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We report levitation force cycles resulting from measurements carried out on a YBCO and a MgB2 disk cooled down in the field of a permanent magnet. In both cases the amplitude of the levitation force tends toward maximum values as the temperature decreases. Otherwise, the cycles are almost closed at low temperature and strongly hysteretic in the high temperature range. The hysteresis of the force cycles is attributed to the distribution of the currents induced in the sample by the field of the magnet. The saturation of the levitation forces at low temperature is related to that of the magnetic moment of the disks. We show that this type of measurement allows for the determination of the critical current density of superconductors in a restricted domain of temperatures.

In contrast to the interaction between two magnets with opposite magnetization directions, the interaction between a permanent magnet and a superconductor can be stable and result in magnetic levitation. This property can be exploited for the development of high velocity rotating bearings with no mechanical contacts and for the development of levitated trains. In this review, we focus on this latter application. After a brief description of the other techniques developed for levitating trains and the resulting achievements, we describe the magnet-superconductor interaction and recall the achievements in this field. We then give insights into the properties of the employed magnets and arrangement of magnets and we detail the characteristics and the fabrication processes of the most frequently used superconductors. Focusing on physics, we detail the procedures generally used for measuring the vertical (levitation) and the lateral (guidance) forces in magnetic levitation and the results obtained from experiments. We detail and give a critical review of the various models proposed for reproducing the force measurements. In the conclusion we discuss the possible future developments of the technology.

Articles you may be interested inDependence of magnetoelectric properties on sintering temperature for nano-SiC-doped MgB2/Fe wires made by combined in situ/ex situ process J. Appl. Phys. 111, 07E135 (2012); 10.1063/1.3677660 Transport properties of superconducting MgB2 composites with carbon-encapsulated Fe nanospheresSpark Plasma Sintering is a promising rapid consolidation technique that allows a better understanding and manipulating of sintering kinetics and therefore makes it possible to obtain MgB 2 -based ceramics with tailored microstructures. Commercial MgB 2 powders were spark plasma sintered with an applied mechanical pressure, leading to MgB 2 pellets with 99% relative density. The obtained samples show a sharp superconducting transition with an onset at 38.5 K. The critical current density was measured equal to 5.6 Â 10 4 A/cm 2 at 20 K under 1 T applied field. Otherwise, the trapped field was also investigated. A 5 mm thick pellet with a 20 mm diameter prepared with optimized processing temperature demonstrated a trapped field of 1.2 T at 15 K and up to 2.5 T at 10 K. V C 2014 AIP Publishing LLC. [http://dx.

The transport properties of superconductive YBa2Cu3O7−δ (YBCO) twinned single crystals and epitaxial thin films are similar in the flux creep regime. In particular, the activation energy of the individual vortices is proportionnal to the temperature in the low temperature range. As a matter of fact, data from the literature show that this energy takes the form Ea=4πνkBT, where ν is an integer. To investigate this behavior, the current–voltage characteristics of epitaxial YBCO thin film microbridges were measured. The activation energy of these samples has proved to take the form given above. This common behavior of single crystals and thin films is interpreted in the framework of a critical state model according to which the vortices move along the sample twin boundaries that behave as arrays of weak links.

Articles you may be interested inInfluence of the ferromagnetic layer on the pair breaking and low alternating current field magnetic response in superconductor/ferromagnet bilayer

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Applications of bulk superconductors concern superconducting motors and generators, the levitation of vehicles, the generation of high magnetic fields with small size cryo-magnets, the shielding of magnetic fields and other applications. For all of them, it is essential to determine the critical current density, and to understand the effect of the shape and size of the bulks on the properties of interest. In this contribution, we show how the combination of levitation force and trapped field measurements allow one to determine the characteristics and the potential performances of superconducting disks using analytical modeling. As examples of applications we detail the effects of the magnetizing field and of the bulk sheet critical current density on the levitation force. An important result of the reported measurements is that in field-cooled samples, the shielding currents possibly do not flow along the whole thickness of the disks.