Magnetic phase diagrams of Dy _0.8Gd _0.2B ₂C ₂ with the antiferroquadrupolar ordering in the antiferromagnetic phase

Gd substitution for Dy in DyB ₂C ₂ that undergoes an antiferroquadrupolar (AFQ) ordering at T _Q = 24:7K and an antiferromagnetic (AFM) one at T _N = 15:3K enhances the AFM interaction and weakens the AFQ interaction. In Dy _0.8Gd _0.2B ₂C ₂, the AFQ transition occurs at T _Q = 17:6K below the AFM one at T _N = 19:0 K. However, the strong competition between the AFQ and AFM interactions supresses the development of the AFQ order parameter below T _Q and causes an anomalous behavior in field-induced transitions. The magnetic phase diagrams of Dy _0.8Gd _0.2B ₂C ₂ drawn from the experimental results of specific heat and magnetization have anomalous phase boundaries originating from this competition.