Vortex motion in superconducting YBa2Cu3O7-delta inferred from the dampingof the oscillations of a levitating magnetic microsphere

The damping of the oscillations of a small permanent magnet (spherical shap e, radius 0.1 mm) levitating between two parallel YBCO surfaces is measured as a function of oscillation amplitude and temperature. The losses in the samples (epitaxial thin films, bulk granular and bulk melt-textured) are an alyzed in terms of oscillating shielding currents flowing through trapped f lux lines whose motion gives rise to electric fields. We find dissipation t o originate from different mechanisms of flux dynamics. At small amplitudes there is a linear regime described by a surface resistance varying from 10 (-9) Omega for the bulk samples down to 10(-13) Omega for the thin films at low temperatures. With increasing amplitude various nonlinear regimes are observed, firstly collective pinning with diverging energy barriers, second ly in bulk samples above 85 K hysteric damping, and finally in thin films e xponentially large losses which can be described by pinning energies vanish ing linearly at large currents.