Enhanced flux pinning of air-processed Gd123 by doping ZrO₂ nanoparticles

Nano-sized centers for magnetic flux pinning were successfully introduced into a bulk superconductor GdBa₂Cu₃O_7-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 (J_c) 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, J_c 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/B_irr < 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.