The total spin of the ground state is calculated in the $U\to \infty$ 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 $\varphi$ and the electron density $n_e$. In particular, the saturated ferromagnetism at $\varphi =n_e$ is induced by the formation of a spin-$1/2$ boson, which is a composite of an electron and the unit flux quantum.

• Received 22 May 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.036406