Nature of the Antiferromagnetic Order in GdCu$_2$

Abstract: A magnetic order in orthorhombic GdCu$2$ was investigated via a single crystal neutron diffraction with thermal neutron. Magnetic peaks were observed at incommensurate positions described by the ordering vector $q$=(${\delta}$,~1,~0) with ${\delta}$=0.678 at 4.6~K. This ordering vector is close to the commensurate one with $q{\rm c}$=(2/3,~1,~0) reported earlier, but clearly deviates. The incommensurate nature of the magnetic order in GdCu$2$ is further corroborated by the peak shift with temperature below $T{\rm N}$.