(X=Cl, Br, I) Monolayers with Enhanced Anisotropy and Curie Temperature*

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.