Temperature Dependence of Magnon-Pair Modes in Antiferromagnets and Paramagnets

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Abstract
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.