Suspension of a field-cooled BiPbSrCaCuO high-Tc superconductor under a toroidal permanent magnet

Magnetic flux measurements of a toroidal magnet revealed a concave-shaped field distribution with a single minimum and a null field along the axis of the torus at the point where the field reversed. The non-linear magnetic field of the toroidal magnet perpendicular to the Ag2O-doped superconducting disc sample with trapped magnetic flux distorted the field line distribution. As a result, the interaction force between the magnet and the sample exhibited regions of repulsive, null, attractive, null and finally repulsive force. The asymmetrical concave-shaped force pattern along the axis with two null force points indicates that the force exerted on the sample changes direction, the transition from repulsive to attractive at the null force point, and the force becomes repulsive again beyond the second null force point as the distance along the axis increases. The magnetic field simulation using the Poisson numerical code for the toroidal magnet of 46 mm OD, 12 mm ID and 10 mm thickness was in close agreement with the force measurements. The lateral stability of a suspended sample under the toroidal magnet is provided by the characteristic symmetrical nature of the field line with respect to the axis of the magnet. The concave-shaped magnetic field forms a vertical magnetic wall around a suspended superconducting sample as demonstrated with lead, a type 1 superconductor. Thus, the toroidal magnet with a concave-shaped magnetic field distribution with respect to the mid-plane of the magnet provides much improved lateral stability for the magnetic bearing. Furthermore, this arrangement provides a loss-free shielding current in the absence of viscous drag from the environment or eddy current or hysteresis. The magnetic moment of an undoped and 2% Ag2O-doped samples was shown to be m = 0.043 emu and 0.06 emu, respectively. The measured suspension force exerted on the doped sample agreed well with that calculated from the magnetostatic force distribution.