  I consider the Hubbard model of graphene in an external magnetic field and in
the Hartree-Fock approximation. In the continuum limit, the ground state energy
at half filling becomes nearly symmetric under rotations of the three-component
vector (N1,N2,m), with the first two components representing the Neel order
parameter orthogonal to and the third component the magnetization parallel with
the external magnetic field. When the symmetry breaking effects arising from
the lattice, Zeeman coupling, and higher Landau levels are included the system
develops a quantum critical point at which the antiferromagnetic order
disappears and the magnetization has a kink. The observed incompressible state
at filling factor one is argued to arise due to a finite third component of the
Neel order parameter at these electron densities. Recent experiments appear
consistent with vanishing N1 and N2, and finite N3, at the filling factors zero
and one, respectively.
