Valence fluctuations in ferromagnetic 4fsystems

We investigate the coexistence of ferromagnetism and intermediate valence in 4f systems which have ferromagnetic ground states in the normal-valence phase and fluctuate between a magnetic (J{≠}0) and a nonmagnetic (J=0) state in the intermediate-valence phase. We use a model which is generated from the well-known s-f model by inclusion of a hybridization term. The alloy analogy of the model is solved for zero temperature in the average T-matrix approximation. For the local magnetic 4f moment mf we derive phase diagrams in terms of the hybridization V, the bandwidth W (closely related to external pressure), the direct exchange J₀, and the s-f exchange g (g>0). The hybridization V, as well as a gap reduction as a consequence of a pressure-induced band broadening, intensify electronic fluctuations and therefore tend to destabilize the local moment, while the exchange integrals g and J₀ favor the opposite. We find regions of saturated (mf=nf), reduced (0mf{n}f$), and quenched f moments (${m}f$=0) even in intermediate-valence phases. Quasiparticle densities of states for typical parameter constellations are derived and discussed. They show an ever-existing hybridization gap.