Neutron scattering investigation of magnetic excitations in CoCl2

Neutron scattering has been used to investigate the energy dispersion relation of spin waves in CoCl 2 at temperatures between 4.4 K and 34 K. CoCl 2 orders antiferromagnetically at T N =24.9 K with the Co 2+ 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 T N 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.