Flux pinning
Encyclopedia
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 superconductor
s, 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).
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.
Magnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...
do not move (become trapped, or "pinned") in spite of the Lorentz force
Lorentz force
In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...
acting on them inside a current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
-carrying
Type II superconductor. The phenomenon cannot occur in Type I superconductor
Type I superconductor
Superconductors cannot be penetrated by magnetic flux lines . This Meissner state breaks down when the applied magnetic field is too large. Superconductors can be divided into two classes according to how this breakdown occurs...
s, since these cannot be penetrated by magnetic fields (Meissner–Ochsenfeld effect
Meissner effect
The Meissner effect is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state. The German physicists Walther Meissner and Robert Ochsenfeld discovered the phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin...
). Flux pinning is only possible when there are defects in the crystalline structure
Crystallographic defect
Crystalline solids exhibit a periodic crystal structure. The positions of atoms or molecules occur on repeating fixed distances, determined by the unit cell parameters. However, the arrangement of atom or molecules in most crystalline materials is not perfect...
of the superconductor (usually resulting from grain boundaries
Crystallite
Crystallites are small, often microscopic crystals that, held together through highly defective boundaries, constitute a polycrystalline solid. Metallurgists often refer to crystallites as grains.- Details :...
or impurities).
Importance of flux pinning
Flux pinning is desirable in high-temperature ceramicCeramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
superconductors to prevent "flux creep", which can create a pseudo-resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
and depress both critical current density
Current density
Current density is a measure of the density of flow of a conserved charge. Usually the charge is the electric charge, in which case the associated current density is the electric current per unit area of cross section, but the term current density can also be applied to other conserved...
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
SQUID
A SQUID is a very sensitive magnetometer used to measure extremely weak magnetic fields, based on superconducting loops containing Josephson junctions....
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.
See also
- Flux pumpingFlux pumpingFlux pumping is a method for magnetising bulk superconductors to fields in excess of 15 teslas. The method can be applied to any type II superconductor and exploits a fundamental property of superconductors. That is their ability to support and maintain currents on the length scale of the...
- Meissner effectMeissner effectThe Meissner effect is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state. The German physicists Walther Meissner and Robert Ochsenfeld discovered the phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin...
- SuperconductivitySuperconductivitySuperconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
- Type II superconductor
- Abrikosov vortexAbrikosov vortexIn superconductivity, an Abrikosov vortex is a vortex of supercurrent in a type-II superconductor. The supercurrent circulates around the normal core of the vortex. The core has a size \sim\xi — the superconducting coherence length...
- Pinning forcePinning forcePinning force is a force acting on a pinned object from a pinning center. In solid state physics, this most often refers to the vortex pinning, the pinning of the magnetic vortices by different kinds of the defects in a type II superconductor...
Other sources
- Flux-Pinning of Bi2Sr2CaCu2O(8 + Delta) High Tc Superconducting Tapes Utilizing (Sr,Ca)14Cu24O(41 + Delta) and Sr2CaAl2O6 Defects (T. Haugan; et al. AFB OH Propulsion Directorate. Air Force Research Lab Wright-Patterson. Oct 2003)