Single-layer metal halides MX₂ (X = Cl, Br, I): Stability and tunable magnetism from first principles and Monte Carlo simulations

Based on first-principles calculations, we investigate a novel class of 2D materials-MX₂ metal dihalides (X = Cl, Br, I). Our results show that single-layer dihalides are energetically and dynamically stable and can be potentially exfoliated from their bulk layered forms. We found that 2D FeX₂, NiX₂, CoCl₂ and CoBr₂ monolayers are ferromagnetic (FM), while VX₂, CrX₂, MnX₂ and CoI₂ are antiferromagnetic (AFM). The magnetic properties of 2D dihalides originate from the competition between AFM direct nearest-neighbor d-d exchange and FM superexchange via halogen p states, which leads to a variety of magnetic states. The thermal dependence of magnetic properties and the Curie temperature of magnetic transition are evaluated using statistical Monte Carlo simulations based on the Ising model with classical Heisenberg Hamiltonian. The magnetic properties of single-layer dihalides can be further tuned by strain and carrier doping. Our study broadens the family of existing 2D materials with promising applications in nanospintronics.