Neutron scattering study of magnetic excitations in intercalated CoCl2

We have studied by inelastic neutron scattering the magnetic excitations of Co${\mathrm{Cl}}_2$ layers intercalated in graphite. At 4.5 K the stage-2 compound exhibits ferromagnetic $\mathrm{xy}$-type interactions within the ${\mathrm{Co}}^{2+}$ sheets, which are weakly antiferromagnetically coupled along the hexagonal $c$ axis. The magnon dispersion curve was measured for spin waves propagating within the basal planes up to the first powder-average Brillouin zone. At small wave vectors, a splitting of the branch occurs, indicative of weak interplanar coupling. A fit to the measured excitation energies yields exchange and anisotropy parameters which are in good agreement with susceptibility data.