Damping of a levitating magnetic microsphere by vortex motion in YBCO

The damping of the oscillations of a small magnet (spherical shape, radius 0.1 mm) levitating between two parallel YBCO surfaces is measured as a func tion of oscillation amplitude and temperature. The losses in the samples (e pitaxial thin films, bulk granular and bulk melt-textured) are analyzed in terms of oscillating shielding currents flowing through trapped flux lines. We find dissipation from different mechanisms of flux dynamics. At small a mplitudes there is a linear regime described by a surface resistance varyin g 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 reg imes are observed, firstly collective pinning with diverging energy barrier s, secondly in bulk samples above 85 K hysteretic damping, and finally in t hin films exponentially large losses which can be described by pinning ener gies vanishing linearly at large currents. (C) 2000 Elsevier Science B.V, A ll rights reserved.