The seminal experimental discovery of the remarkably stable $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$ monolayer has led to a handful of predicted magnetic two-dimensional (2D) materials in the ${\mathrm{MA}}_2{\mathrm{Z}}_4$ 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: ${\mathrm{VSiGeN}}_4$ and ${\mathrm{VSiSnN}}_4$. They are both dynamically and thermally stable, but only the former is thermodynamically stable. Intriguingly, Janus ${\mathrm{VSiGeN}}_4$ and ${\mathrm{VSiSnN}}_4$ monolayers show weak in-plane anisotropy compared with the $\mathrm{V}{\mathrm{Si}}_2{\mathrm{N}}_4$ monolayer. These two emerging Janus magnetic semiconductors offer opportunities for studying 2D magnetism and spin control for spintronics applications.

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• First-principles calculations
• Magnetic anisotropy
• Magnetic phase transitions
• Thermodynamics
• 2-dimensional systems
• Ferromagnets
• Magnetic semiconductors
• Density functional theory
• Electronic structure