INCOMMENSURATE MAGNETIC ORDER IN THE HEAVY-FERMION SUPERCONDUCTOR UNI2AL3

Neutron scattering measurements show UNi2Al3 to be unique among heavy fermion superconductors in that below T(N) approximately 5.2 K it disp lays long range magnetic order which is incommensurate with its chemic al lattice. The ordering wave vector is (1/2 +/- delta, 0, 1/2) where delta = 0.110 +/- 0.003. The measured superlattice intensities are wel l described by the presence of a longitudinal spin density wave within the hexagonal basal plane, polarized along a. The magnitude of the m aximum ordered moment is found to be very small; mu(ord) = (0.24 +/- 0 .10)mu(B).