One-dimensional behaviour of (ND4)2MnF5 and (NH4)2MnF5

We have investigated the one-dimensional magnetic behaviour of single crystals of (ND4)(2)MnF5 and (NH4)(2)MnF5 by means of neutron scattering. The Neel temperature has been determined by monitoring the strongest magnetic Bragg reflection as T-N = 8.5 +/- 0.4 K. Measurements of the magnon dispersion curve resulted in values of D/k = -2.43 K, J/k = -10.6 K and D/k = -2.78 K, J/k = -10.3 K, \J'\/k = 2.7(3) x 10(-4) for the protonated and deuterated sample. respectively. The ratio R = \J'\/J is about 3 x 10(-5). The temperature dependence of the magnetic correlation length shows an exponential behaviour, which is typical for Ising-like systems and can be described by nonlinear excitations (solitons). Neutron diffraction studies revealed the magnetic structure consisting of spins mainly aligned antiferromagnetically, along the chain direction. All results are in very good agreement with magnetic susceptibility measurements and Mossbauer spectroscopy of iron-doped homologue samples. (C) 1998 Elsevier Science B.V. All rights reserved.