Anomalous Magnetic Ordering Phenomena in Tetragonal TbB₂C ₂ Observed by Neutron Diffraction

Detailed neutron diffraction measurements on a single crystalline TbB ₂C₂ in which magnetic field induced antiferroquadrupolar orderings are realised have been performed to understand characteristics of the transition under zero magnetic field. The results indicate that the magnetic transition phenomena consist of development of at least three magnetic components: (1) a dominant antiferromagnetic component which develops below T_N = 21.7 K, (2) a weak long periodic component which develops below about 18 K, and (3) anomalous components with broad magnetic scatterings which develop below about 50 K, which can not be understood by only a short range magnetic ordering. Since these three components develop independently, the transition phenomena in TbB₂C₂ are much more complicated than expected from a typical λ-type anomaly at T_N in the temperature dependence of magnetic specific heat.