Microstructure and field angle dependence of critical current densities in REBa ₂Cu ₃O _y thin films prepared by PLD method

A c-axis oriented epitaxial REBa ₂Cu ₃O _y (RE123) thin film performs excellent superconducting properties in magnetic field. Recently, we reported that a critical current density (J _c) in the RE123 thin film was improved by a deliberate composition control. A Sm _1+xBa _2-xCu ₃O _y (Sm123) thin film with a small amount of Sm/Ba substitution x showed the great J _c ∼ 0.17 MA/cm ² in 5 T at 77.3 K, while the J _c in (Yb _1-zNd _z)Ba ₂Cu ₃O _y (Yb/Nd123) thin films depended on an amount of mixed crystal ratio z. In this report, we studied J _c as a function of magnetic field and field orientation with respect to ab-planes. In low magnetic field, the J _c of Sm123 thin film was almost independent of the applied angle of the field. In the case of Yb/Nd123 thin film, extremely high J _c were observed when the magnetic field was aligned parallel or perpendicular to the surface of the film. Because compositional fluctuations in RE123 thin films were observed by transmission electron microscopy, we found that the pinning centers in RE123 thin films are strongly affected by the composition in the thin films.