Static and Dynamic Magnetic Properties of Rb2CrCl4: Ideal 2D-XYBehaviour in a Layered Magnet

The layered XY ferromagnet Rb 2 CrCl 4 has been studied experimentally for a number of years with the aim of establishing to what extent the characteristic properties of the 2D- X Y model, in particular the Kosterlitz-Thouless-Berezinskii (KTB) phase transition, survive in a system with weak interlayer exchange coupling. We are now able to give a definitive answer to this question, following a deeper understanding of the properties of the 2D- X Y model. Simple length scale arguments and spin wave calculations suggest that the layered magnet is equivalent to an ideal 2D- X Y system of an effective finite size determined by the interlayer exchange. We show that this hypothesis, when combined with a recent renormalisation group approach to the finite size 2D- X Y model, is sufficient to completely explain the results of diverse neutron scattering and other experiments on Rb 2 CrCl 4 . Our conclusion is that, except in a narrow 3D critical region, Rb 2 CrCl 4 behaves as a finite 2D- X Y system, with a broadened KTB transition occuring in the presence of three-dimensional long range order.