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 int eraction, which serves as a simple model for Langmuir-Blodgett films o f Mn (C18H35O2)2. By mapping the system onto a pure easy-plane model w e discuss the corresponding spin wave and vortex dynamics. The additio nal Dzyaloshinsky interaction forces all spins to cant in a certain di rection, which is the same for neighboring spins on different sublatti ces. This canting causes the presence of a second spin wave peak in th e dynamical in-plane correlation function below the Kosterlitz-Thoules s transition temperature T(KT) and a second vortex central peak above T(KT). Using a vortex gas approach we explicitly calculate the contrib ution of the free vortices to several dynamical correlation functions. These results are compared to a combined Monte Carlo-Molecular Dynami cs simulation on square lattices with different sizes. We also discuss the relevance of this simple model for describing the spin dynamics o f Mn (C18H35O2)2.