First-principles study the single-layer transition metal trihalide CrXSe₃ (X  =  Sn, Ge, Si) as monolayer...

Layered transition metal trihalides ABX₃ are promising candidate materials for monolayer magnets. In this paper, we investigated single-layer CrXSe₃ (X  =  Sn, Ge, Si) as monolayer ferromagnetic semiconductors (FMS). Firstly, our calculated interlayer binding energies and mechanical properties demonstrate the feasibility of obtaining the free-standing monolayer CrXSe₃ from the layered van der Waals crystal CrXSe₃ via mechanical exfoliating method. Plus, we find that the ferromagnetic (FM) super-exchange interaction dominates over the anti-ferromagnetic (AFM) direct-exchange interactions, making CrSnSe₃ and CrGeSe₃ monolayers FM with the magnetic moments of 6.0 µ _B per unit cell. Particularly, the FM configurations of CrSnSe₃ and CrGeSe₃ monolayers become more stable under the increasing tensile strain, and CrSiSe₃ converts to FM from AFM under biaxial tensile strain larger than 2%. Additionally, the three monolayers CrXSe₃ are all semiconducting with energy band gaps of 0.76, 0.87 and 1.1 eV for X being Sn, Ge and Si, respectively. Our results suggest CrXSe₃ as monolayer FMS hold promising potential in spintronics.