FERROMAGNETISM IN THE HUBBARD-MODEL ON THE SQUARE LATTICE - IMPROVED INSTABILITY CRITERION FOR THE NAGAOKA STATE

The instability of the fully polarized ferroma,gnetic state (Nagaoka s tate) with respect to single spin flips is re-examined for the Hubbard model on the square lattice with a large family of variational wave f unctions which include correlation effects of the majority spins in th e vicinity of the flipped spin. We find a critical hole density of del ta(cr) = 0.251 for U = infinity and a critical coupling of U-cr = 77.7 t. Both values improve previous variational results considerably.