Ferromagnetism above room temperature in Janus Fe2X (X = S, Se) monolayers

Abstract

Two-dimensional (2D) ferromagnets are the most promising materials for next-generation spintronic devices. However, the low Curie temperatures (T_Cs) of known 2D ferromagnetic materials hinder their applications. In this context, the design of 2D ferromagnets with high T_C becomes rather urgent. Herein we propose that constructing magnetic-ion-rich monolayers with out-of-plane asymmetry (e.g., Janus structure) might be a compelling route to achieve this goal. Based on first-principles structural search calculations, we identify two hexagonal Janus Fe₂X (X = S, Se) monolayers consisting of mixed-valence adjacent Fe atomic layers in sharp contrast with the sandwiched 1T-MoS₂ monolayer. Janus Fe₂S possesses a desirable coexistence of ferromagnetism and ferroelasticity. Its predicted T_C is up to 740 K demonstrating a robust inherent ferromagnetism associated with the itinerant-electron-mediated double-exchange interaction and RKKY interaction. Its high-temperature ferromagnetic order and sizable magnetic anisotropic energy can be preserved under biaxial strain from −4% to 4%. In comparison with Fe₂S, Fe₂Se shows a larger local magnetic moment and a higher T_C of 940 K because of Se having a lower electronegativity. Our study offers valuable guidance for the exploration of Janus magnetic materials.