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 YBa₂Cu₃O_7– disk with common cylinder axis. Precise measurements were made of the vertical force F _z and the magnetic stiffness _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 _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.