Neutron diffraction experiments have been performed on anhydrous Fe${\mathrm{Br}}_2$, Co${\mathrm{Br}}_2$, Fe${\mathrm{Cl}}_2$, and Co${\mathrm{Cl}}_2$ at temperatures from 295°K to 4.2°K to investigate the existence of magnetic ordering in these hexagonal layer-type structures. All four compounds have an antiferromagnetic transition at low temperatures to structures in which the atomic magnetic moments within a metal layer form ferromagnetic sheets and the moments in adjacent layers are antiparallel. In the iron compounds the moments are oriented parallel to the hexagonal $c$ axis and in the cobalt compounds the moment orientation is perpendicular to that axis. Values of the atomic magnetic moments are close to those expected for the divalent metallic ions if the orbital contribution is quenched. Small-angle scattering experiments on Fe${\mathrm{Cl}}_2$ and Co${\mathrm{Cl}}_2$ have shown that the ferromagnetic coupling between moments within a layer is much stronger than the antiferromagnetic coupling between atoms in adjacent layers, and single-crystal investigations on these two compounds have determined the method by which large net magnetization values are obtained at temperatures below $T_N$ in moderate magnetic fields.

• Received 8 September 1958

DOI:https://doi.org/10.1103/PhysRev.113.497