Spin dynamics for 2d Heisenberg antiferromagnets with combined easy-plane and Dzyaloshinsky interactions

We investigate the spin dynamics of the classical two-dimensional easy-plane Heisenberg antiferromagnet with an additional Dzyaloshinsky interaction, which serves as a simple model for Langmuir-Blodgett films of Mn (C18H35O2)2. By mapping the system onto a pure easy-plane model we discuss the corresponding spin wave and vortex dynamics. The additional Dzyaloshinsky interaction forces all spins to cant in a certain direction, which is the same for neighboring spins on different sublattices. This canting causes the presence of a second spin wave peak in the dynamical in-plane correlation function below the Kosterlitz-Thouless transition temperature TKT and a second vortex central peak above TKT. Using a vortex gas approach we explicitly calculate the contribution of the free vortices to several dynamical correlation functions. These results are compared to a combined Monte Carlo-Molecular Dynamics simulation on square lattices with different sizes. We also discuss the relevance of this simple model for describing the spin dynamics of Mn (C18H35O2)2.