THE STATIC AND DYNAMIC CRITICAL-BEHAVIOR OF THE 2-DIMENSIONAL HEISENBERG-ANTIFERROMAGNET KFEF4

KFeF4 is a well characterized planar antiferromagnet with S = 5/2, and is an excellent example of a two-dimensional Heisenberg antiferromagnet. Using neutron-scattering techniques, a study of both the static and dynamic critical behaviour above the Neel temperature, T(N) = 136.75 +/- 0.35 K, has been undertaken. It is found that in the region above 1.04T(N), the static correlation length, the structure-factor peak intensity and the width of the dynamic scattering are all qualitatively described by the theories developed for Heisenberg antiferromagnets. The data have been compared with both the theory for a quantum Heisenberg antiferromagnet, as developed for La2CuO4, and also the model for a classical Heisenberg antiferromagnet. Both these theories describe the data equally well. Below 1.04T(N) the system is expected to behave like a two-dimensional Ising system.