MODEL FOR THE MAGNETIC PHASES OBSERVED AT LOW-TEMPERATURES IN REDUCEDYBA2CU3O6-EARTH IONS FOR BA(X MATERIALS WITH AL SUBSTITUTION FOR CU OR RARE)

From a classical statistical model it is shown that the low temperatur e antiferromagnetic AFII phase, observed in reduced YBa2Cu3O6+x type m aterials with Al in the CuOx 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 pl ane. At higher temperature the ferromagnetic coupling becomes ineffect ive and the spin structure transforms via intermediate Turn Angle phas es into the high temperature AFI phase.