  The newly discovered 2D magnetic materials provide new opportunities for
basic physics and device applications. However, their low Curie temperature
(TC) is a common weakness. In this paper, by combining magnetic Hamiltonian,
Wannier functions and first-principle calculations, we systematically study the
magnetic properties of monolayer CrI3 functionalized by halogen. The magnetic
exchange coupling (EX) and magnetic anisotropy (MA) are found to increase
significantly by X (X=F, Cl and Br) atom adsorption, and increase along with
the coverage of X atom. In the frame work of superexchange theory, the enhanced
EX can be ascribed to the reduced energy difference and increased hopping
strength between Cr d and I p orbitals, due to the states of I ligand are
engineered by X adatom. Besides, the X adatom may provide additional
ferromagnetic superexchange channel. Finally, the CrI3 that one side is fully
adsorbed by F atoms is found to be a room temperature ferromagnetic
semiconductor with TC=650 K. Our results not only give an insightful
understanding for the enhancement of ferromagnetism of CrI3 by atom adsorption,
but also propose a promising way to improve the ferromagnetism of 2D magnetic
materials.
