Determination of Anomalous Superexchange in MnC${\mathrm{l}}_2$ and Its Graphite Intercalation Compound

The low-temperature magnetic structure of ${\mathrm{MnCl}}_2$ graphite intercalation compound has been studied by neutron diffraction. Magnetic peaks occur at wave vectors incommensurate with the ${\mathrm{MnCl}}_2$ and graphene sublattices. The in-plane spin configuration is explained by an exchange Hamiltonian that includes three shells of nearest neighbors in the plane. The nearest-neighbor exchange is ferromagnetic but anomalously weak, and the magnetic behavior is dominated instead by the antiferromagnetic third-neighbor interaction. The exchange parameters are used to explain the spin configuration of bulk ${\mathrm{MnCl}}_2$ after adding an interplanar coupling.