Excitations in a two-dimensional random antiferromagnet

Inelastic neutron scattering studies of the magnetic excitations in the planar Heisenberg random antiferromagnet Rb 2 Mn 0.5 Ni 0.5 F 4 at 7K are reported. Two well-defined bands of excitations are observed. A simple mean crystal model is found to predict accurately the measured dispersion relations using essentially the pure crystal parameters while the zone boundary energy widths are well-accounted for by an Ising cluster mode. Intensities, however, are not properly explained using these simple models.