Structural and magnetic properties of Fe impurities in YBa2Cu3O7−δ

Abstract Samples having the general composition YBa 2 (Cu 1−x Fe x ) 3 O 7−δ have been investigated using x-ray and neutron diffraction, Mossbauer spectroscopy and bulk property measurements. X-ray and neutron data show that samples having x ≥ 0.04 are tetragonal. Neutron diffraction shows that this structure is very similar to that found for undoped YBa 2 Cu 3 O 7−δ when the oxygen concentration is low, although, in the present case, the total oxygen content is found to be ≥7 for all concentrations. Contrary to the common statement that Fe substitutes onto the Cu sites, it is found from neutron diffraction that the Cu1 site is not occupied, and that Fe appears to have a substantial substitution for Ba. Both Mossbauer spectroscopy and heat capacity measurements show the presence of magnetic transitions at temperatures well below the superconducting transition temperature, with evidence for a large variation in magnetic transition temperatures.