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 $\pi$ 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.

• Received 26 August 2022
• Revised 20 November 2022
• Accepted 28 November 2022

DOI:https://doi.org/10.1103/PhysRevB.106.235402