ONE-DIMENSIONAL BEHAVIOR OF (ND4)(2)MNF5 AND (NH4)(2)MNF5

We have investigated the one-dimensional magnetic behaviour of single crystals of (ND4)(2)MnF5 and (NH4)(2)MnF5 by means of neutron scatteri ng. The Neel temperature has been determined by monitoring the stronge st magnetic Bragg reflection as T-N = 8.5 +/- 0.4 K. Measurements of t he 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 co rrelation length shows an exponential behaviour, which is typical for Ising-like systems and can be described by nonlinear excitations (soli tons). Neutron diffraction studies revealed the magnetic structure con sisting of spins mainly aligned antiferromagnetically, along the chain direction. All results are in very good agreement with magnetic susce ptibility measurements and Mossbauer spectroscopy of iron-doped homolo gue samples. (C) 1998 Elsevier Science B.V. All rights reserved.