Enhancing ferromagnetic coupling in CrXY (X = O, S, Se; Y = Cl, Br, I) monolayers by turning the covalent character of Cr-X bonds

On the basis of first-principles calculations, we investigate the electronic and magnetic properties of 1T phase chromium sulfide halide CrXY (X = O, S, Se; Y = Cl, Br, I) monolayers in CrCl2 structure with the P (3) over bar m1 space group. Except for the CrOI monolayer, all CrXY monolayers are stable and ferromagnetic semiconductors. Our results show that the ferromagnetic coupling is dominated by the kinetic exchange between the empty e(g)-orbital of Cr atoms and the p-orbital of anions under the three-fold rotational symmetry. In this context, the coupling strength allows for being greatly enhanced by turning the nature of Cr-X bonds, i.e., increasing the covalent contribution of the bonds by minimizing the energy difference of the coupled orbitals. As we illustrate for the example of CrOY, the Curie temperature (T-c) is nearly tripled by substituting O by S/Se ion, eventually reaching the highest T-c in CrSeI monolayer (334 K). The high stabilities and Curie temperature manifest these monolayer ferromagnetic materials feasible for synthesis and applicable to 2D spintronic devices.