Octupolar ordering of Γ8 ions in magnetic field

Motivated by the recent suggestion that the 25K phase transition of NpO2 may be explained as octupolar ordering, we study f -electron lattice models which are capable of supporting octupolar, as well dipolar and quadrupolar order. Analyzing the properties of the Γ8 ground state quartet, we find that (111)-type combinations of the Γ5 octupoles T β 111 = T β x +T β y +T β z are the best candidates for octupolar order parameters. Octupolar ordering induces Γ5-type quadrupoles as secondary order parameter. Octupolar order is to some extent assisted, but in its basic nature unchanged, by allowing for the presence of quadrupolar interactions. The bulk of our paper is devoted to a study of the effect of an external magnetic field on octupolar ordering. We found that octupolar order survives up to a critical magnetic field if the field is lying in specific directions, while for general field directions, the underlying symmetry of the model is destroyed and therefore the phase transition suppressed even in weak fields. Field-induced multipoles and field-induced couplings between various order parameters are discussed on the basis of a group theoretical analysis of the Helmholtz potential. We also studied the effect of octupolar ordering on the non-linear magnetic susceptibility which satisfies novel Ehrenfest-type relations at continuous octupolar transitions.