Two-dimensional binary transition metal nitride MN4 ( M = V, Cr, Mn, Fe, Co) with a graphenelike structure and strong magnetic properties

Binary transition metal nitride with a graphene-like structure and strong magnetic properties is rare. Based on the first-principles calculations, we design two kinds of $M{\mathrm{N}}_{4}$ ($M$ = transition metal) monolayers, which are transition metal nitrides with a planar structure, made up of $M{\mathrm{N}}_{4}$ units aligned in the rhombic and square patterns. The two structural lattices have robust stability and good compatibility with different metal atoms, and the underlying mechanism is the combination of $s{p}^{2}$ hybridization, coordinate bond, and $\ensuremath{\pi}$ conjugation. With the metal atom changing from V, Cr, Mn, Fe to Co, the total charge of $M{\mathrm{N}}_{4}$ system increases by one electron in turn, which results in continuous adjustability of the electronic and magnetic properties. The planar ligand field is another feature of the two $M{\mathrm{N}}_{4}$ lattices, which brings about the special splitting of five suborbitals of $3d$ metal atom and gives rise to strong magnetism. Moreover, room-temperature ferromagnetism in square-${\mathrm{CoN}}_{4}$ monolayer with the Curie temperatures of 321 K is determined by solving the Heisenberg model combined with Monte Carlo method.