Temperature Dependence of Magnon-Pair Modes in Antiferromagnets and Paramagnets

The temperature dependence of magnon-pair excitations has been obtained in the simple antiferromagnets RbMnF3 and FeF2, by the technique of inelastic light scattering. At low temperatures the pair-mode frequency is nearly twice that of a Brillouin-zone boundary magnon, decreased slightly by magnon-magnon interaction effects. As the temperature is increased, the magnon-pair modes broaden and decrease in frequency. The excitation evolves smoothly as T is raised above TN into the paramagnetic phase. In both materials, light scattering from the paramagnetic vestiges of the magnon-pair modes could be observed up to 300°K. Together with earlier results on NiF2, these data indicate the trends in magnon-pair temperature behavior for materials with different spins and different magnon-magnon interaction effects. These trends may be useful in guiding theories for magnon-pair excitations.