On the stability of ferromagnetism in intermediate valence systems

Abstract

We study the stability of ferromagnetism in intermediate valence systems which fluctuate between a magnetic (J≠0) and a non-magnetic (J=0) state as e.g. EuO. We consider thes-f model extended by a hybridization term as a good description of the normal valence as well as the intermediate valence phase of such materials. Special attention is devoted to the competing influence ofs-f exchange ands-f hybridization on the Curie temperatureT _c. For large gaps between the localizedf-level and the conduction band (normal valence phase!)s-f exchange ands-f hybridization both tend to stabilize magnetism giving rise to an effective exchange interaction betweenf-moments. This effective exchange increases with decreasing gap leading to a substantialT _c-enhancement for small gaps. In the intermediate valence phase, however, electronic fluctuations destroy the collective magnetism, and that twofold, namely by reducing the local magnetic moment and by enhancing entropy-influences. The latter leads toT _c→0, therewith determining the boundary between the ferromagnetic and paramagnetic phase. A corresponding phase diagramm, in dependence of hybridizationV andf-level positionE _f, is derived and discussed.