Magnetic Flux Pinning
Superconductors are materials, often ceramics, which possess the unusual property that their electrical resistance vanishes below a critical temperature. Magnetic fields near a superconductor excite exactly opposed currents by induction, and these supercurrents repel the applied magnetic field (a phenomenon called the Meissner Effect).
However, when a sufficiently strong magnetic field is applied to a Type II supercondcutor (our lab favorite is yttrium barium copper oxide, or YBCO), it penetrates the superconductor at the locations of impurities in the crystal lattice. The magnetic flux lines end up "pinned" on these impurities in such a way that the magnetic field source feels a macroscopic force - in fact, the interaction is very similar (for small displacements and motions) to familiar spring and damping forces.
The equilibrium position of the "virtual spring" is determined by the initial position of the magnet when the superconductor first cools below its critical temperature. One of the more dramatic demonstrations of these effects - the restoring force and initial equilibrium - is the stable levitation of a magnet against gravity. No combination of magnetic fields, electric fields, and gravitational fields can produce such a stable equilibrium: the free body will eventually be sucked in or repelled unless the system has active control. However, flux pinning does not require any power or control to maintain the equilibrium - the effect relies only on material properties of the magnet and superconductor.
You can read more about this and other research at www.spacecraftresearch.com
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