Magnetic neutron-scattering study of ${\mathrm{MnCl}}_2$-graphite intercalation compound

The in-plane spin order of ${\mathrm{MnCl}}_2$ graphite intercalation compound has been studied between 63 mK and 30 K by elastic neutron scattering. This compound approximates a classical XY antiferromagnet on a triangular lattice. Below 7.5 K magnetic critical scattering peaks are observed at wave vectors incommensurate with the ${\mathrm{MnCl}}_2$ and graphene sublattices. As the temperature is raised, the cell contracts towards a commensurate 2$\sqrt{3}$×2$\sqrt{3}$ magnetic unit cell. The ground-state in-plane spin configuration is explained by an exchange Hamiltonian that includes no fewer than three shells of nearest neighbors in the plane. The magnetic peaks have a Lorentzian shape and are broader than resolution down to 63 mK, well below the peak in the magnetic susceptibility at 1.1 K. No evidence of three-dimensional magnetic correlations was found at any temperature.