The effects of relatively low concentrations of impurities on the magnetic excitation spectra of antiferromagnetic insulators are now rather well understood. Essentially the entire spectral function can be studied directly by neutron scattering, and we describe the results of such studies on several single crystals of the (Co,Mn)F2, K(Co,Mn)F3, K(Mn,Ni)F3 and (Mn,Zn)F2 systems. In Co‐doped MnF2 (5%) and KMnF3 (20%), weakly perturbed band modes and localized impurity modes were observed. The spatial extent of the local modes has also been determined. In Ni‐doped KMnF3 (3%), weakly perturbed band modes, localized impurity modes, and also shell modes associated with the nearest neighbors to impurities were observed. These results and the resonant perturbation of spin waves observed in Zn‐doped MnF2 (5%) are adequately described by low‐concentration Green‐function theory. Recent measurements at higher concentration have shown the rather surprising existence of two branches of well defined propagating excitations. Although these results are not adequately described by existing theories, the character (localized or propagating) of the excitations is approximately described by an extension of the Anderson localization criterion for electron bands. Ising theory allows a simple interpretation of several features of the results for all materials studied.
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Svensson et al. (1972) studied this question.