Spin Correlations in an Isotropic Spin-5/2Two-Dimensional Antiferromagnet

We report a neutron scattering study of the spin correlations for the spin- $5/2$ two-dimensional antiferromagnet $\mathrm{Rb}{}_2\mathrm{MnF}{}_4$ in an external magnetic field. Choosing fields near the system's bicritical point, we tune the effective anisotropy in the spin interaction to zero, constructing an ideal $S=5\mathrm{}/2$ Heisenberg system. The correlation length and structure factor amplitude are closely described by the semiclassical theory of Cuccoli et al. over a broad temperature range, but show no indication of approaching the low-temperature renormalized classical regime of the quantum nonlinear sigma model.