Near-room-temperature antiferromagnetism in Janus Fe$X$F ($X$ = O, S) monolayers

TL;DR

This work tackles the challenge of achieving robust 2D antiferromagnetism at higher temperatures by substituting Janus Fe$X$F monolayers for FeF$_2$. Using first-principles DFT (PBE+U with $U=4.0$ eV and HSE06) plus SOC, phonon, AIMD, and Monte Carlo simulations, the authors map how chemical substitution and strain modulate electronic and magnetic properties. FeF$_2$ is found to be an AFM semiconductor with a direct gap of $E_g=3.37$ eV and $T_N=18$ K, while FeOF and FeSF reach $T_N$ up to $207$ K and $248$ K with $E_g$ of $2.35$ eV (direct) and $1.13$ eV (indirect), respectively; biaxial compression can raise $T_N$ further to $244$ K and $274$ K, with semiconducting behavior preserved. This work establishes Janus engineering as an effective route to enhance 2D AFM order and identifies Fe-based oxyhalides as promising spintronic materials for higher-temperature operation.

Abstract

Inspired by the recently synthesized hexagonal layered phase of FeF$_2$, we studied the magnetic properties of the 1T-FeF$_2$ monolayer and its Janus Fe$X$F ($X$ = O, S) derivatives by first-principles calculations. Our results confirm that these materials are antiferromagnetic semiconductors, and that anion substitution effectively tunes their material properties: the band gap shifts from 3.37 eV (direct, FeF$_2$) to 2.35 eV (direct, FeOF) and 1.13 eV (indirect, FeSF); the magnetic moment of Fe ions increases; and the Néel temperature ($T_N$) rises dramatically to 248 K (FeSF) and 207 K (FeOF). Janus structures exhibit enhanced magnetic moment and direct AFM coupling. Under compression, $T_N$ is further optimized to 274 K ($-2$\% strain, FeSF) and 244 K ($-5$\% strain, FeOF). Both Janus materials retain their semiconducting nature and direction of easy magnetization axis under $\pm5$\% strain. This study validates the Janus structure as a viable approach to enhance 2D antiferromagnetism and highlights Fe-based oxyhalides as promising spintronic materials.

Figures (4)

• Figure 1: (a) The top and (b) side view of FeF$_2$ monolayer and Janus Fe$X$F ($X$ = O, S) monolayer. The brown, grey, and blue balls represent the Fe, F, and $X$ atoms, respectively. The primitive cell is labeled by bash lines. The exchange interaction $J_1$, $J_2$, and $J_3$ are displayed by arrows. (c) Four possible magnetic configurations, including FM, AFM1, AFM2, and AFM3 states. Here, the red and blue colors represent Fe atoms with up and down spins, respectively.
• Figure 2: (a) Band structure and total density of states (TDOS) of the FeF$_2$ monolayer. (b) Orbital-projected contribution of Fe-$d$ orbitals to the MAE. (c) The normalized magnetic moment ($M_z$) and heat capacity ($C_v$) of Fe atoms as function of temperature, which is obtained through the MC simulation.
• Figure 3: Band structures and density of states diagrams of (a) (b) FeOF and (d) (e) FeSF monolayers. Orbital-projected contribution to the MAE from the Fe-$d$ orbitals in (c) FeOF and (f) FeSF monolayers.
• Figure 4: (a) (d) Normalized magnetic moment ($M_Z$) and heat capacity ($C_v$) versus temperature; (b) (e) Evolution of exchange constants ($J_1$, $J_2$, $J_3$) under equibiaxial strain from $-5\text{\%}$ to $5\text{\%}$; (c) (f) Strain dependence of Néel temperature ($T_{\mathrm{N}}$) and magnetocrystalline anisotropy energy (MAE). (a-c) correspond to FeOF monolayer, (d-f) to FeSF monolayer.