  We report direct imaging of boundary magnetization associated with
antiferromagnetic domains in magnetoelectric epitaxial Cr2O3 thin films using
diamond nitrogen vacancy microscopy. We found a correlation between magnetic
domain size and structural grain size which we associate with the domain
formation process. We performed field cooling, i.e., cooling from above to
below the N\'eel temperature in the presence of magnetic field, which resulted
in the selection of one of the two otherwise degenerate 180 degree domains.
Lifting of such a degeneracy is achievable with a magnetic field alone due to
the Zeeman energy of a weak parasitic magnetic moment in Cr2O3 films that
originates from defects and the imbalance of the boundary magnetization of
opposing interfaces. This boundary magnetization couples to the
antiferromagnetic order parameter enabling selection of its orientation.
Nanostructuring the Cr2O3 film with mesa structures revealed reversible edge
magnetic states with the direction of magnetic field during field cooling.
