Enhancing magnetic pinning by BaZrO₃ nanorods forming coherent interface by strain-directed Ca-doping in YBa₂Cu₃O_7−x nanocomposite films

BaZrO₃ (BZO) one-dimensional artificial pinning centers (1D-APCs) aligned along the c-axis of the YBa₂Cu₃O₇ (YBCO) have been adopted to enhance the magnetic vortex pinning in BZO/YBCO nanocomposite films. However, the pinning force density F _p of the BZO 1D-APCs remains moderate at temperatures near 77 K. A hypothesis of the major limiting factor is the defective BZO 1D-APCs/YBCO interface as a direct consequence of the large interfacial strain originated from the BZO/YBCO lattice mismatch of ∼7.7%. Herein, we explore enlarging the c-axis of the YBCO dynamically to reduce the lattice mismatch and hence to prevent formation of the defective BZO 1D-APCs/YBCO interface. Specifically, the c-axis enlargement was achieved by partial replacement of Cu with Ca on the YBCO lattice using strain-directed Ca diffusion into YBCO from two Ca_0.3Y_0.7Ba₂Cu₃O_{7− x} (CaY-123) spacers of only 10 nm in thickness inserted into the 2 vol% BZO 1D-APC/YBCO nanocomposite thin films of ∼150 nm in total thickness. The achieved elongated c-axis is attributed to the formation of stacking faults induced by Ca-replacement of Cu on YBCO lattice. The reduced BZO/YBCO lattice mismatch allows formation of a coherent BZO 1D-APC/YBCO interface with negligible defects. This leads to an enhanced F _p value up to 98 GN m⁻³ at 65 K, which is 70% higher than that of the reference 2 vol% BZO 1D-APC/YBCO sample. Furthermore, the benefit of the enhanced pinning of the BZO 1D-APCs with a coherent interface with YBCO can be extended to a large angular range of the magnetic field orientation. This study reveals the significant effect of the BZO/YBCO interface on the pinning efficiency of BZO 1D-APCs and provides a promising approach to achieve a coherent interface in BZO/YBCO nanocomposite films. (paper)