Strong out-of-plane magnetic anisotropy and elevated Curie temperatures in 2D ferromagnets NiSi2N4, NiGe2N4, and Janus NiSiGeN4

Abstract

We investigate the electronic, magnetic, and thermal properties of NiSi${}_2$N${}_4$, NiGe${}_2$N${}_4$, and NiSiGeN${}_4$ monolayers using DFT, $DFT+U$, and Monte Carlo simulations. Phonon dispersion calculations and ab initio molecular dynamics confirm their dynamical and thermal stability. All compounds exhibit metallic character with large out-of-plane magnetic anisotropy energies (467.03, 440.49, and 250.44 345 $\mu$eV, respectively) and Curie temperatures ranging from 375–405 K, which can be further enhanced to 435–440 K under an external magnetic field. We also explore the impact of carrier doping on their magnetic properties. Remarkably, hole doping drives NiSi${}_2$N${}_4$, NiGe${}_2$N${}_4$, and NiSiGeN${}_4$ monolayers from the ferromagnetic ground state into an antiferromagnetic order, whereas electron doping increases the critical temperature. These results underscore their promise as robust, room-temperature spintronic materials capable of operating under extreme conditions.