EVIDENCE FOR COLLECTIVE FLUX PINNING FROM MAGNETIC-RELAXATION COMBINED WITH I-V-CHARACTERISTICS IN YBCO THIN-FILMS

Magnetic relaxation and current-voltage (I-V) characteristic measurements have been performed on YBa2Cu3O7-x thin films. The magnetic relaxation M(t) was measured on samples patterned into a well-defined ring geometry allowing I-V data to be calculated from the M(t) curves. Whereas the direct transport measurements give a typical electric field sensitivity of 10(-6) V/cm, the M(t) data lead to a sensitivity of better than 10(-14) V/cm. Although the conventional flux creep model might serve as a first approximation to the two sets of data separately, it is unable to describe the data combined to span ten decades in an electric field. Rather, the measurements are well described by the relation E = E0 exp[-U0/kT(J0/J)1/alpha], as predicted by the collective pinning theory and in the vortex glass model.