  The total spin of the ground state is calculated in the U -> infinity Hubbard
model with uniform magnetic flux perpendicular to a square lattice, in the
absence of Zeeman coupling. It is found that the saturated ferromagnetism
emerges in a rather wide region in the space of the flux density \phi and the
electron density n_e. In particular, the saturated ferromagnetism at \phi = n_e
is induced by the formation of a spin-1/2 boson, which is a composite of an
electron and the unit flux quantum.
