Biquadratic Exchange inCsMnxMg1−xBr3

Abstract

The importance of higher-order exchange interaction in localized $S$-state systems is discussed. The equation of motion is solved for a one-dimensional antiferromagnet with bilinear and biquadratic exchange coupling. It is shown that the biquadratic exchange interaction usually cannot be derived from the observed spin-wave dispersion, but is susceptible to experimental determination from the excitation spectra of small clusters of magnetic ions as exemplified for $\mathrm{Cs}{\mathrm{Mn}}_x{\mathrm{Mg}}_{1-x}{\mathrm{Br}}_3$ by inelastic neutron scattering.