Magnetic Order in Intermediate Valence Systems

Abstract

AbstractA „hybridized”︁ s—f model is used in order to investigate the possibility of a coexistence of ferromagnetism and intermediate valence in systems which fluctuate between a magnetic and a nonmagnetic 4f configuration. The Curie temperature Tc, is determined, the local magnetic f‐moment mf, the valence nf and the static susceptibility χ as functions of the f‐level position Ef, the hybridization V, the s—f exchange g, and the Bloch bandwidth W for semiconducting as well as metallic 4f systems. It is found that electronic fluctuations destablize the local moments, but enhance the coupling between them. This leads to a Tc‐maximum just in the intermediate valence phase. Many results remarkably depend on, how the gap between the 4f‐level and the lower band edge (or Fermi‐edge) is closed, by a shift of the f‐level towards the conduction band (alloying!) or by a respective broadening of the band (external pressure!). Striking differences between semiconducting and metallic 4f‐systems are observed, while qualitatively similar behaviour is found for systems with ferromagnetic and systems with antiferro magnetic s—f exchange coupling. Magnetic phase diagrams in terms of Ef, W, V, and conduction band occupation n are derived and discussed. Qualitative agreement is found with recently performed experiments on systems like EuO, EuRh3B2, CeNixPt1−x, Ce(Rh1−xOsx)3B2,‥.