Magnetic Levitation: Materials and Processes
Magnetic levitation is one of the most outstanding properties of superconductors, which in addition has launched the development of several electromechanical systems. The simplest experiment consists of a single permanent magnet levitating in a stable position over a superconducting pellet refrigerated with liquid nitrogen.
It is indeed the characteristic of stable levitation that gives rise to great interest in devices such as magnetic bearings, rotating machinery, linear transport systems, etc.
The axial vertical force of a permanent magnet over a superconductor stems from the flux exclusion associated with materials with high critical currents (Moon 1994). In this case, the following can be written:
where M is the magnetization of the superconductor, B ext is the magnetic field generated by the permanent magnet, and the integral is extended over the whole volume of the superconductor.
Therefore a body may magnetically levitate if the vertical gradient of the magnetic induction displays a positive F z component, which eventually overcomes the weight of the permanent magnet. From the superconducting material point of view, the generation of a large magnetization requires current loops shielding the magnetic field generated by the permanent magnet. According to the critical state model, a partial flux penetration occurs in which the flux gradient obeys the Ampere law rot B = 0 J c, where J c is the critical current of the superconductor. Hence, the magnetization of the superconductor increases only if J c or...