Dynamic effects in one-dimensional magnets with an easy plane

Dynamic properties of classical one-dimensional magnets with ferro- or antiferromagnetic exchange coupling and single-site anisotropy B(S z ) 2 are discussed in terms of a theory which utilizes exact expressions for static spin correlation functions e.g. the wavevector-dependent susceptibility. The theory is shown to provide a quantitative interpretation of the dispersion and damping of collective modes observed in the ferromagnetically coupled compounds CsNiF 3 . Marked differences are predicted between the dynamic properties of ferro- and antiferromagnetically coupled models. A gap in the dispersion of the out-of-plane collective mode at low temperatures for antiferromagnetic coupling leads to the persistence of a zone-boundary mode up to moderate temperature with only a small anisotropy. The associated lineshape is studied as a function of wavevector and temperature, and the intensity is found to decrease rapidly with temperature.