AMPLITUDE DEPENDENCE OF MAGNETIC STIFFNESS IN BULK HIGH-TEMPERATURE SUPERCONDUCTORS

Magnetic stiffness has been measured in several vibratory systems composed of a permanent magnet elastically suspended above a stationary high-temperature superconductor at 77 K. For Y-Ba-Cu-O, Ag/Y-Ba-Cu-O, (Pb-Bi)-Sr-Ca-Cu-O, and Tl-Ba-Ca-Cu-O sintered disks, both the vertical and horizontal magnetic stiffnesses increase strongly as the magnet's oscillation decays, down to amplitudes of 1-mu-m. Vertical stiffness is about twice as strong as horizontal stiffness, and both are much stronger than the gradient of the levitation force for large monotonic changes in magnet height over the superconductor. These results are not deducible from contemporary magnetization measurements, and elastic pinning of flux lines are believed to contribute significantly to the stiffness.