Magnetic moment and valence fluctuations in a model for rare earth compounds

The ionic model proposed by Jefferson and Stevens (J. Phys. C.: Solid St. Phys.; 9:2151 (1976)) for the magnetic properties of rare earth compounds that display intermediate valence behaviour is studied. The f-electrons of the rare earth ions are described by localised atomic-like configurations which are hybridised with a conduction band. The free energy and susceptibility are calculated perturbationally in powers of the hybridisation matrix element V to order V4. This enables the various contributions that cause a deviation of the susceptibility from a Curie law, on approaching from the high-temperature limit, to be distinguished. When only virtual valence transitions are possible the leading correction to the Curie law susceptibility is due to the Kondo moment flip scattering. An effective interaction between the f- and conduction electrons is derived in this regime which is a generalisation of that given by Coqblin and Schrieffer (Phys. Rev.; 185:847 (1969)). In the intermediate valence or valence fluctuation regime, the susceptibility at high temperatures is consistent with a Curie-Weiss law chi varies as (T + Tsub(f))-1, Tsub(f) varies as V2, change of chemical pote effects being taken into account to order V2. The one-electron Green function is also calculated perturbationally and the results indicate that the spectral density has a virtual bound state resonance due to the incoherent scattering of the f-moments similar to that caused by impurity scattering. (author)