Antiferro-Quadrupolar Ordering and Multipole Interactions in PrPb₃

抄録

Antiferro-quadrupolar (AFQ) ordering in the cubic Γ ₃ ground state system PrPb₃ has been studied by means of magnetization and specific heat measurements under magnetic fields H. Field variation of the AFQ transition temperature T_Q (0.4 K at H=0) has been determined for the three crystallographic directions ‹100›, ‹110› and ‹111›. The obtained T_Q(H) is strongly anisotropic, with T_Q^‹100›(H) the largest (∼0.7 K at 6 T). The AFQ phase diagram and the magnetization behavior observed for H allel ‹100› are well explained by a model that incorporates both antiferromagnetic and antiferroquadrupolar interactions. The interactions between field-induced staggered magnetic moments stabilize the AFQ order. This simple model, however, seriously fails to explain the observed anisotropy: T_Q^‹100›(H)>T_Q^‹110›(H)>T_Q^‹111›(H). Reconsidering the field-induced multipole moments in the AFQ phase, we propose that inclusion of a weak ferro-octupolar interaction between Γ ₅ type octupole moments is important in reproducing the anisotropic phase diagram of PrPb₃.