Anisotropy of critical correlations in moderately delocalized cerium and actinide systems

The equilibrium and excitation magnetic behavior of a class of cerium and light actinide compounds have been explained previously, in a theory first developed by Siemann and Cooper, in terms of a band-f-electron anisotropic hybridization-mediated two-ion interaction of the Coqblin-Schrieffer type. Using the same theory, we present here a calculation, within the random-phase approximation, of the longitudinal component of the static wave-vector-dependent susceptibility in the paramagnetic phase. The calculations have been performed in the presence of a cubic crystal field (CF) and yield results for the ratio of inverse critical correlation lengths, κ?/κ_⊥, parallel and perpendicular to the moment direction, that compare well with those of diffuse critical neutron scattering experiments. In Ce³⁺ (f¹) compounds, we find that as the CF interaction (Γ₇ ground state) predominates over the two-ion interaction, the relative strength of the coupling within the ferromagnetic {001} planes (with moments perpendicular to the planes) and that between the {001} planes is gradually reversed, resulting in a ratio κ?/κ_⊥ smaller than unity, as is experimentally observed. We also present results for the effect of differing intraionic (L-S, intermediate, and j-j) coupling on κ?/κ_⊥ for the case of Pu³⁺(f⁵) and U³⁺(f³) compounds.