Static and Dynamic Magnetic Properties of Rb_2CrCl_4: Ideal 2D-XY Behaviour in a Layered Magnet

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The layered XY ferromagnet Rb₂CrCl₄ has been studiedexperimentally for a number of years with the aim of establishing towhat extent the characteristic properties of the 2D-XY model, inparticular the Kosterlitz-Thouless-Berezinskii (KTB) phase transition, survive in a system with weak interlayer exchange coupling. We are nowable to give a definitive answer to this question, following a deeperunderstanding of the properties of the 2D-XY model. Simple lengthscale arguments and spin wave calculations suggest that the layeredmagnet is equivalent to an ideal 2D-XY system of an effective finitesize determined by the interlayer exchange. We show that thishypothesis, when combined with a recent renormalisation group approachto the finite size 2D-XY model, is sufficient to completely explainthe results of diverse neutron scattering and other experiments onRb₂CrCl₄. Our conclusion is that, except in a narrow 3D criticalregion, Rb₂CrCl₄ behaves as a finite 2D-XY system, with abroadened KTB transition occuring in the presence of three-dimensionallong range order.