Antiferromagnetic phase diagram of YBa2(Cu1−xFex)3Oy

Neutron‐diffraction measurements have been performed in the YBa2(Cu1−xFex)3Oy system with 0.01≤x≤0.12 and y≂6.2. At low iron content, two magnetic transitions are observed with decreasing temperature and a noncollinear phase (AF1+AF2) at low temperature. This results from the occurrence of a very small magnetization μCu1 on the oxygen‐deficient layers. Around x=0.03, the AF1 phase disappears, μCu1 remaining very small. At higher iron content, in the AF2 phase an increase of μCu1 with increasing x is observed, together with a spin‐glass‐like freezing of the iron spins. The results suggest that charge‐transfer effects play a crucial role in the change of magnetic phases at low x values, whereas the magnetic effects of iron substitution dominate at higher concentrations.