Investigations of the magnetic and lattice vibrations in CoBr 2 using neutron scattering

Inelastic neutron scattering has been used to investigate both the acoustic phonons and the magnetic excitations in CoBr2. The acoustic phonon dispersion curves were measured in three principal symmetry directions at room temperature. From the initial slopes the sound velocities have been derivated and the elastic constants calculated. The two dimensional nature of the crystallographic structure is apparent in the difference between the sound velocities parallel and perpendicular to the closed packed layers although the difference is less than in other halides. Furthermore, the order parameter of the magnetic lattice was found to be β = 0.3 a value not inconsistent with that expected for a 3 D Ising model. The magnon dispersion was investigated in the (100) and (001) directions at 5 K. The observed dispersion could be explained using a model containing an isotropic exchange within the ground state doublet of the 4T 1 term. Renormalization of the spin waves revealed a strong magnon-phonon coupling in the (001) direction.