ANTIFERROMAGNETIC PHASE-DIAGRAM OF YBA2(CU1-XFEX)3OY

Neutron-diffraction measurements have been performed in the YBa2(Cu1-x Fex)3Oy system with 0.01 less-than-or-equal-to x less-than-or-equal-to 0.12 and y congruent-to 6.2. At low iron content, two magnetic transi tions are observed with decreasing temperature and a noncollinear phas e (AF1 + AF2) at low temperature. This results from the occurrence of a very small magnetization mu(Cu1) on the oxygen-deficient layers. Aro und x=0.03, the AF1 phase disappears, mu(Cu1) remaining very small. At higher iron content, in the AF2 phase an increase of mu(Cu1) with inc reasing x is observed, together with a spin-glass-like freezing of the iron spins. The results suggest that charge-transfer effects play a c rucial role in the change of magnetic phases at low x values, whereas the magnetic effects of iron substitution dominate at higher concentra tions.