Co₂C Nanoparticle-Decorated Grain Boundaries: A Source of Robust, Thermally Stable Vortex Pinning in Bi-2223 High <i>T</i>_c Superconductors

In this study, we investigate the impact of cobalt carbide (Co2C) nanoparticle incorporation on the vortex pinning properties of Bi-2223 high-temperature superconductors (HTSC). Three batches of Bi-2223 pellets, containing 0%, 0.05%, and 2% by weight of Co2C (average particle size ∼40 nm), were analyzed. We identify two distinct Co2C pinning centers: larger intergranular clusters (Pin-I, ∼0.1 to 0.2 μm in size) and smaller intragranular speckles (Pin-II, ∼30 to 40 nm in size). By analyzing the magnetization response, we extract the behavior of the critical current density (Jc) and pinning force (Fp) as functions of the field and temperature. While the δTc pinning mechanism, intrinsic to Bi-2223, was observed, our analysis also revealed additional stronger pinning sources, which dominate at different magnetic field regimes. A Josephson-junction model showed that Co2C clusters (Pin-I) are the source of robust grain-boundary pinning at low fields, while at higher fields, collective pinning from Co2C speckles (Pin-II) becomes significant. We find the average pinning potentials due to the magnetic Co2C to be in Pin-I at ∼3000 meV and in Pin-II at ∼200 meV. Furthermore, these potentials show minimal thermal degradation, even at 77 K, thereby enhancing the pinning performance of Bi-2223 in high-temperature environments. We also estimate the range of the pinning force (Lp) due to Co2C. The strong pinning force range due to magnetic Co2C particles is estimated to remain up to a few nanometers even at temperatures as high as 80 K. The superconducting ferromagnetic properties of the Josephson-junctions at Co2C-decorated grain boundaries contribute to these robust magnetic pinning features at high T. Our findings highlight the potential of transition metal carbide–HTSC nanocomposites to enhance the performance of HTSC materials, particularly in applications operating at elevated liquid nitrogen temperatures.