  Among the recently widely studied van der Waals layered magnets CrX3 (X=Cl,
Br, I), CrCl3 monolayer (ML) is particularly puzzling as it is solely shown by
experiments to have an in-plane magnetic easy axis and, furthermore, all of
previous first-principles calculation results contradict this. Through
systematical first-principles calculations,we unveil that its in-plane shape
anisotropy that dominates over its weak perpendicular magnetocrystalline
anisotropy is responsible for the in-plane magnetic easy axis of CrCl3 ML. To
tune the in-plane ferromagnetism of CrCl3 ML into the desirable perpendicular
one, we propose substituting Cr with isovalent tungsten (W). We find that
CrWCl6 has a strong perpendicular magnetic anisotropy and a high Curie
temperature up to 76 K. Our work not only gives insight into understanding the
two-dimensional ferromagnetism of van der Waals MLs but also sheds new light on
engineering their performances for nanodevices.

Materials Science