Effects of Y dilution on the antiferroquadrupolar ordering in the tetragonal compound DyB₂C₂

Antiferroquadrupolar (AFQ) and magnetic transitions in Dy_1-xY_xB₂C₂ (0 ≤ x ≤ 0.6) have been investigated on the single-crystalline samples by magnetization and specific heat measurements. As the Y concentration increases, the magnetic transition temperature decreases from 15.3 K at x = 0 to 4.5 K at x = 0.6. The AFQ transition temperature also decreases from 24.7 K at x = 0 to 7.4 K at x = 0.6, although it becomes unclear for x ≥ 0.5 in the magnetization. A λ-type peak of specific heat at T_Q becomes broader as x increases, and changess to a small upheaval for x ≥ 0.5. On the other hand, the field-induced transitions at the boundary between the AFQ and paramagnetic phases are clearly observed in the magnetization processes at 1.5 K x ≤ 0.6. The behavior of specific heat implies that the AFQ interaction acts strongly in a rather shorter distance than the antiferromagnetic interaction in DyB₂C₂. The Y dilution causes in a wide distribution of the AFQ interaction strength acting on the Dy quadrupoles. Then the AFQ order developes in a wide temperature range, and is almost completed above the magnetic transition temperature.