Flux pinning is the trapping of Abrikosov vortices by spatial variations in a superconductor, such as defects or regions where superconductivity is already weakened. A vortex can lower its energy by placing its suppressed-order core in such a region, producing a barrier to displacement.
This matters for transport because an electric current exerts a force on vortices. Moving vortices produce an electric field and hence dissipative electrical response; pinning resists that motion and supports useful superconducting critical current density. Pinning also explains magnetic hysteresis and trapped magnetic flux. Thermal activation can still permit slow motion through flux creep, while stronger drive can produce vortex depinning. Thus pinning improves practical current carrying capacity without making every finite-temperature relaxation rate exactly zero.
Flux pinning is a phenomenon observed in type-II superconductors where magnetic flux lines (or vortices) are "pinned" in place within the superconducting material. This occurs due to defects, impurities, or microstructures within the superconductor that impede the movement of these magnetic vortices. In type-II superconductors, when exposed to a magnetic field above a certain critical level, the material allows magnetic flux to penetrate in discrete packets known as flux vortices.
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