SPIN-S BILAYER HEISENBERG MODELS - MEAN-FIELD ARGUMENTS AND NUMERICAL-CALCULATIONS

Spin-S bilayer Heisenberg models (nearest-neighbor square-lattice anti ferromagnets in each layer, with antiferromagnetic interlayer coupling s) are treated using dimer mean-field theory for general S and high-or der expansions about the dimer limit for S=1,3/2,...,4. We suggest tha t the transition between the dimer phase at weak intraplane coupling a nd the Neel phase at strong intraplane coupling is continuous for all S, contrary to a recent suggestion based on Schwinger boson mean-field theory. We also present results for S=1 layers based on expansions ab out the Ising limit: In every respect the S=1 bilayers appear to behav e like S=1/2 bilayers, further supporting our picture for the nature o f the order-disorder phase transition.