  It is shown that the semimetallic state of the two-dimensional honeycomb
lattice with a point-like Fermi surface is unstable towards a canted
antiferromagnetic insulator upon application of an in-plane magnetic field.
This instability is already present at the mean-field level; the magnetic field
shifts the up- and the down-spin cones in opposite directions thereby
generating a finite density of states at the Fermi surface and a perfect
nesting between the up- and the down-spin Fermi sheets. This perfect nesting
triggers a canted antiferromagnetic insulating state. Our conclusions, based on
mean-field arguments, are confirmed by auxiliary field projective quantum Monte
Carlo methods on lattices up to $12 \times 12$ unit cells.
