The static and dynamic critical behaviour of the two-dimensional Heisenberg antiferromagnet KFeF4

Abstract

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+or-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.