Neutron Scattering Investigation of the Magnetic Excitations in CoBr 2

In order to investigate the two-dimensionality and the easy plane anisotropy, the spin waves propagating along the [h, 0, 0], [h, h, 0], [h, 0.5h, 0] and [0, 0, l ] directions in CoBr 2 were observed using inelastic neutron scattering technique. The spin dynamics of CoBr 2 can be described by the anisotropic exchange Hamiltonian for S =1/2 with intraplanar exchange constants J 1 =2.01 meV, J 2 =0.019 meV and J 3 =-0.338 meV, interplanar exchange J '=-0.97 meV, and planar anisotropies D 1 =1.14 meV and D '=-0.06 meV. As shown by these numerical values, CoBr 2 has fairly strong XY characters, but the two-dimensionality is definitely poor. We point out, however, that in a planar antiferromagnet, the easy plane type anisotropy should remove the degeneracy of the excitation spectrum for the `in-plane' and the `out-of-plane' correlations, and in fact, these excitations are observed separately in this experiment.