Measurement of the magnetic hysteresis cycle of bulk superconductor / ferromagnet hybrids

Large grain, bulk YBa2Cu3O7-δ (YBCO) high temperature superconductors (HTS) can be potentially used as powerful permanent magnets for magnetic levitation. The magnetic flux distribution above a bulk HTS magnet, however, is strongly non-uniform compared to the rather flat distribution above a ferromagnet. In the present work, we study experimentally how cylindrical pieces of FeNi soft ferromagnetic alloys can be combined with a cylindrical, bulk, large grain YBCO superconductor to improve the distribution of the trapped field at the surface or its average value through the volume of the sample.
The magnetic properties of each hybrid structure are measured at 77 K under axial magnetic field up to 3 T. The remanent induction distribution near the top and bottom surfaces is determined by miniature Hall probe mapping. Pick-up coils wound around the superconductor are used to measure the average flux density inside the superconductor. This technique relies on low level voltage measurements (typically a few microvolts) and allows non-destructive measurement of the average magnetic hysteresis curve of the entire bulk superconductor.