Antiferroquadrupolar ordering and magnetic properties of the tetragonal DyB2C2 compound

Magnetic properties were investigated on a single-crystalline DyB2C2 compou nd with the tetragonal structure. Spontaneous magnetizations appear along t he a- and [110]-directions below T-c = 15.3 K. A very small sholder is obse rved only along the c-axis at T-Q = 24.7 K, although large lambda-type anom alous specific heats are observed at T-C and T-Q. There exists a large magn etic anisotropy in the tetragonal basal plane which is observed in magnetiz ation processes below T-C. Neutron powder diffraction experiments reveal th at magnetic reflections are observed only below T-C. In the magnetically or dered phase, the Dy moments are arranged to he perpendicular to neighbours along the c-axis. The results are well interpreted by postulating that the phae between T-C and T-Q is an antiferroquadrupolar ordered one. DyB2C2 is a novel compound with a high antiferrocluadrupolar ordering temperature of 24.7 K.