INFLUENCE OF A DEOXYGENATION PROCESS ON THE MAGNETIC DIAGRAM OF IRON-DOPED YBA2CU3OY PHASES - A NEUTRON-DIFFRACTION STUDY

Neutron powder diffraction measurements (NPD) and Mossbauer spectrosco py (MS) have been performed to determine the magnetic phase diagram of the YBa2(Cu1-xFex)(3)O-y system with 0 less than or equal to x less t han or equal to 0.16 and an oxygen content y close to 6.3. These phase s were deoxygenated from the initial oxygen-rich state (y approximate to 7), by a low temperature process. This thermal treatment leaves the iron distribution identical to the one of the original oxygenated com pound, whereas the usual high temperature method leads to a modificati on of the iron distribution. The distribution of iron on the two coppe r sites, Cu(1) and Cu(2), was carefully checked by both methods, The t hermal dependence of the mean magnetic moments on the Cu(1) and Cu(2) sites was determined from Rietveld refinements. For x > 0.04, a collin ear phase (AF'2) is observed with a doubling of the cell along the c a xis and a ferromagnetic coupling between Cu(1) and Cu(2) sites. At low er iron content, several transitions are observed as the temperature d ecreases, from a paramagnetic to a collinear structure without any mom ent on the Cu(1) site, then to a canted structure and at low temperatu re the AF'2 structure is stabilized. (C) 1997 Elsevier Science B.V.