  We address the effective tight-binding Hamiltonian that describes the
insulating Mott state of twisted graphene bilayers at a magic angle. In that
configuration, twisted bilayers form a honeycomb superlattice of localized
states, characterized by the appearance of flat bands with four-fold
degeneracy. After calculating the maximally localized superlattice Wannier
wavefunctions, we derive the effective spin model that describes the Mott
state. We suggest that the system is an exotic ferromagnetic Mott insulator,
with well defined experimental signatures.
