Magnons, Excitons and Light Scattering in Antiferromagnetic CoF₂

Spin waves and low energy excitons in antiferromagnetic CoF ₂ are studied theoretically on the basis of the effective Hamiltonian including all degrees of freedom in the ionic \(^{4}\varGamma_{4}\) state (effective L =1, S =3/2). Inter-ionic exchange interactions is taken into account in a simple but reasonable way. Calculated dispersions of the low-lying excitons (including magnons) compare well with the results of neutron scattering experiment. The light scattering by these excitons are treated by extending the previous theory, which makers use of the spin-dependent electric polarizability, to include the degrees of freedom of the orbital angular momentum. Symmetry arguments as well as quantum mechanical calculation of the L - and S -dependent electric polarizability are given. The polarization characteristics and scattering efficiency of the Raman scattering by the long wave excitons in CoF ₂ are predicted. The Raman scattering by simultaneous excitations of two excitons are discussed qualitatively.