Quadrupole ordering and multipole interactions in Pr-based compounds

Field-temperature phase diagrams of cubic compounds PrPb₃ and PrOs₄Sb₁₂ with non-magnetic crystalline electric field (CEF) ground state have been examined by means of low-temperature magnetization measurements. PrPb₃ is a typical Γ₃ doublet CEF ground state system which undergoes an antiferro-quadrupole (AFQ) transition at T_Q = 0.4 K. In a magnetic field T_Q(H) exhibits re-entrant behaviour depending on the field direction, which can be interpreted in terms of interactions between induced dipole (and possibly octupole) moments in the AFQ phase. In the heavy-electron superconductor PrOs₄Sb₁₂, a field-induced ordered state has been observed above 4 T for all the principal directions. The results are in favour of a Γ₁ CEF ground state model in which a level crossing with a Γ₅ triplet excited state induces a long-range (possibly quadrupolar) ordering in a magnetic field.