Novel Pr-Cu magnetic phase at low temperature in PrBa2Cu3O6+x observed by neutron diffraction

We have studied by neutron diffraction the magnetic ordering in Al-free crystals of PrBa2Cu3O6+x (x = 0.35 and 0.92) that do not display the AFII Cu magnetic phase. Wt find that the Pr ordering below 20 K is accompanied by a counterrotation of the Cu antiferromagnetism on each plane of the bilayer. The maximum turn angle between the two planes is 60 degrees +/- 9 degrees for the x = 0.92 crystal, and 40 degrees +/- 11 degrees for the x = 0.35 crystal. This is the first observation of a noncollinear ordering of Cu moments in the bilayer, and is evidence for significant magnetic coupling between the Cu and Pr sublattices.