Incommensurate Longitudinal SDW State with a Long Periodicity in UCu2Si2

Neutron diffraction experiment on a high quality single crystal of UCu 2 Si 2 has been carried out in order to clarify magnetic structures of this compound. At T = 101 K, we have observed incommensurate magnetic satellite reflections with the propagation q = [0 0 δ], δ= 0.116, indicating the magnetic amplitude modulation with a long periodicity, Λ= 85.7 A corresponding to roughly 17 uranium basal plane layers. From a careful analysis of integrated intensities of the magnetic satellites, we determined that the magnetic structure of antiferromagnetic phase is the longitudinal and nearly sinusoidal spin density modulation with the first and third order modulation amplitude µ 1 = 1.3 µ B /U and µ 3 = 0.1 µ B /U, respectively. The mean field behavior of the antiferromagnetic order parameter, µ 1 ∝( T N - T ) 1/2 is consistent with the gap formation of a SDW/CDW state. These results can be illustrated with the strong itinerant character of 5 f electrons in this compound.