TRICRITICAL BEHAVIOR OF A HEISENBERG-MODEL WITH DZYALOSHINSKI-MORIYA INTERACTION

The phase diagram of a spin-1/2 anisotropic Heisenberg model with Dzyaloshinski-Moriya (DM) interaction is studied in the mean-field approximation. It is shown that for the easy direction (D = Dz; where D is the DM vector coupling) the model exhibits tricritical point (TCP). The TCP is obtained explicitly and the tricritical temperature, T-t, is independent of the exchange anisotropy parameter Delta (Delta=0 and Delta=1, corresponds to the isotropic Heisenberg and Ising models, respectively), while the tricritical parameter, D-t, is dependent on Delta. We obtain T-t=2/3(z-1) an d D-t(Delta=0)<D-t(Delta=1), where z is the lattice coordinate number.