Neutron-scattering study of incommensurate magnetic order in the heavy-fermion superconductor UNi2Al3

Elastic neutron scattering from a single-crystal sample of the heavy-fermion superconductor UNi2Al3 has revealed the onset of long-range magnetic order below TN=4.6 K. This order is characterized by an incommensurate (IC) ordering wave vector given by (12±τ, 0, 12) with τ=0.11±0.003. The intensity of several magnetic satellite Bragg peaks within the (h,0,l) plane is well described by a model in which the spins lie within the basal plane and are modulated in amplitude from site to site. By applying a magnetic field to select from all the possible domains, we find that the moment is polarized along the a direction, with a maximum amplitude of 0.21±0.1μB per uranium atom. The order-parameter exponent β associated with this transition is 0.34±.03, which is typical of three-dimensional ordering transitions. Measurements down to ∼0.3 K show that the magnetic order coexists with superconductivity below TC∼1.2 K, and that these states are coupled as shown by anomalous behavior of the magnetic order parameter around TC. Measurements were also made in magnetic fields of up to 8 T applied perpendicular to the (h,0,l) plane, along (-1,1,0), a near-neighbor direction within the hexagonal basal plane. While the field does not influence TN, it does increase the intensity of the magnetic Bragg peaks by a factor of ∼1.5, as well as increase the IC part of the ordering wave vector at low temperatures.