SPIN CORRELATIONS IN THE 2-DIMENSIONAL SPIN-5 2 HEISENBERG-ANTIFERROMAGNET RB2MNF4/

We report a neutron scattering study of the instantaneous spin correla tions in the two-dimensional spin S = 5/2 square-lattice Heisenberg an tiferromagnet Rb2MnF4. The measured correlation lengths are quantitati vely described, with no adjustable parameters, by high-temperature ser ies expansion results and by a theory based on the quantum self-consis tent harmonic approximation. Conversely, we find that the data, which cover the range from about 1 to 50 lattice constants, are outside of t he regime corresponding to renormalized classical behavior of the quan tum non-linear a model. In addition, we observe a crossover from Heise nberg to Ising critical behavior near the Neel temperature; this cross over is well described by a mean-field model with no adjustable parame ters.