STATIC AND DYNAMIC MAGNETIC-PROPERTIES OF RB2CRCL4 - IDEAL 2D-XY BEHAVIOR IN A LAYERED MAGNET

The layered XY ferromagnet Rb-2 CrCl4 has been studied experimentally for a number of years with the aim of establishing to what extent the characteristic properties of the 2D-XY 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-XY 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-XYmodel, is sufficient to completely explain the results of diverse neutron scattering and other experiments on Rb2CrCl4. Our conclusion is that, except in a narrow 3D critical region, Rb2CrCl4 behaves as a finite 2D-XY system, with a broadened KTB transition occuring in the presence of three-dimensional long range order.