Flux pinning

Flux pinning is the phenomenon that magnetic flux lines do not move (become trapped, or "pinned") in spite of the Lorentz force acting on them inside a current-carrying Type II superconductor. The phenomenon cannot occur in Type I superconductors, since these cannot be penetrated by magnetic fields (Meißner-Ochsenfeld effect). Flux pinning is only possible when there are defects in the crystalline structure of the superconductor (usually resulting from grain boundaries or impurities).

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Importance of flux pinning

Flux pinning is desirable in high-temperature ceramic superconductors in order to prevent "flux creep", which can create a pseudo-resistance and depress both critical current density and critical field.

Degradation of a high-temperature superconductor's properties due to flux creep is a limiting factor constraining the use of these superconductors. SQUID magnetometers suffer reduced precision in a certain range of applied field due to flux creep in the superconducting magnet used to bias the sample, and the maximum field strength of high-temperature superconducting magnets is drastically reduced by the depression in critical field.

References

• Future Science introduction to high-temperature superconductors.
• American Magnetics tutorial on magnetic field exclusion and flux pinning in superconductors.