Flux line depinning in a magnet-superconductor levitation system

The AC loss characteristics of a magnet-superconductor system were studied with the magnet fixed to the free end of an oscillating cantilever located near a stationary melt-textured YBCO pellet. Below a threshold AC field amp litude approximate to2 Oe, the dissipation of the oscillator is amplitude-i ndependent, characteristic of a linear, non-hysteretic regime. Above thresh old, dissipation increases with amplitude, reflecting the depinning and hys teretic motion of flux lines. The threshold AC field is an order of magnitu de higher than that measured for the same YBCO material via AC susceptometr y in a uniform DC magnetic field. A partial lock-in of flux lines between Y BCO ab planes is proposed as the mechanism for the substantial increase of the depinning threshold.