Nano-sized centers for magnetic flux pinning were successfully introduced into a bulk superconductor GdBa2Cu3O7-δ (Gd123) by mixing ZrO2 nanoparticles with precursor prior the melt growth in air. Superconducting properties and structure of doped Gd123 were studied by magnetization and transmission electron microscopy, respectively. The critical current density (Jc) and critical temperature indicate that the optimum addition amount is 0.2-0.8% ZrO2 in molar ratio to Gd123 as composition. With 0.4 mol % ZrO2 addition, Jc was increased to 100,000 A/cm2 at 77 K under self-field. The normalized pinning force deduced from the magnetization as a function of applied magnetic field shows single peak. Thus, the nano-sized ZrO2 addition enhanced the normal core pinning under low magnetic field, B/Birr < 0.2. Particles with <50 nm in diameter were observed and assigned as BaZrO3 particles locating inside single domain Gd123. The present result may assess that BaZrO3 nanoparticles or associated defects plays a role of pinning centers within a comparable range with the coherence length ξ.
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