VI3: A two-dimensional Ising ferromagnet

Two-dimensional (2D) magnetic materials are of great current interest for their promising applications in spintronics. Here we propose the van der Waals (vdW) material VI3 to be a 2D Ising ferromagnet (FM), using density functional calculations, crystal field level diagrams, superexchange model analyses, and Monte Carlo simulations. The a(1g) (1)e'(1)_ground state in the trigonal crystal field gives rise to the 2D Ising FM due to a significant single ion anisotropy (SIA) and enhanced FM superexchange both associated with the S-z = 1 and L-z = -1 states of V3+ ions. We find that a tensile strain on the VI3 monolayer further stabilizes the a(1g) (1)e'(1)_ground state, and its Curie temperature (T-c) would increase from 70 to 90-110 K under a 2.5%-5% tensile strain. Moreover, we suggest a group of spin-orbital states with a strong SIA which may help to search more 2D Ising magnets.