Co₂C Nanoparticle-Decorated Grain Boundaries: A Source of Robust, Thermally Stable Vortex Pinning in Bi-2223 High T_c Superconductors

In this study, we investigate the impact of cobalt carbide (Co 2 C) 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 Co 2 C (average particle size ∼40 nm), were analyzed. We identify two distinct Co 2 C 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 ( J c ) and pinning force ( F p ) as functions of the field and temperature. While the δ T c 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 Co 2 C clusters (Pin-I) are the source of robust grain-boundary pinning at low fields, while at higher fields, collective pinning from Co 2 C speckles (Pin-II) becomes significant. We find the average pinning potentials due to the magnetic Co 2 C 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 ( L p ) due to Co 2 C. The strong pinning force range due to magnetic Co 2 C 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 Co 2 C-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.