PHASE-TRANSITIONS OF THE BILAYERED SPIN-S HEISENBERG-MODEL

We study the ground state and the phase transitions of the bilayered s pin-S antiferromagnetic Heisenberg model using the Schwinger boson mea n-field theory. The interplane coupling initially stabilizes but event ually destroys the long-range antiferromagnetic order. The transition to the disordered state is continuous for small S, and first order for large S. The latter is consistent with an argument based on spin-wave theory.