Understanding Enhanced Pinning Efficiency of 1D BaZrO₃ Artificial Pinning Centers by Ca Ion Diffusion into YBa₂Cu₃O_7-x Matrix

C-axis aligned BaZrO₃ nanorods are regarded as strong one-dimensional artificial pinning centers (1D-APCs) in YBa₂Cu₃O_7-x films. In this work, we report significantly improved pinning in 2-8 vol.% BZO-doped YBa₂Cu₃O₇ nanocomposite films made in multilayers (ML) consisting of three BZO/YBa₂Cu₃O_7-x layers stacking alternatively with two thin Ca-containing spacers to enable diffusion of Ca ions from the spacers to BZO/YBa₂Cu₃O₇ layers dynamically during the film growth. Evidence has shown that the diffused Ca ions induce Ca (30% larger)/Cu substitution on the Cu-O planes of tensile strained YBCO near the BZO/YBa₂Cu₃O₇ interface, resulting in stacking faults and elongation of the c-lattice constant and hence reduction of the BZO/YBa₂Cu₃O₇ lattice mismatch from 7.7% to 1.4%. This leads to a coherent BZO/YBCO interface and significantly enhanced critical current density J_c (B) by up to five-fold at magnetic field B=9.0 T//c-axis and 20-80 K in the ML samples as compared to their BZO/YBCO single-layer counterpart’s. Further studies on the effect of Ca diffusion have been carried out by varying the Ca-containing spacers thickness in the range of 1 nm-10 nm for control of the amount of the Ca source and the optimal thickness was found to be 5-10 nm. At fixed 10 nm thickness of the spacers, the BZO/YBCO thickness was varied in the range of 50-330 nm (total film thickness of 170-1000 nm) to probe the Ca diffusion range. A similar enhanced pinning was found achievable in BZO/YBa₂Cu₃O₇ ML films of all thicknesses. Interestingly, the thicker BZO/YBa₂Cu₃O₇ ML films outperform their thinner counterparts in both higher value and less anisotropy of J_c (B). This result illustrates strain-mediated Ca diffusion and substitution on YBCO may provide a facile method to engineer the BZO 1D-APC/YBa₂Cu₃O₇ interface for improved pinning.

Keywords: YBCO nanocomposite multilayer films, Ca ion diffusion, vortex pinning efficiency, interface engineering

Acknowledgements
This research was supported in part by US NSF (DMR-2413044 and ECCS-2314401), the US AFRL and AFOSR (LRIR #23RQCOR008, and #24RQCOR004), and the U.S. Office of Naval Research (ONR, N00014-22-1-2160), and the US NSF DMR-2016453 for TEM characterization.