Magnetic Studies on the 1-D Antiferromagnetic Chains of enH₂Mn(Fe)F₅

Measurements of the ⁵⁷Fe Mössbauer effect and of magnetic susceptibilities have been performed on the ⁵⁷Fe doped or pure quasi-1-d antiferromagnetic chain compound enH₂MnF₅ as a function of temperature. Particular attention was paid to the regions very near the Néel point at T_N = 14.5(5) K. 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 non-linear excitations was applied. Experimental data confirm the predicted exponential temperature dependence of the thermal excitation of moving domain walls. From the activation energy E_A/k = 184(5) K and the 1-d exchange energy J/k = -13.6(2) K a local anisotropy energy D/k of -2.4 (3) K was obtained which is smaller than D/k = -3.5 K derived from our single crystal measurements of magnetic susceptibilities. Within the 3-d magnetic ordering the interchain interaction amounts to J′/k ≈ 0.22(4) K (J′/J = 15.8 10^-3). A zero-spin reduction ⊿S/S equal to 17% was obtained. 1-d correlations are observed for T_N<T< |J|S (S + 1)/k ≈ 78 K, and a crystallographic phase transition at 216 K was indicated in the magnetic properties as well as by DSC and X-ray powder diffraction measurements.