Observation of a localized magnetic excitation in Co-doped KMnF3

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Abstract
A magnetic local mode associated with the cobalt impurities in a single-crystal specimen of the antiferromagnet KMn0.8Co0.2F3 has been observed by means of neutron inelastic scattering. At 20 °K, its frequency is 6.55 ± 0.15 THz. If it is assumed that (JCo–Mn)2 = JCo–Co × JMn–Mn, and allowance is made for the exchange mixing of the levels of the cobalt ion, then a calculation of the frequency of the local mode using molecular-field theory gives 6.8 THz, in satisfactory agreement with experiment. From the variation of the intensity of the local mode with wave-vector transfer, the probability that the localized excitation is on one of the nearest neighbors to a cobalt ion is estimated to be 0.08 ± 0.03. Measurements of the acoustic spin waves propagating along the [00ζ], [ζζ0], and [ζζζ] directions at 20 °K have also been carried out. Their frequencies are essentially the same as those of pure KMnF3 except near the center of the magnetic Brillouin zone where they are slightly lower. The maximum frequency of the acoustic spin waves is 2.30 ± 0.04 THz.

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