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.
Boothroyd, A. T., Longmore, A., Andersen, N. H., Brecht, E., & Wolf, T. (1997). Novel Pr-Cu magnetic phase at low temperature in PrBa2Cu3O6+x observed by neutron diffraction. Physical Review Letters, 78(1), 130-133. https://doi.org/10.1103/PhysRevLett.78.130