Large Magnetoresistance and Perfect Spin-Injection Efficiency in Two-Dimensional Strained VSi2N4 -Based Room-Temperature Magnetic-Tunnel-Junction Devices

Two-dimensional (2D) materials provide a promising platform for exploring spintronic devices. However, the low Curie temperature of most 2D magnetic materials limits their development and application. Based on density-functional theory and the nonequilibrium-Green's-function formalism, we present a systematic simulation study of room-temperature strained ${\mathrm{VSi}}_{2}{\mathrm{N}}_{4}$-based magnetic tunnel junctions (MTJs), using a high-accuracy Heyd-Scuseria-Ernzerhof (HSE) functional. In contrast to Perdew-Burke-Ernzerhof results, a spin-conductance match is observed in $\mathrm{Ag}/{\mathrm{VSi}}_{2}{\mathrm{N}}_{4}$-${\mathrm{VSi}}_{2}{\mathrm{N}}_{4}/\mathrm{Ag}$ MTJs at the HSE06 level. Thus, a 1200% tunnel-magnetoresistance (TMR) ratio and a perfect spin-injection efficiency are theoretically predicted in $\mathrm{Ag}/{\mathrm{VSi}}_{2}{\mathrm{N}}_{4}$-${\mathrm{VSi}}_{2}{\mathrm{N}}_{4}/\mathrm{Ag}$ MTJs. Interestingly, the coexistence of a Weyl semimetal and a half-metal state is found in 1.965% tensile-biaxial-strained monolayer ${\mathrm{VSi}}_{2}{\mathrm{N}}_{4}$. In addition, a slight compressive strain leads to a boost of the TMR ratio by 2 orders of magnitude, up to ${10}^{5}\mathrm{%}$ in strained ${\mathrm{VSi}}_{2}{\mathrm{N}}_{4}$-based MTJs. Other MTJs based on ${\mathrm{VSi}}_{2}{\mathrm{P}}_{4}$, ${\mathrm{VSi}}_{2}{\mathrm{As}}_{4}$, and ${\mathrm{Nb}\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ are investigated. Our results show that these spin-polarized magnetic silicide compounds, with Curie temperatures close to room temperature, are promising candidates for next-generation spintronic devices.