Nonlinear excitations (solitons) in the antiferromagnetic chain of (NH4)2Mn0.98Fe0.02F5

Measurements of the⁵⁷Fe Mössbauer effect and of the magnetic susceptibilities on a single crystal have been performed on the quasi-1-d antiferromagnetic chain of (NH₄)₂Mn_0.98Fe_0.02F₅ as a function of temperature. Particular attention was paid to the region very near the Néel point. The Mössbauer spectra fitted by the Blume-Tjon model show definite relaxation effects, which are attributed to short-range order with temperature-dependent relaxation times. The soliton model of nonlinear excitations was applied. Experimental data confirm the predicted exponential temperature dependence of the thermal excitation of moving domain walls. From the activation energy a local anisotropy energyD/k of –3.9 K was derived.