Elementary excitations of antiferromagnetic CoCl2

The dispersion relations of magnons and excitons within the ${}_{}{}^4{}_{}{}^{}T_{1g}^{}{}_{}{}^{}$ ground term of Co${\mathrm{Cl}}_2$ are investigated both theoretically and by Raman scattering. The exchange interaction between coupled ${\mathrm{Co}}^{2+}$ ions is taken to be isotropic in the ionic spins, with parameters $2J_1=5.11\mathrm{}\pm 0.28$ ${\mathrm{cm}}^{-1\mathrm{}}$ for the intralayer (nearest-neighbor) coupling and $2J_2=-0.36\mathrm{}\pm 0.02$ ${\mathrm{cm}}^{-1\mathrm{}}$ for the interlayer (next-next-nearest-neighbor) coupling. These parameters are obtained by fitting the experimentally determined magnon and exciton energies at $\overrightarrow{\mathrm{k}}=0\mathrm{}$. Satisfactory agreement is found between calculation and observed spin-wave dispersion curves, antiferromagnetic resonance, and electronic Raman scattering. Two of the observed Raman bands are identified as simultaneous magnon-exciton scattering, involving zone-edge excitations propagating on opposite sublattices.