Effects of intersite Coulomb interactions on ferromagnetism: Application to Fe, Co, and Ni

We reanalyze the condition for metallic ferromagnetism in the framework of the tight-binding approximation and investigate the consequences of the intersite Coulomb interactions using the Hartree-Fock approximation. We first consider a nondegenerate s band, we show that the intersite interactions modify the occurrence of ferromagnetism, and we derive a generalized Stoner criterion. We analyze the main effects due to the renormalization of the hopping integrals by the intersite Coulomb interactions. These effects are strongly dependent on the relative values of the intersite electron-electron interactions and on the shape of the density of states as illustrated by a study of cubic crystals which allowed us to establish general trends. We then investigate a realistic spd tight-binding model, including intra (Coulomb and exchange) and intersite charge-charge Coulomb integrals. This model is used to study the electronic structure (band structure, densities of states, magnetic moment) of bulk ferromagnetic $3d$ transition metals Fe(bcc), Co(hcp and fcc), and Ni(fcc). An excellent agreement with local spin-density-functional calculations is obtained for the three metals, in particular concerning the different widths of the majority and minority spin bands. Thus the tight-binding Hartree-Fock model is capable to describe this effect and provides its consistent interpretation.