Hybridization-mediated anisotropic coupling in plutonium compounds

The magnetic behavior of a class of cerium and light actinide compounds containing moderately delocalized f electrons has been explained on the basis of an anisotropic two-ion interaction that arises from the hybridization of band electrons and the f electrons. This theory, first developed by Siemann and Cooper for cerium compounds using the treatment of Coqblin and Schrieffer for the hybridization, was later generalized by Thayamballi and Cooper to fⁿ systems in the L-S and j-j coupling limits. We here extend the theory to the case of intermediate intraionic coupling and further include the possibility of long-period antiferromagnetic structures. In particular, we have considered the Pu³⁺(f⁵) ion in PuSb. The theory reproduces the experimentally observed magnetic behavior of PuSb quite closely, predicting a phase transition from a low-temperature ferromagnetic phase to a long-period antiferromagnetic phase at about 75 K, for a fitting to a N\'eel temperature of 85 K, with ordered moments close to the experimental values. However, while the modulation in the long-period antiferromagnetic phase has been experimentally observed to be longitudinal, the theory predicts a transverse modulation with moments aligned along the cube edge. We also present the $T=0$ magnetic excitation spectrum in the ferromagnetic phase calculated on the basis of this theory using the random-phase approximation.