Prediction of colossal magnetocrystalline anisotropy for transition metal triiodides

In virtue of first principle calculations based on density functional theory, we have investigated the magnetism of transition metal triiodides XI₃ (X  =  Cr, Mn, Fe, Mo, Tc, Ru, W, Re, Os) monolayers. Our results indicate that CrI₃, TcI₃, RuI₃, ReI₃ and OsI₃ monolayers are ferromagnetic (FM), while MnI₃, FeI₃, MoI₃ and WI₃ monolayers are antiferromagnetic (AFM). Interestingly, TcI₃, RuI₃, ReI₃ and OsI₃ monolayers have considerable magnetic anisotropy energy (MAE). Especially, ReI₃ monolayer exhibits the largest MAE (-36.22 meV/ReI₃) in known two-dimensional (2D) van der Waals (vdW) crystals. We further demonstrate that biaxial strain can greatly change MAEs of ReI₃ and OsI₃ monolayers. From the electronic structure analysis, the change in MAE is mainly attributed from the charge transfer between the a and e ₂ states induced by biaxial strain. In addition, we have also found that a tensile strain can lead to a phase transition of ReI₃ from FM to AFM. We predicted that 2D FM XI₃ monolayers are promising candidates for the application in tunable magnetic storage technology.