THEORY OF THE MAGNETIC FORM-FACTOR IN REDUCED-MOMENT KONDO SYSTEMS

The magnetic form factor in Kondo systems, as measured by polarized ne utron elastic scattering, is uniformly reduced (with respect to the at omic form factor in the crystal field) due to the Kondo effect, and mo reover it shows deviations at small scattering vector due to a spin po larization of conduction electrons: the sign of the deviations in Ce c ompounds is positive, which is opposite to what is observed in normal rare earths of the first half of the series, and expected from the s-f exchange Hamiltonian. In the present work the above two anomalies are interpreted within a theory for the magnetic form factor in Kondo sys tems. The uniform reduction of the form factor follows from a variatio nal calculation for the one-impurity Anderson model in an effective ma gnetic field: Kondo reduction of the localized moment is distinguished here from the presence of a screening cloud, which cannot be observed by neutron scattering. The conduction electron polarization is first studied for the Coqblin-Schrieffer Hamiltonian, which we find to lead again to negative deviations of the form factor. Within the Anderson i mpurity model, the conduction electron contribution to the form factor is found to be the sum of two terms: a negative RKKY-like polarizatio n, and a positive superexchange term, which is nonzero even for a fill ed (or empty) band. Dominance of the superexchange polarization could explain the observed sign of the deviations in Ce compounds. A variati onal basis which describes both Kondo reduction and conduction electro n polarization effects is studied: the two effects are clearly separat ed, since they appear in different orders of the calculations. The con duction electron polarization is shown to be reduced due to the Kondo effect by the same spin-fluctuation factor which reduces the localized moment.