Co<sub>2</sub>C\nNanoparticle-Decorated Grain Boundaries:\nA Source of Robust, Thermally Stable Vortex Pinning in Bi-2223 High <i>T</i><sub>c</sub> Superconductors

In this study, we investigate the impact of cobalt carbide\n(Co<sub>2</sub>C) nanoparticle incorporation on the vortex pinning\nproperties\nof Bi-2223 high-temperature superconductors (HTSC). Three batches\nof Bi-2223 pellets, containing 0%, 0.05%, and 2% by weight of Co<sub>2</sub>C (average particle size ∼40 nm), were analyzed. We\nidentify two distinct Co<sub>2</sub>C pinning centers: larger intergranular\nclusters (Pin-I, ∼0.1 to 0.2 μm in size) and smaller\nintragranular speckles (Pin-II, ∼30 to 40 nm in size). By analyzing\nthe magnetization response, we extract the behavior of the critical\ncurrent density (<i>J</i><sub>c</sub>) and pinning force\n(<i>F</i><sub>p</sub>) as functions of the field and temperature.\nWhile the δ<i>T</i><sub>c</sub> pinning mechanism,\nintrinsic to Bi-2223, was observed, our analysis also revealed additional\nstronger pinning sources, which dominate at different magnetic field\nregimes. A Josephson-junction model showed that Co<sub>2</sub>C clusters\n(Pin-I) are the source of robust grain-boundary pinning at low fields,\nwhile at higher fields, collective pinning from Co<sub>2</sub>C speckles\n(Pin-II) becomes significant. We find the average pinning potentials\ndue to the magnetic Co<sub>2</sub>C to be in Pin-I at ∼3000\nmeV and in Pin-II at ∼200 meV. Furthermore, these potentials\nshow minimal thermal degradation, even at 77 K, thereby enhancing\nthe pinning performance of Bi-2223 in high-temperature environments.\nWe also estimate the range of the pinning force (<i>L</i><sub>p</sub>) due to Co<sub>2</sub>C. The strong pinning force range\ndue to magnetic Co<sub>2</sub>C particles is estimated to remain up\nto a few nanometers even at temperatures as high as 80 K. The superconducting\nferromagnetic properties of the Josephson-junctions at Co<sub>2</sub>C-decorated grain boundaries contribute to these robust magnetic\npinning features at high <i>T</i>. Our findings highlight\nthe potential of transition metal carbide–HTSC nanocomposites\nto enhance the performance of HTSC materials, particularly in applications\noperating at elevated liquid nitrogen temperatures.