  In the present work, the structural, magnetic, and electronic properties of
the two- and one-dimensional honeycomb structures of recently synthesized MnO
[Zhang et al. Nat. Commun., 20, 1073-1078 (2021)] are investigated by using
first principles calculations. Our calculations show that the single layer 2D
MnO crystal has a degenerate antiferromagnetic (AFM) ground state and a
relatively less favorable ferromagnetic (FM) state. In addition, magnetic
anisotropy calculations unveil that the easy-axis direction for magnetism
originating from unpaired electron states in manganese atoms is normal to the
crystal plane. Electronically, while the FM-MnO is a direct semiconductor with
a narrow bandgap, AFM phases display large indirect bandgap semiconducting
behavior. Moreover, calculations on nanoribbons of MnO reveal that zigzag edged
ribbons display metallic bahavior, whereas armchair edged nanoribbons are
semiconductors. Magnetically, for both zigzag- or armchair-edged nanoribbons,
AFM order perpendicular to the nanoribbon growth direction is found to be
favorable over the other AFM and FM orders. Moreover, depending on the edge
symmetry and ribbon width, forbidden band gap values of nanoribbons display
distinct family behaviors.
