Restrictive thermodynamic stability and intrinsic ferromagnetism in MA₂N₄ and Janus VSiGeN₄ monolayers

A handful of predicted magnetic two-dimensional (2D) materials in the MA₂Z₄ family (M = transition metals, A = Si, Ge, and Z = N, P, As) were predicted to be dynamically stable, but none of them has been synthesized to date. In this work, from the first-principles thermodynamic stability analysis, we demonstrate that only the nitrides are thermodynamically stable under N-rich conditions. In the group of 2D magnetic nitrides, VSi₂N₄ has been found to be the most stable monolayer, from which two Janus ferromagnetic monolayers, namely, VSiGeN₄ and VSiSnN₄, have been designed. They are both dynamically and thermally stable, but only the former is thermodynamically stable. Intriguingly, these two semiconducting Janus monolayers show weak in-plane anisotropy compared to the VSi₂N₄ monolayer. A large valley polarization can be induced in VSiGeN₄ and VSiSnN₄ monolayers through the valley splitting of the bottom conduction band. The magnetic properties of these 2D ternary layered vanadium-based Janus semiconductors make them emerging candidates for spintronics and optoelectronics applications.