INSTABILITY OF FERROMAGNETISM AT FINITE HOLE DENSITY IN THE U = INFINITY HUBBARD-MODEL

It is shown that the Nagaoka ferromagnetic state is unstable at finite density of holes in the U= Hubbard model. A new estimate for the critical density xc is derived using a mean-field approach. The origin of xc is the discrete nature of the lattice, and results from the competition between two effects: The stabilization of a Fermi sea in the presence of a uniform magnetic field, and the increase in kinetic energy due to the spin-field distortions. © 1990 The American Physical Society.