Thermodynamic properties of a Heisenberg antiferromagnet

A finite temperature perturbation expansion is employed to obtain an approximate description of the thermodynamic behaviour of a Heisenberg antiferromagnet for spin half. The calculations are carried out using the drone-fermion representation, and contributions are classified with respect to a high density parameter 1/z, where z is the number of spins interacting with any given spin. This leads to the results of molecular field theory in lowest order (1/z) 0 . The spin fluctuation effects, which are absent in molecular field theory, are introduced in higher orders. Calculations of the sublattice magnetization, free energy, and internal energy are carried out up to and including (1/z) 1 . The results are valid provided the fluctuation effects are sufficiently small, which is the case for all temperatures below the Neel temperature T N and also for higher temperatures if there is a suitable applied magnetic field or anisotropy field acting on the system. In a low temperature approximation the results are found to be consistent with linear spin wave theory, including zero point effects.