Ferromagnetism in the Hubbard model on the square lattice: Improved instability criterion for the Nagaoka state



The instability of the fully polarized ferromagnetic state (Nagaoka state) with respect to single spin flips is re-examined for the Hubbard model on the square lattice with a large family of variational wave functions which include correlation effects of the majority spins in the vicinity of the flipped spin. We find a critical hole density of δcr = 0.251 for U = ∞ and a critical coupling of Ucr = 77.7t. Both values improve previous variational results considerably.