Critical behaviour of a two-dimensional random antiferromagnet: Rb₂Mn0.5Ni0.5F₄

A neutron scattering study of the order parameter, correlation length and staggered susceptibility of the two-dimensional random antiferromagnet Rb 2 Mn 0.5 Ni 0.5 F 4 is reported. The system is found to exhibit a well-defined phase transition with critical exponents identical to those of the isomorphous pure materials K 2 NiF 4 and K 2 MnF 4 . Thus, in these systems, which have the asymptotic critical behaviour of the two-dimensional Ising model, randomness has no measurable effect on the phase-transition behaviour.