MAGNETIC-LEVITATION WITH HIGH-T-C SUPERCONDUCTING THIN-FILMS

The levitation force between a permanent magnet and a superconducting thin film was investigated experimentally. The configuration consisted of a cylindrical NdFeB permanent magnet placed above a circular YBa2C u3O7-delta disk with common cylinder axis. Precise measurements were m ade of the vertical force F-z and the magnetic stiffness kappa(z) as a function of the magnet-superconductor separation at 77 K. Several fea tures contrasting the levitation force produced using bulk superconduc tors were observed. Thin films produced very high values for F-z and k appa(z) per unit volume of superconducting material. The hysteretic be havior of F-z during decreasing and increasing separation resulted in loops of nearly symmetrical shape, which also contain a peak in the re pulsive force branch. The observations are analyzed and explained with good quantitative agreement using recent theories for flux penetratio n in thin superconductors in transverse magnetic fields.