Abstract. The authors present analytical and numerical simulation studies on the two-dimensional XY classical ferromagnetic model with weak in-plane magnetic field (the reduced field is h=0.05 and it is applied along the x axis). The structure and dynamics of vortex spin configurations are considered. The simulation data show a strong crossover at T_c approximately=1.0 JS²; the data for T<T_c are well interpreted by one- and two-spin wave processes with an additional anomalous central peak at S_xx(q, omega ) structure function which is suggested as a contribution from domain walls. The data for T>or=T_c can be interpreted by vortex gas theory. The in-plane data can be compared to recent experiments performed on CoCl₂-intercalated into graphite. The S_zz(q, omega ) shows unusual behaviour as T is increased to T>or=1.2 JS² indicating that another phase transition or crossover, related to the out-of-plane spin component, can occur.

Print publication: Issue 7 (19 February 1990)