Previously, the spin dynamics of the magnetic quasi-one-dimensional systems ${(\mathrm{N}\mathrm{H}}_4{)}_2{\mathrm{Mn}}_{0.98}{\mathrm{Fe}}_{0.02}{\mathrm{F}}_5$ and ${(\mathrm{N}\mathrm{D}}_4{)}_2{\mathrm{Mn}}_{0.98}{\mathrm{Fe}}_{0.02}{\mathrm{F}}_5$ has been studied with the aid of Mössbauer spectroscopy. These results were interpreted on the basis of classical soliton theory. In order to confirm this concept, we have independently performed neutron scattering experiments on large single crystals (about 1.4 g). We discuss the results obtained on thermal and cold three-axis spectrometers, which probe the magnon spin-wave excitations and the existence of nonlinear excitations in the quasi-one-dimensional antiferromagnetic chains of ${(\mathrm{N}\mathrm{D}}_4{)}_2{\mathrm{MnF}}_5,$ respectively. Additionally, we include elastic neutron diffraction and dc single-crystal susceptibility measurements to determine the magnetic structure. From the width of the quasielastic scattering signal, the temperature dependence of the inverse magnetic correlation length was derived, resulting in a soliton activation energy of $E_s/k=81\mathrm{}\left(3\right)\mathrm{}\mathrm{K},$ which is in good agreement with the soliton energy of $E_s/k=77\mathrm{}\left(5\right)\mathrm{}\mathrm{K}$ obtained by our high-resolution inelastic neutron scattering experiment. In contrast to this result, the Mössbauer spectroscopy gives twice the value of the soliton energy, caused by soliton pair excitations.

• Received 9 November 1999

DOI:https://doi.org/10.1103/PhysRevB.61.15221