Abstract
Neutron scattering has been used to investigate the energy dispersion relation of spin waves in CoCl2 at temperatures between 4.4 K and 34 K. CoCl2 orders antiferromagnetically at TN=24.9 K with the Co2+ spins lying within ferromagnetic planes which are antiferromagnetically stacked along the hexagonal c axis. The interplanar exchange interactions are much weaker than those within the planes, giving rise to the 'metamagnetic' property of ferromagnetic saturation in relatively weak magnetic fields, and suggesting that above TN there could be correlations between spins appropriate to a two-dimensional ferromagnet. The observed spin waves at low temperature indeed only exhibit strong dispersion within the basal plane and measurement of this dispersion enables an accurate value for the nearest-neighbour exchange constant to be determined.