NEUTRON-SCATTERING INVESTIGATION OF THE STATIC CRITICAL PROPERTIES OF RB2CRCL4

We have investigated the static zero-field critical properties of the ionic ferromagnet Rb2CrCl4, using neutron scattering, under conditions such that the quasi-static approximation is valid. A general consideration of the spin Hamiltonian of Rb2CrCl4, with S = 2, shows that it approximates the two-dimensional (2D) XY model, with additional weak perturbations. Rb2CfCl4 orders three dimensionally at T(C) = 52.3 K, and very close to T(C) there is a regime of three-dimensional critical behaviour. However, both below and above T(C) there is a crossover to regimes of two-dimensional critical fluctuations. In these regimes, there is excellent agreement between experiment and theoretical predictions for a finite-sized 2D XY model, suggesting that RbCrCl4 is an ideal realization of this system.