Annealing Effect on Structure and Superconductivity in FeSr2YCu2O6+δ Magnetic Superconductor

The FeSr2YCu2O6+δ compound with the tetragonal Ba2YCu3O6+δ-type structure exhibits superconductivity around 60 K and antiferromagnetism around 20 K, only after the reduction annealing and subsequent oxidization annealing. The reduction annealing causes the transition from the disorder to the order of Cu and Fe perpendicularly to the CuO2 sheet, and the oxidization annealing causes the supply of oxygen onto the oxygen-deficient FeOδ sheet. Although the onset Tc is almost independent of the reduction- annealing temperature, Ta, the zero-resistance Tc is dependent on Ta. The compound reduction-annealed at 780oC has the highest zero-resistance Tc value. The neutron diffraction study indicates that the reduction annealing at 780oC causes the highest degree of order of Cu and Fe while the oxygen content, 6+δ, is independent of of Ta. The subtle difference of order of Cu and Fe affects the electrical conductivity because of much lower critical current density than other high-Tc superconductors.