Embedded Magnetism in YBa₂Cu₃O₇ Associated with Cu–O Vacancies within Nanoscale Intergrowths: Implications for Superconducting Current Performance

Stacking faults are ubiquitous structural defects in YBa2Cu3O7 high-Tc superconductors. Far from being detrimental, the control of the defect landscape, i.e., the length, density, and distribution of YBa2Cu4Oy (Y124) intergrowths, changing the nanostrain of the material can be used to enhance the vortex pinning properties of the superconductor, as required for applications. Recently, the use of advanced scanning transmission electronic microscopy has unveiled the presence of complex point nanodefects (clusters of Cu and O vacancies) within Y124, with an associated magnetic moment predicted by density functional calculations. In this work, we investigate the nature of defect-driven embedded magnetism in YBCO samples (thin films, nanocomposites, and a single crystal) containing different distributions of Y124 intergrowths using X-ray magnetic circular dichroism (XMCD), combined with scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The dichroic Cu signal correlates with the density of Y124 intergrowths, demonstrating the defect-driven origin of the embedded magnetism. Remarkably, a Cu XMCD signal is observed even in the single crystal, associated with the small presence of defects at the surface. Our work thus demonstrates the ubiquity of embedded magnetism in YBCO associated with faulted Y124 defects.