STRONG SHIFT OF THE IRREVERSIBILITY LINE IN HIGH-T-C SUPERCONDUCTORS UPON VORTEX SHAKING WITH AN OSCILLATING MAGNETIC-FIELD

Torque magnetometry is a powerful method for probing superconducting a nisotropy in the mixed state. In order to use the three-dimensional an isotropic London model to analyze torque data, the vortex lattice must be in a reversible state, a state normally restricted to a narrow ran ge close to the upper critical-field boundary H-c2(T) because of large pinning effects that set in at lower temperatures T. We show that the application of an additional oscillating magnetic field perpendicular to the main field B leads to a fast depinning of the vortex lattice. This vortex-shaking process dramatically extends the reversible domain in the (H,T) phase diagram, and thus the range in which torque invest igations can be made.