High-energy spin excitations up to above 8000 ${\mathrm{cm}}^{\mathrm{-}1}$ were measured by polarized Raman scattering in the insulating phases of S=1/2 copper oxides, ${\mathrm{La}}_2$${\mathrm{CuO}}_4$, ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.2}$, ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{0.5}$${\mathrm{Y}}_{0.5}$${\mathrm{Cu}}_2$${\mathrm{O}}_{8+\mathit{y}}$, ${\mathrm{Nd}}_2$${\mathrm{CuO}}_4$, and ${\mathrm{Pr}}_2$${\mathrm{CuO}}_4$, and an S=1 nickel oxide ${\mathrm{La}}_2$${\mathrm{NiO}}_4$. All the copper oxides commonly show anomalous spectra, especially the secondary scattering peaks near 4J (J is the two-spin superexchange energy). Those anomalies are caused by the large four-spin cyclic exchange interactions. The present experiments demonstrated that the four-spin exchanges show distinctive characteristics in cuprates as compared to other materials.

• Received 21 March 1990

DOI:https://doi.org/10.1103/PhysRevB.42.1045