Electronic Mechanism for the Coexistence of Ferroelectricity and   Ferromagnetism
  We study the strong coupling limit of a two-band Hubbard Hamiltonian that also includes an inter-orbital on-site repulsive interaction $U_{ab}$. When the two bands have opposite parity and are quarter filled, we prove that the ground state is simultaneously ferromagnetic and ferroelectric for infinite intra-orbital Coulomb interactions $U_{aa}$ and $U_{bb}$. We also show that this coexistence leads to a singular magnetoelectric effect. 
C. D. Batista, J. E. Gubernatis, and Wei-Guo Yin