Instability of ferromagnetism at finite hole density in theU=∞ Hubbard model

Abstract

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 ${\mathit{x}}_{\mathit{c}}$ is derived using a mean-field approach. The origin of ${\mathit{x}}_{\mathit{c}}$ 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.