MODEL FOR THE LOW-TEMPERATURE MAGNETIC PHASES OBSERVED IN DOPED YBA2CU3O6+X

A classical statistical model for the antiferromagnetic ordering of th e Cu spins in the CuO2 planes of reduced YBa2Cu3O6+x-type materials is presented. The magnetic phases considered are the experimentally obse rved high-temperature antiferromagnetic (AFI) phase with ordering vect or Q(I)=(1/2 1/2 0), and the low- temperature phases: antiferromagneti c (AFII) with Q(II)=(1/2 1/2 1/2) and intermediate Turn-Angle (TA) pha ses TAI, TAII, and TAIII with components of both ordering vectors. It is shown that the AFII and TA phases result from an effective ferromag netic-type coupling mediated by free spins in the CuOx basal plane. Go od agreement with experimental data is obtained for realistic model pa rameters. [S0163-1829(97)08041-7].