  Low-dimensional semiconducting ferromagnets have attracted considerable
attention due to their promising applications as nano-size spintronics.
However, realizing robust ferromagnetic couplings that can survive at high
temperature is restrained by two decisive factors: super-exchange couplings and
anisotropy. Despite widely explored low-dimensional anisotropy, strengthening
super-exchange couplings has rarely been investigated. Here, we found that
ligands with lower electronegativity can strengthen ferromagnetic
super-exchange couplings and further proposed the ligand modulation strategy to
enhance the Curie temperature of low-dimensional ferromagnets. Based on the
metallic CrX2 (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 super-exchange
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 orbits. While the second nearest
anti-ferromagnetic 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 super-exchange couplings in
low-dimension.
