Spontaneous and persistent rotation of cylindrical magnets levitated by Y-Ba-Cu-O superconductors

Abstract

When a cylindrical permanent magnet is levitated with a superconductor under certain conditions, it will rotate spontaneously and persist for an indefinite period of time. The results of this study elucidating the nature of this behavior are presented.

Spontaneous spinning of a magnet levitating over a superconductor

“…In agreement with previous work [2,3] we find that the tendency to spinning is strongest when the magnet is situated in a region of large temperature gradient. In the setup of Fig.…”

“…We have found this by experimentation with 8 different magnets, some with a high degree of homogeneity and some with less. The torque that makes the magnet oscillate and spin is to lowest order proportional to the magnet's angular velocity: if the magnet is initially at rest with zero angular velocity it is in unstable equilibrium, and any small perturbation will initiate the motion .In agreement with previous work [2,3] we find that the tendency to spinning is strongest when the magnet is situated in a region of large temperature gradient. In the setup of Fig.…”

“…We argue that it is a necessary consequence of the existence of a temperature gradient in the region where the magnet is and of the fact that the magnetization of the N d 2 F e 14 B magnet is a decreasing function of temperature in the temperature range where the magnet is, as discussed in Refs. [2,3]. Qualitatively it works as follows: the superconductor exerts a force on each element of the magnet that is an increasing function of its magnetization.…”

Superconducting bearings

“…A phenomenon related to bearing dynamics is the possibility for self-oscillation that may lead to rotation of a PM disc initially levitated without motion over a HTS [29,139,140]. The cause of the phenomenon appears to be a temperature gradient across the PM, in which the temperature dependence of the magnetization causes the centre of levitation force to reside below the centre of gravity, a situation that is mechanically unstable.…”

The dynamics of a levitated cylindrical permanent magnet above a superconductor

“…Experiments show that the cylindrical magnet starts to oscillate around its horizontal axis with increasing amplitude and will eventually rotate continuously with a nearly constant angular speed. The effect was explained independently in [5], [7], and [10] (see also [9]). The energy for the rotation is due to temperature differences of the surrounding air.…”