Antiferro-quadrupolar (AFQ) ordering in the cubic Γ3 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 TQ (0.4 K at H = 0) has been determined for the three crystallographic directions 〈100〉. 〈110〉 and 〈111〉. The obtained TQ(H) is strongly anisotropic, with T〈100〉Q (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〈100〉Q (H) > T〈110〉Q (H) > T〈111〉Q (H). Reconsidering the field-induced multipole moments in the AFQ phase, we propose that inclusion of a weak ferro-octupolar interaction between Γ5 type octupole moments is important in reproducing the anisotropic phase diagram of PrPb3.
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