Comparison of the non-linear spin dynamics in antiferromagnetic chain compounds Na2MnF5 and (ND4)2MnF5

Na2Mn0.98Fe0.02F5 and(ND4)(2)Mn0.98Fe0.02F5 was 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 performed neutron scattering experiments on large single crystals of the pure compounds. We discuss the results obtained on a thermal and cold three-axis spectrometer, which probe the magnon spin wave excitations and the existence of the nonlinear excitations in the quasi 1-d antiferromagnetic chains of Na2MnF5 and (ND4)(2)MnF5, 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 lengths was derived resulting in a soliton activation energy of E-s/k = 65(3) K and E-s/k - 81(3) K, respectively, which are in good agreement with the soliton energies obtained by our high resolution inelastic neutron scattering experiment. In contrast to these results the Mossbauer spectroscopy gives twice the value of the soliton energy caused by soliton pair or interband excitations.