Observation of Magnetic Order in a ${\rm YBa_2Cu_3O_{6.6}}$   Superconductor
  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. 
H.A. Mook, Y. Sidis, B. Fauqu\'e, V. Bal\'edent, and P. Bourges