“Influence of valence instabilities on the Curie-temperature of ferromagnetic 4f-systems”

We inspect the stability of ferromagnetism in 4f-systems, which have an insulating, ferromagnetic ground state in the integral valent phase and fluctuate between a magnetic and a nonmagnetic configuration in the intermediate valent phase. It turns out that for large gaps between 4f-level and lower edge of the empty conduction band ferromagnetism is created by a certain interplay between the s-f hybridization, which allows real or virtual electron transitions between f-level and conduction band, and the s-f exchange interaction, which mediates an indirect coupling between the localized 4f-moments. When the gap is reduced by external pressure or by alloying with proper impurities the local f-moment becomes steadily quenched while simultaneously the coupling between the more and more reduced moments is drastically enhanced. These two competitive effects lead to a distinct maximum of the Curie-temperature just in the intermediate valence phase.