Low-dimensional semiconducting ferromagnets have attracted considerable attention due to their promising applications as nanosize spintronics. However, realizing robust ferromagnetic couplings that can survive at high temperature is restrained by two decisive factors: superexchange couplings and anisotropy. Despite widely explored low-dimensional anisotropy, strengthening superexchange couplings has rarely been investigated. Here, we found that ligands with lower electronegativity can strengthen ferromagnetic superexchange couplings and further proposed the ligand modulation strategy to enhance the Curie temperature of low-dimensional ferromagnets. Based on the metallic $\mathrm{Cr}{X}_2$ ($X$ = S, Se, Te) family, substituting ligand atoms by halides can form stable semiconducting phase as CrSeCl, CrSeBr and CrTeBr. It is interesting to discover that, the nearest ferromagnetic superexchange couplings can be strengthened when substituting ligands from S to Se and Te. Such evolution originates from the enhanced electron hopping integral and reduced energy intervals between $d$ and $p$ orbitals, while the competing second nearest antiferromagnetic couplings are also benefitted due to delocalized $p-p$ interactions. Finally, ligand modulation strategy is applied in other ferromagnetic monolayers, further verifying our theory and providing a fundamental understanding on controlling superexchange couplings in low dimension.

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• Electronic structure
• Exchange interaction
• Ferromagnetism
• First-principles calculations
• Magnetic interactions