THERMODYNAMICAL PROPERTIES OF A HEISENBERG-MODEL WITH DZYALOSHINSKI-MORIYA INTERACTIONS

Within the framework of a new correlated effective-field theory (CEF) the effects of the Dzyaloshinski-Moriya (DM) interactions on magnetic properties of the spin-1/2 anisotropic Heisenberg model are discussed. The CEF theory is based on a generalized but approximate Callen-Suzuk i spin relation for cluster with two spins, and makes use of the Honmu ra-Kaneyoshi exponential operator technique. The phase diagram and the thermal behavior of magnetization are analyzed for the simple cubic l attice, and compared with the corresponding two-spin cluster mean-fiel d (MFA) predictions. It is shown that for the easy direction (D=Dz; wh ere D is the DM vector coupling), the model exhibit a tricritical poin t (TCP), at which the phase transition changes from second to first or der, The TCP is explicitly obtained, and the tricritical temperature, T(t), is independent of the exchange anisotropy parameter DELTA (DELTA = 0 and DELTA = 1, correspond the isotropic Heisenberg and Ising mode ls, respectively), while the tricritical parameter, D(t), has dependen ce on DELTA. In spite of its simplicity, the present CEF formalism yie lds results, which represent a remarkable improvement on the usual MFA treatment.