Periodic arrangement of quadrupolar moments induced by the antiferromagnetic order in TbB₂C₂: Observation by resonant X-ray scattering

We have investigated the antiferromagnetic ground state, phase IV, of TbB₂C₂ by resonant X-ray scattering to study the background of its anomalous magnetic structure. Polarization analysis was carried out to clarify the origin of the superlattice and forbidden reflections that appear below the Néel temperature. We observed magnetic reflections of (1 0 0.5) and (0 0 0.5), consistently with the neutron diffraction experiment by Kaneko et al. In addition, we have detected the (1 0 0) forbidden reflection in the unrotated σ-σ′ channel, which was not observed by neutron diffraction. From the detailed analysis of the resonant X-ray scattering, we conclude that this reflection originates from the ordering of the quadrupolar moment that accompany the canted antiferromagnetic structure of TbB₂C₂ because of the strong spin-orbit coupling. No lattice distortion was detected.