  Two-dimensional (2D) ferromagnets with high Curie temperature have long been
the pursuit for electronic and spintronic applications. CrI3 is a rising star
of intrinsic 2D ferromagnets, however, it suffers from weak exchange coupling.
Here we propose a general strategy of self-intercalation to achieve enhanced
ferromagnetism in bilayer CrI3. We showed that filling either Cr or I atoms
into the van der Waals gap of stacked and twisted CrI3 bilayers can induce the
double exchange effect and significantly strengthen the interlayer
ferromagnetic coupling. According to our first-principles calculations, the
intercalated native atoms act as covalent bridge between two CrI3 layers and
lead to discrepant oxidation states for the Cr atoms. These theoretical results
offer a facile route to achieve high-Curie-temperature 2D magnets for device
implementation.
