Spin fluctuations in random magnetic-nonmagnetic two-dimensional antiferromagnets. I. Dynamics

The magnetic excitations in the system ${\mathrm{Rb}}_2{\mathrm{Mn}}_c{\mathrm{Mg}}_{1-c}{\mathrm{F}}_4$ with $c=1, 0.57, \mathrm{and} 0.54$ have been measured using neutron-inelastic-scattering techniques. In pure ${\mathrm{Rb}}_2$Mn${\mathrm{F}}_4$ the magnon dispersion relation is accurately described by simple spin-wave theory with dominant Heisenberg antiferromagnetic nearest-neighbor exchange, a much weaker antiferromagnetic next-nearest-neighbor interaction, and a small single-ion anisotropy field. In the specimens with $c=0.57\mathrm{} \mathrm{and} 0.54$ the scattering shows a broad response but with substructure which may be associated with the different Ising energies of Mn ions in different environments. The measurements are in excellent accord with the results of computer simulations but theories based on the coherent-potential approximation do not give a satisfactory description of the measured spectra.