  We have investigated the growth of antiferromagnetic Mn3N2(001) on MgO (001)
by molecular beam epitaxy and scanning tunneling microscopy . The images show
smooth terraces and atomic steps. On some of the terraces a unique and new
reconstruction is seen, resolved as square Mn tetramers in a c(4x2) structural
arrangement. Two domains of the tetramer reconstruction, rotated by 90$^\circ$
to each other, occur. A model is presented for this square Mn tetramer
reconstruction, in which the Mn atoms of the tetramer layer belong to the Mn
layer at the surface in the MnN-MnN-Mn stacking sequence.
