Levitation force from high-T_csuperconducting thin-film disks

Experimental studies and theoretical modeling of the levitation force between a permanent magnet and superconducting thin film are reported. Measurements of the force F_zand magnetic stiffness κ_z=δF_z/δ_z as functions of the magnet-superconductor separation z, show several features contrasting all previous levitation force data for bulk superconductors. In particular, the F_z(z) curves measured for decreasing and increasing separation form hysteresis loops of nearly symmetrical shape, also displaying a peak in the repulsive force branch. Recent theories for flux penetration in thin type-II superconductors in transverse magnetic fields are invoked to explain the results, which were obtained using a cylindrical Nd-Fe-B magnet and a YBa₂Cu₃O_7-δcircular disk made by laser ablation. We derive explicit formulas for both F_zand κ_z, reproducing quantitatively all the features seen experimentally.