The phase diagram of the Hubbard model in an external magnetic field is
examined by extrapolation of small-cluster exact-diagonalization calculations.
Using a general expression for the hopping matrix elements (tij∼q∣i−j∣) the influence of long-range hopping (band asymmetry) on
ferromagnetism in this model is studied. It is found that the long-range
hopping (nonzero q) stabilizes ferromagnetism in an external magnetic field
for n>1. In the opposite limit n≤1 the fully polarized ferromagnetic
state is generally suppressed with increasing q. The critical value of
magnetic field h below which the ferromagnetic state becomes unstable is
calculated numerically.Comment: 8 pages, 3 Postscript figures, Late