The classical two-dimensional XY model with in-plane magnetic field

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 Tor=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.