  We study the ground state and the phase transitions of the bilayered spin-$S$
antiferromagnetic Heisenberg model using the Schwinger boson mean field theory.
The interplane coupling initially stabilizes but eventually 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 the spin wave theory. The phase diagram
and phase transitions in corresponding model in fractional dimensions are also
discussed.

Condensed Matter