Statistical Mechanics of Uniaxial Antiferromagnet with First- and Second-Neighbor Exchanges

Abstract

Low-temperature neutron-scattering study together with the zero-field AFMR in MnF2 furnished a reliable set of values of first- and second-neighbor exchange integrals and the uniaxial anisotropy energy. It is the purpose of this paper to see to what extent one can understand the high-temperature properties such as the Néel temperature and the anisotropic susceptibilities in terms of the known parameters obtained at low temperatures. For this purpose the cluster-variation method is used in which up to the two-spin correlation is taken into account. One of the main tasks of this method is to diagonalize effective one- and two-spin Hamiltonians. It is noted here that the anisotropy energy could be as big, at least, as the second-neighbor exchange energy. It is, therefore, not justifiable to treat the anisotropy energy as a small perturbation. In this paper diagonalization of the effective two-spin Hamiltonian is achieved for the actual spin, S = 52, and the Néel temperature and the anisotropic susceptibilities are calculated. Calculations of the sublattice magnetization and spin-flop field are in progress.