mathit{B}}$.