  Recent experiments have demonstrated that neutral graphene sheets have an
insulating ground state in the presence of an external magnetic field. We
report on a $\pi$-band tight-binding-model Hartree-Fock calculation which
examines the competition between distinct candidate insulating ground states.
We conclude that for graphene sheets on substrates the ground state is most
likely a field-induced spin-density-wave, and that a charge density wave state
is possible for suspended samples. Neither of these density-wave states support
gapless edge excitations.
