Ternary MgZ₂ N₄ (Z = Ge, Si) and Janus MgGeSiN₄ monolayers: high stability with distinctive electronic and magnetic properties

The recent synthesis of two-dimensional (2D) MoSi₂N₄ and WSi₂N₄ crystals has given rise to a new class of 2D materials with distinctive properties and significant potential for advanced technologies. The transition metal (TM) elements can potentially be substituted with other elements from the periodic table, enabling the introduction of new members into this emerging 2D family. Building on this framework, we propose a new family of light-element-based 2D ternary compounds of MgZ₂N₄ (Z = Si, Ge) and their Janus derivative MgGeSiN₄ using first-principles density functional theory (DFT) calculations. All three monolayers are found to be dynamically, thermally, and mechanically stable. MgGe₂N₄ exhibits a metallic antiferromagnetic (AFM) ground state within the PBE functional, while more accurate HSE-level calculations reveal a weak ferrimagnetic (FiM) configuration. Conversely, MgSi₂N₄ is a robust half-metallic ferromagnet with 100% spin polarization and a sizable magnetic moment of 5.62<i>μ</i> _B per unit cell. Janus MgGeSiN₄ shows FiM metallic behavior, driven by structural asymmetry. All systems favor an in-plane easy axis magnetization, and our analysis shows that their magnetic behavior follows a unique low-temperature Berezinskii-Kosterlitz-Thouless (BKT) transition, characteristic of the 2D XY-like model. The estimated BKT transition temperatures for MgGe₂N₄, MgSi₂N₄, and MgGeSiN₄ are approximately 19 K, 4 K, and 16 K, respectively. This study presents the first report of intrinsic FM, AFM, and FiM in this monolayer family, highlighting their potential for advanced low-temperature magnetic device applications.