Two-dimensional binary transition metal nitride $M$N$_4$ ($M$ = V, Cr, Mn, Fe, Co) with a graphene-like structure and strong magnetic properties

TL;DR

This study designs stable two-dimensional $M$N$_4$ monolayers with graphene-like structures that exhibit strong magnetic properties, including room-temperature ferromagnetism, using first-principles calculations.

Contribution

It introduces novel $M$N$_4$ monolayers with unique structural and magnetic features, expanding the understanding of 2D magnetic materials.

Findings

Stable $M$N$_4$ monolayers with rhombic and square patterns.

Continuous tuning of electronic and magnetic properties with different metals.

Room-temperature ferromagnetism in square-CoN$_4$ with T$_c$ of 321 K.

Abstract

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 $M$N$_4$ ($M$ =transition metal) monolayers, which are transition metal nitrides with a planar structure, made up of $M$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 $sp^2$ hybridization, coordinate bond, and $\pi$ conjugation. With the metal atom changing from V, Cr, Mn, Fe to Co, the total charge of $M$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 $M$N$_4$ lattices, which brings about the special splitting of five suborbitals of 3$d$ metal atom and gives rise to strong magnetism. Moreover, room-temperature ferromagnetism in square-CoN$_4$ monolayer with the Curie temperatures of 321 K is determined by solving the Heisenberg model combined with Monte Carlo method.