Abstract
In an easy-plane spin system such as CsNiF3, the (SxSx+SySy) 'in-plane correlation components' (IPC) behave differently from the (SzSz) 'out-of-plane correlation components' (OPC). The linewidths of spin waves in the paramagnetic state with one-dimensional short-range order in the z direction was investigated by inelastic scattering of neutrons at 4.9<T<12K. The IPC and the OPC contributions could be separated by measuring the same magnon at different momentum transfers Q of the neutron. The origin of the linewidth in a short-range ordered system is discussed showing that the main contribution to the width of IPC is due to the finite correlation length. The behaviour of the IPC is similar to the magnons in the isotropic one-dimensional antiferromagnet TMMC.