a 2D 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_1g¹e′₋¹ state in the trigonal crystal field turns
out to be the ground state, and it gives rise to the 2D Ising FM due to a significant single ion
anisotropy (SIA) associated with the Sz=1 and Lz=–1 state of V³⁺ ions. Moreover, a tensile strain
on the VI3 monolayer further stabilizes the a_1g¹e′₋¹ ground state, and the enhanced FM superexchange
and the strong SIA would raise the Curie temperature of the VI3 monolayer from 70 K to
90-110 K under a 2.5-5% tensile strain. This prediction is worth a prompt experimental verification.