  The low temperature correlation length and the static structure factor are
computed for bilayer antiferromagnets such as YBa_2Cu_3O_6. It is shown that
energy integrated two-axis scan in neutron scattering experiments can be
meaningfully interpreted to extract correlation length in such bilayer
antiferromagnets despite intensity modulation in the momentum perpendicular to
the layers. Thus, precise measurements of the spin-spin correlation length can
be performed in the future, and the theoretical predictions can be tested. It
is also shown how the same correlation length can be measured in nuclear
magnetic resonance experiments.
