Magnetic levitation with high-Tc 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 YBa2Cu3O7-delta disk with common cylinder axis. Precise measurements were made of the vertical force F-z and the magnetic stiffness kappa(z) as a function of the magnet-superconductor separation at 77 K. Several features contrasting the levitation force produced using bulk superconductors were observed. Thin films produced very high values for F-z and kappa(z) per unit volume of superconducting material. The hysteretic behavior of F-z during decreasing and increasing separation resulted in loops of nearly symmetrical shape, which also contain a peak in the repulsive force branch. The observations are analyzed and explained with good quantitative agreement using recent theories for flux penetration in thin superconductors in transverse magnetic fields.