Neutron Scattering Investigation of the Magnetic Excitations in CoBr₂

Abstract

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 CoBr₂ were observed using inelastic neutron scattering technique. The spin dynamics of CoBr₂ can be described by the anisotropic exchange Hamiltonian for S=1⁄2 with intraplanar exchange constants J₁=2.01 meV, J₂=0.019 meV and J₃=−0.338 meV, interplanar exchange J′=−0.97 meV, and planar anisotropies D₁=1.14 meV and D′=−0.06 meV. As shown by these numerical values, CoBr₂ 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.