Nonlinear spin dynamics in (ND4)(2)MnF5

Previously, the spin dynamics of the magnetic quasi-one-dimensional systems (NH4)(2)Mn0.98Fe0.02F5 and (ND)(2)Mn0.98Fe0.02F5 has been studied with the aid of Mossbauer spectroscopy. These results were interpreted on the basis of classical soliton theory. In order to confirm this concept, we have ind ependently performed neutron scattering experiments on large single crystal s (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 ex istence of nonlinear excitations in the quasi-one-dimensional antiferromagn etic chains of (ND4)(2)MnF5, respectively. Additionally, we include elastic neutron diffraction and de single-crystal susceptibility measurements to d etermine the magnetic structure. From the width of the quasielastic scatter ing signal, the temperature dependence of the inverse magnetic correlation length was derived, resulting in a soliton activation energy of E-s/k = 81( 3) K, which is in good agreement with the soliton energy of E-s/k = 77(5) K obtained by our high-resolution inelastic neutron scattering experiment. I n contrast to this result, the Mossbauer spectroscopy gives twice the value of the soliton energy, caused by soliton pair excitations.