Harmonic magnons in ${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te and ${\mathrm{Zn}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te

Studies of spin dynamics in frustrated fcc antiferromagnets ${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Mn}}_{\mathrm{x}}$Te and ${\mathrm{Zn}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Mn}}_{\mathrm{x}}$Te are reported. Inelastic neutron scattering measurements carried out for single-crystal specimens with compositions ${\mathrm{Cd}}_{0.33}$ ${\mathrm{Mn}}_{0.67}$Te, ${\mathrm{Zn}}_{0.35}$ ${\mathrm{Mn}}_{0.65}$Te, and ${\mathrm{Zn}}_{0.59}$ ${\mathrm{Mn}}_{0.41}$Te reveal that at low temperatures (T=4.2–5 K) the magnetic excitation spectra of these systems are dominated by magnonlike modes despite the absence of long-range spin order. Theoretical modeling has been carried out by numerical techniques utilizing the formalism of conventional harmonic magnon theory. The results reproduce remarkably well the dispersion and the line-shape characteristics of the excitation modes seen in the experiments. As shown using another theoretical approach, the observed shift in the inelastic peak positions mirror the dispersion characteristics of spin waves in a perfect type-III antiferromagnetic structure. Fits of the numerically calculated line shapes to the experimental spectra yield information about the exchange parameters in the systems, which is especially difficult to obtain by other methods in the region of high Mn concentrations.