Magnon-pair modes have been studied in the quadratic layer antiferromagnet ${\mathrm{K}}_2$Ni${\mathrm{F}}_4$ at temperatures between 5 and 150°K ($T_{\mathrm{N}}=97.1\mathrm{}°$K) using the technique of second-order Raman scattering. At low temperatures the effect of magnon-magnon interactions is observed to be in excellent agreement with theoretical predictions. The temperature evolution of the pair-mode spectrum demonstrates striking differences between two and three dimensions in the frequency renormalization and damping of short-wavelength magnons.

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