Prediction of FeS₃ Monolayer with Antiferromagnetic Semiconductor Properties and Janus‐Tunable Altermagnetism

The development of two‐dimensional (2D) antiferromagnetic materials operating above room temperature with large magnetic anisotropy energy (MAE) is crucial for next‐generation spintronic devices. Herein, a stable FeS 3 monolayer featuring a nonequilateral hexagonal Fe arrangement by structure search method and first principles calculations is predicted. This material exhibits robust dynamical, thermal, and mechanical stability, an indirect bandgap of 1.12 eV (HSE06), and intrinsic antiferromagnetism with a large MAE of 272 μeV Fe −1 and Néel temperature ( T N ) of 222 K, while biaxial strain further modulates MAE (up to 384 μeV Fe −1 at −6% strain). By constructing Janus Fe 2 S 3 X 3 (X = Se, Te) monolayers, this structure also exhibits altermagnetism and a high Néel temperature. The research offers value guidance for the exploration of 2D magnetic materials.