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.
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