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

We have investigated the anti ferromagnetic 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 Neel 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 sigma-sigma' 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.