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, ${\ensuremath{\kappa}}_{?}$/${\ensuremath{\kappa}}_{\ensuremath{\perp}}$, parallel and perpendicular to the moment direction, that compare well with those of diffuse critical neutron scattering experiments. In ${\mathrm{Ce}}^{3+}$ (${f}^{1}$) compounds, we find that as the CF interaction (${\ensuremath{\Gamma}}_{7}$ 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 ${\ensuremath{\kappa}}_{?}$/${\ensuremath{\kappa}}_{\ensuremath{\perp}}$ 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 ${\ensuremath{\kappa}}_{?}$/${\ensuremath{\kappa}}_{\ensuremath{\perp}}$ for the case of ${\mathrm{Pu}}^{3+}$(${\mathrm{f}}^{5}$) and ${\mathrm{U}}^{3+}$(${\mathrm{f}}^{3}$) compounds.