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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Flux pinning is desirable in high-temperature ceramic superconductors 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 in 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.