Extraordinary Hall-effect in ferromagnetic (rare-earth)Al2

The extraordinary Hall effect of ferromagnetic metals is ascribed to the asymmetric scattering of the conduction electrons through two basic mechanisms, namely the skew-scattering effect and the side-jump effect. For the pure rare-earth metals the explanation of these effects was first given from the Coulomb interaction between conduction electrons and localized $4f$ electrons. We present here Hall-effect measurements on some intermetallic compounds of the form $R{\mathrm{Al}}_2$ ($R=\mathrm{P}\mathrm{r},\mathrm{N}\mathrm{d},\mathrm{G}\mathrm{d},\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{T}\mathrm{b}$). The same qualitative features previously observed for the rare earths are also found. However the Coulomb interaction alone fails to give an extraordinary Hall effect in the case of ferromagnets like Gd and Gd${\mathrm{Al}}_2$ where there is an $S$-state magnetic ion and the other possible sources in the formation of the effect, particularly covalent mixing interaction and intrinsic spin-orbit coupling of the conduction band, are briefly considered.