Raman Scattering by Four Magnons in NiO and KNiF3

We have observed $\Gamma _3^{}{}_{}{}^{+}$ excitations by Raman scattering in KNi${\mathrm{F}}_3$ at 1270 ${\mathrm{cm}}^{-1\mathrm{}}$ ($17.9J$), and in NiO at 2800 ${\mathrm{cm}}^{-1\mathrm{}}$ ($18.9J$). These excitations are identified as four-magnon excitations by agreement of the peak frequencies with calculated values, and in the case of NiO by the temperature shift. Scattering by four magnons is the highest-order nonresonant Raman scattering yet observed. The excitation of four magnons is interpreted in terms of excited-state propagation effects.