Influence of defect-induced biaxial strain on flux pinning in thick YBa ₂Cu ₃O ₇ layers

This work reports a detailed structural study by synchrotron x-ray diffraction of several sets of thick YBa ₂Cu ₃O ₇ layers. The samples represent recent advances in flux-pinning design, containing various concentrations of artificial pinning centers: (i) BaZrO ₃ nanorods, (ii) BaZrO ₃ nanoparticles, and (iii) Y ₂O ₃ nanoparticles. A statistical analysis was performed in order to separate the effects of defect-induced and intrinsic pinning. We report a statistically significant correlation between the orthorhombic distortion of the YBCO matrix and the pinning strength. Our result implies that the in-plane ordering of oxygen ions in the chain positions accounts for approximately 60% of the pinning force. The strain-induced pinning mechanism analysis, based on the Eshelby model of elastically strained composites, predicts that small YBCO grain size is a critical component of a strong pinning architecture that can enable critical current density values approaching the depairing limit.