Unique electronic state in ferromagnetic semiconductor FeCl${}_2$ monolayer

Abstract

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$${}_{z+}^{\downarrow }$ electronic ground state is achieved. Then, both the orbital and spin contributions (0.59 $\hat{I}{¼}_{\mathrm{B}}$ plus 3.56 $\hat{I}{¼}_{\mathrm{B}}$) to the total magnetic moment well account for, for the first time, the experimental perpendicular moment of 4.3 $\hat{I}{¼}_{\mathrm{B}}$/Fe. Moreover, we find that a compressive strain further stabilizes the $d$${}^{5\uparrow }$$l$${}_{z+}^{\downarrow }$ ground state, and that the enhanced magnetic anisotropy and exchange coupling would boost the Curie temperature ($T_{\mathrm{C}}$) 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.

6 pages, 6 figures, 1 table