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 ${\mathrm{VI}}_3$ to be a 2D Ising ferromagnet (FM), using density functional calculations, crystal field level diagrams, superexchange model analyses, and Monte Carlo simulations. The $a_{1g}{}^1{e}^{\prime }_{-}^1$ 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 ${\mathrm{V}}^{3+}$ ions. We find that a tensile strain on the ${\mathrm{VI}}_3$ monolayer further stabilizes the $a_{1g}{}^1{e}^{\prime }_{-}^1$ ground state, and its Curie temperature $\left(T_{\mathrm{C}}\right)$ 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.

• Received 12 September 2019
• Accepted 27 February 2020

DOI:https://doi.org/10.1103/PhysRevB.101.100402