Ferromagnetism in the Hubbard model on a lattice of three coupled chains

Abstract We study the Hubbard model which is reduced to Mielke's flat-band model on the line graph of the two-leg ladder in a special case. In the flat-band case, the model exhibits saturated ferromagnetism when the electron number corresponds to 1 6 -filling +1 . We show that, even in the non-flat-band case, the model with that number of electrons exhibits ferromagnetism when the on-site interaction is large, and that this is true for 1 6 -filling. We also give numerical evidence that the model with the nearly flat-band exhibits ferromagnetism for less than 1 6 -filling.