Enhanced flux pinning in a high-T_c superconducting film by a ferromagnetic buffer layer

A YBa₂Cu₃O₇ (30 nm) superconductor layer and a YBa₂Cu₃O₇ (30 nm)/ Pr_0.67Sr_0.33MnO₃ (30 nm) superconducting/ferromagnetic bilayer were epitaxially grown on the single-crystal (100) SrTiO₃ substrates. Magnetic characterizations on both samples were very carefully performed over wide temperature and field ranges. The enhanced flux pinning by the ferromagnetic layer in the bilayer sample is clearly seen in the comparison of the temperature-dependent magnetization data obtained in the zero-field-cooled and field-cooled processes for both samples. The fact that the temperature-dependent remanent magnetization in the bilayer shows a sharper transition near the T_C than that in the YBCO single layer is also consistent with the picture of the enhanced flux pinning. The above observations are confirmed by the enhanced critical current density, J_c ∞ ΔM, where ΔM is the magnetic irreversibility in the hysteresis loops.