Temperature and field dependence of the magnetisation of Rb2CrCl4: a two-dimensional easy-plane ionic ferromagnet

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Abstract
The magnetisation of a single crystal of the two-dimensional (2D) easy-plane ferromagnet Rb2CrCl4 has been measured as a function of temperature (5-160K) and as a function of magnetic field (0-7T) applied within the easy (001) plane. Isothermal critical behaviour of the form M varies as H1 delta / was observed at low temperatures with delta decreasing slowly with increasing temperature; this is characteristics of Kosterlitz-Thouless (KT) (2D) XY behaviour. An effective KT transition temperature, TKT, can be defined as 45.5K, the temperature at which delta =15. Above TC the susceptibility, found from the initial slope of the magnetisation with field, could be accounted for equally well by an exp(B'/(tKT)12/) dependence, where tKT=(T-TkT)/TKT, as predicted by KT theory, or by the usual power-law behaviour in t=(T-TC)/TC. The variation of the magnetic properties of Rb2CrCl4 with field and temperature was found to be qualitatively very similar to that of K2CuF4, whose properties have been described by the KT theory. The difficulties of isolating unambiguous evidence for KT behaviour in such experiments is discussed.