2-DIMENSIONAL XY TYPE MAGNETISM IN INTERCALATED GRAPHITE - AN ELASTICAND INELASTIC NEUTRON-SCATTERING STUDY

Stage-2 CoCl2-GIC approximates a two-dimensional easy-plane (XY) ferro magnet on a triangular lattice. It has been found in prior work to ord er in two steps, with the intermediate phase showing long-ranged ferro magnetic correlations within the intercalate plane, but no correlation s between neighboring planes. We have probed the wave vector and tempe rature dependence of the static and dynamic spin correlations in detai l, including measurements of the critical scattering, the quasielastic scattering from vortex diffusion and the spin wave excitations with a nd without an external magnetic field. Some of the predictions for a K osterlitz-Thouless type transition are met in this compound, at least qualitatively, including an apparent jump in the spin stiffness at the critical point and the existence of a diffusive central peak in the s cattering function possibly originating from vortex autocorrelations. However, there are some inconsistencies between our observations and r ecent analytical studies as well as Monte Carlo - molecular dynamics s imulations of the vortex dynamics that prevent unambigous assignment o f the upper critical temperature as a vortex-binding transition.