Antiferro-Quadrupolar Ordering and Multipole Interactions in PrPb3

Abstract

Antiferro-quadrupolar (AFQ) ordering in the cubic Γ 3 ground state system PrPb 3 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 ∥<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 PrPb 3 .