Anomalous magnetic scattering in tetragonal RB₂C₂ (R = rare earth) observed by means of neutron diffraction

Detailed neutron diffraction experiments have been performed on tetragonal R¹¹B₂C₂ (R = rare-earth) compounds which show characteristic antiferroquadrupolar orderings. TbB₂C₂, ErB₂C₂ and HoB₂C₂, which have magnetic long-periodic states, show anomalous magnetic diffuse scattering with flat intensity. The anomalous diffuse scattering exists in a small region in the reciprocal space, surrounded by satellite peaks, due to the long-periodic magnetic structures. The profile and temperature dependence of the anomalous diffuse scattering indicate that the origin cannot be understood on the basis of magnetic short-range correlations.