Among the recently widely studied van der Waals layered magnets $\mathrm{Cr}{X}_3$ ($X=\mathrm{Cl}$, Br, I), $\mathrm{CrC}{\mathrm{l}}_3$ monolayer (ML) is particularly puzzling: it is shown by experiments to have only an in-plane magnetic easy axis and, furthermore, all previous first-principles calculation results contradict this. Through systematic first-principles calculations, we unveil that the in-plane shape anisotropy that dominates over the weak perpendicular magnetocrystalline anisotropy is responsible for the in-plane magnetic easy axis of $\mathrm{CrC}{\mathrm{l}}_3$ ML. To tune the in-plane ferromagnetism of $\mathrm{CrC}{\mathrm{l}}_3$ ML into the desirable perpendicular one, we propose substituting Cr with isovalent tungsten (W). We find that $\mathrm{CrWC}{\mathrm{l}}_6$ 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.

• Received 4 October 2019

DOI:https://doi.org/10.1103/PhysRevB.100.224429