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Unique electronic state in ferromagnetic semiconductor FeCl$_{2}$ monolayer

Two-dimensional (2D) van der Waals (vdW) magnetic materials could be an ideal platform for ultracompact spintronic applications. Among them, FeCl$_{2}$ monolayer in the triangular lattice is subject to a strong debate. Thus, we critically examine its spin-orbital state, electronic structure, and magnetic properties, using a set of delicate first-principles calculations, crystal field level analyses, and Monte Carlo simulations. Our work reveals that FeCl$_{2}$ monolayer is a ferromagnetic (FM) semiconductor in which the electron correlation of the narrow Fe $3d$ bands determines the band gap of about 1.2 eV. Note that only when the spin-orbit coupling (SOC) is properly handled, the unique $d$$^{5\uparrow}$$l$$^\downarrow_{z+}$ electronic ground state is achieved. Then, both the orbital and spin contributions (0.59 $μ_{\rm B}$ plus 3.56 $μ_{\rm B}$) to the total magnetic moment well account for, for the first time, the experimental perpendicular moment of 4.3 $μ_{\rm B}$/Fe. Moreover, we find that a compressive strain further stabilizes the $d$$^{5\uparrow}$$l$$^\downarrow_{z+}$ ground state, and that the enhanced magnetic anisotropy and exchange coupling would boost the Curie temperature ($T_{\rm C}$) from 25 K for the pristine FeCl$_{2}$ monolayer to 69-102 K under 3$\%$-5$\%$ compressive strain. Therefore, FeCl$_{2}$ monolayer is indeed an appealing 2D FM semiconductor.