Giant Valley Polarization in Bipolar Ferromagnetic Semiconductors SmX ₂ (X = Cl, Br, I) Monolayers under Strain

The discovery and characterization of two-dimensional (2D) ferrovalley materials represent a significant advancement in the field of materials science and electronics. Herein, we utilize first-principles calculations to predict and investigate a class of 2D ferrovalley materials, SmX2 (X = Cl, Br, I) monolayers (MLs), and explore their potential as magnetic semiconductors with varying halide elements. All three monolayers exhibit excellent mechanical, dynamic, and thermal stability. They exhibit intrinsic ferromagnetic (FM) ground states with Curie temperatures (TC) exceeding room temperature and show significant magnetic anisotropy energy (MAE). Unexpectedly, both SmBr2 and SmI2 display significant valley polarization. This polarization arises from the interplay between magnetic exchange and spin–orbit coupling effects. The SmX2 MLs are typical bipolar magnetic semiconductors that can transform into semimetals under hole or electron doping. Additionally, applying modest strains enhances both the MAE and valley polarization. Notably, the Curie temperature, MAE, and valley polarization all increase significantly with rising halogen atomic number. These theoretical findings broaden the application prospects of 2D rare-earth halide MLs and advance the design of high-performance, multifunctional spintronic devices.