Stabilization of d-band ferromagnetism by hybridization with uncorrelated bands

We investigate the influence of s-d or p-d hybridization to d-band ferromagnetism to estimate the importance of hybridization for the magnetic properties of transition metals. To focus our attention on the interplay between hybridization and correlation we investigate a simple model system consisting of two nondegenerate hybridized bands, one strongly correlated and the other one quasifree. To solve this extended Hubbard model, we apply simple approximations, namely the spectral density approach and the modified alloy analogy, that, concerning ferromagnetism in the single-band model, are known to give qualitatively satisfactory results. This approach allows us to discuss the underlying mechanism by which d-band ferromagnetism is influenced by hybridization on the basis of analytical expressions. The latter clearly display the order and the functional dependencies of the important effects. It is found that spin-dependent interband particle fluctuations cause a spin-dependent band shift and a spin-dependent band broadening of the Hubbard bands. The shift stabilizes and the broadening tends to destabilize ferromagnetism. Stabilization requires relatively high band distances and small hybridization matrix elements. Superexchange and Ruderman-Kittel-Kasuya-Yosida coupling are of minor importance.