Magnetic phase diagrams of Dy_0.8Gd_0.2B₂C₂

Gd substitution in DyB₂C₂, which undergoes an antiferroquadrupolar (AFQ) ordering at T_Q = 24.7 K and an antiferromagnetic (AFM) one at T_N = 15.3 K, enhances the AFM interaction and weakens the AFQ interaction, and the transition sequence at 20% Gd is the reverse of that in DyB₂C₂. The AFQ transition occurs at T_Q = 17.5 K, below the AFM one at T_N = 19.0 K in Dy_0.8Gd_0.2B₂C₂. The present work provides some experimental results on the specific heat and magnetization of Dy_0.8Gd_0.2B₂C₂. The magnetic phase diagrams of Dy_0.8Gd_0.2B₂C₂ have some resemblance to those of HoB₂C₂ and Ce_0.75La_0.25B₆. There is an anomalous magnetic phase (named phase IV) that is adjacent to the AFQ phase in each compound.