Polarized beam neutron scattering measurements on a highly perfect crystal of ${rm YBa_2Cu_3O_{6.6}}$ show a distinct magnetic transition with an onset at about 235K, the temperature expected for the pseudogap transition. The moment is found to be about 0.1 $mu_B$ for each sublattice and have a correlation length of at least 75 AA. We found the critical exponent for the magnetic neutron intensity to be 2$beta$ =0.37$pm$ 0.12. This is the proper range for the class of transition that has no specific heat divergence possibly explaining why none is found at the pseudogap transition.
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