PHASE-TRANSITIONS IN BILAYER HEISENBERG-MODEL WITH GENERAL COUPLINGS

The ground state properties and phase diagram of bilayer Heisenberg mo del on the square lattice are studied in a wide range of parameter spa ce of intralayer exchange couplings, assuming an antiferromagnetic cou pling between constituent layers. In the classical limit, the model ex hibits three phases: two of these are ordered phases specified by the ordering wave vectors (pi, pi; pi) and (0, 0; pi); where the third com ponent of each indecates the antiferromagnetic orientation between lay ers, and another one is ''canted'' phase, stabilized by competing inte ractions. The effects of quantum fluctuations in the model with S = 1/ 2 have been explored in terms of dimer mean-field theory exact diagona lization of 2 root 2 x 2 root 2 x 2 clusters, and higher-order perturb ation expansions about the interlayer dimer limit.