Tunable magnetic anisotropy and phase transitions in 2D Janus transition-metal chalcogenide halides

Two-dimensional (2D) magnetic semiconductors have received increasing interest for spintronic applications due to their tunable magnetic properties. Using first-principles calculations, we design a series of 2D novel ferromagnetic semiconductors, including VSBr and Janus M₂S₂BrX (M = Cr/V, X = F, Cl, I), which exhibit various exotic properties. We realize an indirect-to-direct bandgap transition in Cr₂S₂BrF, a giant perpendicular magnetic anisotropy in V₂S₂BrI, and a significant T_C enhancement in both V₂S₂BrF and V₂S₂BrI. Notably, we reveal that biaxial tensile strain can induce a phase transition from ferromagnetic to antiferromagnetic states in the VSBr, V₂S₂BrCl and V₂S₂BrI, which is driven by the sign reversal of the third nearest-neighbor spin interaction J₃. Meanwhile, under tensile strain, the T_C of VSBr and V₂S₂BrCl reach 264 K and 297 K, respectively. These findings highlight our designed materials for great potential applications in advanced electronic and spintronic devices, showcasing their unique capabilities and promising performance.