  In doped lamellar cuprates, localized holes create ferromagnetic bonds and
cause spin canting similar to that caused by magnetic dipoles. At low
temperatures, these dipoles are frozen, behaving like quenched correlated
random fields. Renormalization group methods are used to show that such
impurities cause a strong reduction of the two-dimensional antiferromagnetic
correlations in the non-linear $\sigma$ model, and a related decrease in the
three dimensional N\'eel temperature, in quantitative agreement with
experiments.
