Spin dynamics of the one-dimensional magnet Na2MnF5

The spin dynamics of the one-dimensional (1D) magnet Na2MnF5 have been investigated by inelastic neutron scattering. Measurements of the spin-wave dispersion resulted in values of the exchange constant J/k(B)=-8.5(1) K and the anisotropy constant D/k(B)=-2.70(2) K, respectively. The temperature dependence of the magnetic correlation length showed significant deviations from the Heisenberg/ Fischer behavior at low temperatures. The observed exponential increase of the inverse correlation length upon temperature is a typical feature of strongly anisotropic systems. Soliton excitations have been measured directly by high-resolution neutron spectroscopy. From these measurements, a soliton activation energy of E-s/k(B)=65(5) K has been determined. The results are discussed with respect to other 1D magnetic systems among the fluoromanganates