Flux pinning is a phenomenon that occurs when flux

vortices In fluid dynamics, a vortex (: vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...

in a

type-II superconductor In superconductivity, a type-II superconductor is a superconductor that exhibits an intermediate phase of mixed ordinary and superconducting properties at intermediate temperature and fields above the superconducting phases. It also features the ...

are prevented from moving within the bulk of the superconductor, so that the magnetic field lines are "pinned" to those locations. The superconductor must be a

because

type-I superconductor The interior of a bulk superconductor cannot be penetrated by a weak magnetic field, a phenomenon known as the Meissner effect. When the applied magnetic field becomes too large, superconductivity breaks down. Superconductors can be divided into ...

s cannot be penetrated by

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

s. Some type-I superconductors can experience the effects of flux pinning if they are thin enough. If the material's thickness is comparable to the

London penetration depth In superconductors, the London penetration depth (usually denoted as \lambda or \lambda_L) characterizes the distance to which a magnetic field penetrates into a superconductor and becomes equal to e^ times that of the magnetic field at the surface ...

, the magnetic field can pass through the material. The act of magnetic penetration is what makes flux pinning possible. At higher magnetic fields (above lower critical field but below upper critical field ) the superconductor allows magnetic flux to enter in quantized packets surrounded by a superconducting current vortex (see

Quantum vortex In physics, a quantum vortex represents a quantized flux circulation of some physical quantity. In most cases, quantum vortices are a type of topological defect exhibited in superfluids and superconductors. The existence of quantum vortices was ...

). These sites of penetration are known as

flux tube A flux tube is a generally tube-like (cylindrical) region of space containing a magnetic field, B, such that the cylindrical sides of the tube are everywhere parallel to the magnetic field lines. It is a graphical visual aid for visualizing a magn ...

s. The number of flux tubes per unit area is proportional to the magnetic field with a constant of proportionality equal to the

magnetic flux quantum The magnetic flux, represented by the symbol , threading some contour or loop is defined as the magnetic field multiplied by the loop area , i.e. . Both and can be arbitrary, meaning that the flux can be as well but increments of flux can be ...

. On a simple 76 millimeter diameter, 1-micrometer thick disk, next to a magnetic field of 28 kA/m, there are approximately 100 billion flux tubes that hold 70,000 times the superconductor's weight. At lower temperatures the flux tubes are pinned in place and cannot move. This pinning is what holds the superconductor in place thereby allowing it to levitate. This phenomenon is closely related to the

Meissner effect In condensed-matter physics, the Meissner effect (or Meißner–Ochsenfeld effect) is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state when it is cooled below the critical temperature. Th ...

, though with one crucial difference — the Meissner effect shields the superconductor from all magnetic fields causing repulsion, unlike the pinned state of the superconductor disk which pins flux, and the superconductor in place.

Importance

Flux pinning is desirable in high-temperature

ceramic A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...

superconductors to prevent "flux creep", which can create a pseudo- resistance and depress both critical

current density In electromagnetism, current density is the amount of charge per unit time that flows through a unit area of a chosen cross section. The current density vector is defined as a vector whose magnitude is the electric current per cross-sectional ...

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 A squid (: squid) is a mollusc with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also ...

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.

Potential applications

The worth of flux pinning is seen through many implementations such as lifts, frictionless joints, and transportation. The thinner the superconducting layer, the stronger the pinning that occurs when exposed to magnetic fields. Since the superconductor is pinned above the magnet away from any surfaces, there is the potential for a frictionless joint. Transportation is another area in which flux pinning technology could revolutionize and reform. ''MagSurf'' was developed by the

Paris Diderot University Paris Diderot University, also known as Paris 7 (), was a French university located in Paris, France. It was one of the inheritors of the historic University of Paris, which was split into 13 universities in 1970. Paris Diderot merged with Pari ...

utilizing flux pinning to create a

hoverboard A hoverboard (or hover board) is a fictional levitating board used for personal transportation, first described in science-fiction, and made famous by the appearance of a skateboard-like hoverboard in the film ''Back to the Future Part II''. Ma ...

-like effect that could transport a person, demonstrating the usefulness of the technology.PBS news 30 October 2013: http://www.mpq.univ-paris-diderot.fr/spip.php?article1709 The

Federal University of Rio de Janeiro The Federal University of Rio de Janeiro (, UFRJ) is a public university, public research university in Rio de Janeiro, Brazil. It is the largest federal university in the country and is one of the Brazilian centers of excellence in teaching and r ...

has also been developing a flux pinning-based

MagLev Maglev (derived from '' magnetic levitation'') is a system of rail transport whose rolling stock is levitated by electromagnets rather than rolled on wheels, eliminating rolling resistance. Compared to conventional railways, maglev trains h ...

system called Maglev Cobra, which aims for a smaller form factor than existing urban rail systems. There has also been some research into using the flux pinning effect to isolate vibrations for microdevices. The ability to fix the superconductor in space can be used as a damping device like a spring. This idea has proposed for isolating vibrations for parts in satellites.

References

Other sources

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

Stability of superconductors. *
Flux-Pinning of ''Bi₂Sr₂CaCu₂O_{(8 + Delta)}'' High T_c 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. October 2003)
Superconducting Magnetic Levitation (MagLev) on a Magnetic Track
Ithaca College Physics demonstration of the Meissner effect and flux pinning. {{Use dmy dates, date=February 2021 Magnetic levitation Superconductivity Quantum magnetism

Importance

Potential applications

See also

References

Other sources