Inelastic neutron scattering investigation of the magnetic excitations in the helimagnetic state of NiBr2

We report on an inelastic neutron scattering study of the spin waves in NiBr2 at T = 4.2 K, performed in order to determine the exchange and anisotropy parameters. These are needed in order to understand the unusual helimagnetic-antiferromagnetic transition which occurs at TIC = 22.8 K. We find that the spin dynamics of NiBr2 can be described by an isotropic exchange hamiltonian for S =1 with in-plane exchange constants J1 = 1.56 meV, J2 = - 0.018 meV and J3 = - 0.41 meV, a weak inter plane coupling J' = - 0.182 meV, and an anisotropy of XY-type D = 0.15 meV. We point out that in a helimagnetically ordered phase the out-of-plane and the in-plane spin components may give rise to peaks in the neutron scattering intensity at energies ω(q) and ω(q ± k0), respectively. These three peaks have actually been observed in our experiments on NiBr 2.