Magnetic flux distribution and hysteresis properties of bulk high temperature ferromagnet / superconductor / ferromagnet hybrid structures

Bulk (RE)BCO materials can be used potentially as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings where their large flux density / volume ratio makes them more efficient than traditional ferromagnetic permanent magnets. However, the magnetic flux distribution above a bulk HTS magnet is strongly non-uniform (conic profile predicted by the Bean model) and influenced by the finite length of the superconductor (curvature of the flux lines through the volume of the superconductor).

In the present work, we study how ferromagnetic materials can be combined with a bulk, large grain YBCO superconductor to improve the average value of the magnetic induction through the volume of the superconductor and to shape the magnetic induction at the surface.

Various pieces of different shapes were machined in two high permeability ferromagnetic alloys with different saturation magnetizations (0.8 T and 1.4 T). These pieces were placed on (i) the top surface of the bulk HTS cylinder to form bulk ferromagnet / superconductor (F/S) hybrid structures and (ii) the top and bottom surfaces to form bulk F/S/F structures. The magnetic properties of each hybrid structure subjected to an axial magnetic field were measured at 77 K. Pick-up coils were used to measure the average magnetic induction inside the superconductor and miniature Hall probes were used to map the remanent induction near the top and bottom surfaces of the assemblies.

The modifications of the hysteresis curves and flux distributions were examined taking into account that flux lines are trapped by the ferromagnetic component, which drives the return flux lines towards the outside of the superconductor. The superconductor hysteresis curve can be analysed as a combination of a diamagnetic and a ferromagnetic behaviour depending on the ferromagnet intrinsic properties (intrinsic permeability, saturation) and geometrical properties (size and volume). The bulk average remanent magnetization increases in presence of the ferromagnets (F/S/F structures give the best results) and is only slightly dependent on the saturation magnetization. When the ferromagnet is fully saturated, its saturation value dictates the magnetic behaviour of the hybrid structure which shows an addition of the superconductor and ferromagnet magnetic effects.