  The bilayer Heisenberg antiferromagnet is known to exhibit a quantum-critical
transition at a particular value of the inter-layer coupling. Using a new type
of coherent state, appropriate to the special order parameter structure of the
bilayer, we map the problem onto the quantum non-linear sigma model. It is
found that the bare coupling constant diverges at the classical transition of
Chubukov and Morr, so that in any finite dimension the actual transition occurs
inside the ordered phase of the classical theory.
