Model for the magnetic phases observed at low temperatures in reduced YBa 2Cu 3O 6+x materials with Al substitution for Cu or rare earth ions for Ba

From a classical statistical model it is shown that the low temperature antiferromagnetic AFII phase, observed in reduced YBa ₂Cu ₃O _6+x type materials with Al in the CuO _x basal plane or rare earth on the Ba site, results from an effective ferromagnetic coupling between adjacent CuO ₂ double layers mediated by polarization of free spins in the basal plane. At higher temperature the ferromagnetic coupling becomes ineffective and the spin structure transforms via intermediate Turn Angle phases into the high temperature AFI phase.