Magnetic solitons in the 1-D antiferromagnetic chains of Li2Mn0.98Fe0.02F5 and Na2Mn0.98Fe0.02F5

Measurements of the Fe-57 Mossbauer effect and magnetic susceptibility have been performed on Fe-57-doped quasi-1-d antiferromagnetic chains of Li2MnF5 and Na2MnF5 as a function of temperature. The Mossbauer spectra, fitted by the Blume-Tjon model, show definite relaxation effects near the Neel temperature, which are attributed to short-range order with temperature-dependent relaxation times. The soliton model of non-linear excitations was applied. Experimental data confirm the predicted exponential temperature dependence of the thermal excitation of moving domain walls. From the activation energies E(A)(2)/k(2) as a function of 1-d exchange energies J/k the local anisotropy energy D/k of -3.7(2)K was derived on the basis of sine-Gordon theory. This result is in fair agreement with -3.5(1)K previously derived from magnetic susceptibility measurements. In the light of our results there is some evidence that at low bridging angle beta approximate to 102(5)degrees a phase-transition from antiferromagnetic to ferromagnetic ordering occurs.