Long-period, longitudinal spin density modulation in an itinerant 5f magnetic compound UCu₂Si₂

The magnetic structure of UCu₂Si₂ has been studied by means of neutron scattering. We have observed the first-and very weak third-order incommensurate satellite reflections for 100 K<T<106 K with the propagation q ≤ [0 0 δ], δ ≤ 0.116 at T ≤ 101 K, indicating nearly sinusoidal magnetic modulation with long periodicity of Λ ≤ 85.7 Å. The intensity of the first-order satellite reflections is consistent with a moment direction parallel to the c-axis. The second-order satellite due to lattice modulation accompanied by a charge density wave could not be detected within our experimental sensitivity. The long-period magnetic modulation in UCu₂Si₂ cannot be explained in terms of an ANNNI model based on frustrated antiferromagnetic short-range interactions. The nearly sinusoidal long-period modulation is evidence for a spin density wave state as a consequence of itinerant behaviour of the 5f electrons in UCu₂Si₂. The ferromagnetic ground state and the very weak upper critical field for the incommensurate phase are consistent with the long periodicity.