Intrinsic ferromagnetism and restrictive thermodynamic stability in MA$_2$N$_4$ and Janus VSiGeN$_4$ monolayers

TL;DR

This study uses first-principles calculations to analyze the stability and magnetic properties of MA$_2$N$_4$ and Janus VSiGeN$_4$ monolayers, identifying thermodynamically stable nitrides and proposing new ferromagnetic semiconducting Janus monolayers for spintronics.

Contribution

It reveals thermodynamic stability conditions in the MA$_2$Z$_4$ family and proposes two new Janus monolayers with potential for 2D magnetism and spintronics.

Findings

Only nitrides are thermodynamically stable under N-rich conditions.

VSiGeN$_4$ and VSiSnN$_4$ are dynamically and thermally stable; only VSiGeN$_4$ is thermodynamically stable.

Janus VSiGeN$_4$ and VSiSnN$_4$ show weak in-plane anisotropy.

Abstract

The seminal experimental discovery of the remarkably stable MoSi$_2$N$_4$ monolayer has led to a handful of predicted magnetic two-dimensional (2D) materials in the MA$_2$Z$_4$ family (M = transition metals, A = Si, Ge, and Z = N, P, As). These magnetic monolayers were predicted to be dynamically stable, but none of them has been synthesized to date. In this Research Letter, from first-principles thermodynamic stability analysis, we demonstrate that only the nitrides are thermodynamically stable and this occurs under N-rich conditions. Based on this finding, we propose two ferromagnetic, semiconducting Janus monolayers in the family: VSiGeN$_4$ and VSiSnN$_4$. They are both dynamically and thermally stable, but only the former is thermodynamically stable. Intriguingly, Janus VSiGeN$_4$ and VSiSnN$_4$ monolayers show weak in-plane anisotropy compared with the VSi$_2$N$_4$ monolayer. These two emerging Janus magnetic semiconductors offer opportunities for studying 2D magnetism and spin control for spintronics applications.