Electron Correlation in a Doubly Degenerate Band

The effect of intra‐atomic Hund's rule coupling on the occurrence of ferromagnetism is investigated by considering two orbitally degenerate narrow bands with arbitrary density of states and strength of intra‐atomic interaction. By using the Green's function technique the two orbitally degenerate narrow bands split due to intra‐atomic Coulomb and exchange interaction into Hubbard‐like spin‐polarized sub‐bands. To discuss the stability of the ferromagnetic and paramagnetic state the energy of the system is calculated for a square density of states. In the strongly correlated limit eight Hubbard‐like spin‐polarized sub‐bands appear and the ferromagnetic state is favoured if the difference between the Coulomb repulsion U and exchange energy J on an atom is finite. In the case where the ratio U1/Δ between U1 = U — J and bandwidth Δ of the unperturbed bands tend to infinity the ferromagnetic and paramagnetic state are degenerated energetically.