THEORETICAL-STUDY OF THE SPIN DYNAMICS IN CSNIF3, A ONE-DIMENSIONAL FERROMAGNET WITH PLANAR ANISOTROPY, IN AN EXTERNAL MAGNETIC-FIELD

Using a Green's function technique we have studied the spin dynamics i n the one-dimensional ferromagnetic CsNiF3 with an external field. Tak ing into account damping effects, the longitudinal mode becomes purely imaginary and the transverse mode is underdamped for low temperatures and overdamped near and above the critical temperature. The spin wave energy and the damping strongly depend on the anisotropy energy and t he magnetic field. We have calculated the dynamic structure factors S- xx, S-yy, and S-zz via the imaginary part of the Green's functions. Th e coupling between the transverse mode and the relaxing longitudinal m ode produces a central peak in the dynamic structure factor, i.e., we obtain two spin wave scattering components and a central component. Th e temperature, magnetic field, anisotropy energy, and wave vector depe ndence of the three peaks are discussed and compared with the experime ntal data.