Origin of ferromagnetic interactions in NaMnCl3 : How the response theory reconciles with Goodenough-Kanamori-Anderson rules

The on-site Coulomb repulsion $U$ is the key ingredient for describing the magnetic properties of Mott insulators, leading to a popular belief that many limitations of the density-functional theory based methods can be cured by artificially incorporating such on-site interactions for localized electrons in the model form. The layered antiferromagnet $\mathrm{Na}\mathrm{Mn}{\mathrm{Cl}}₃$ reveals quite a different story: while the Coulomb $U$ on the Mn sites controls the strength of antiferromagnetic superexchange interactions, an equally important parameter is the Stoner coupling ${\mathcal{I}}_{\mathrm{Cl}}$ on the ligand sites. The latter is responsible for large ferromagnetic contributions to the interatomic exchange interactions, which in $\mathrm{Na}\mathrm{Mn}{\mathrm{Cl}}₃$ nearly cancel the effect of the superexchange interactions. Although such behavior is anticipated from the phenomenological Goodenough-Kamanori-Anderson rules, the quantitative description of the ligand-related contributions remains disputable. Considering $\mathrm{Na}\mathrm{Mn}{\mathrm{Cl}}₃$ as an example, we discuss how they can be generally taken into account in the linear response theory to regain the dependence of the exchange interactions on ${\mathcal{I}}_{\mathrm{Cl}}$. The problem is complicated by the fact that, for the nearly filled Cl $3p$ shell, the parameters ${\mathcal{I}}_{\mathrm{Cl}}$ are sensitive to the model assumptions.