Magnetic relaxation of type-II superconductors in a mixed state of entrapped and shielded flux

Abstract

The magnetic relaxation has been investigated in type-II superconductors when the initial magnetic state is realized with entrapped and shielded flux contemporarily. This flux state is produced by an inversion in the magnetic field ramp rate due to, for example, a magnetic field overshoot or undershoot. The investigation has been faced both numerically and by measuring the magnetic relaxation in BSCCO tapes. Numerical computations have been performed in the case of an infinite thick strip and of an infinite slab, showing a quickly relaxing magnetization in the first seconds. As verified experimentally, the effects of the overshoot (or the undershoot) cannot be neglected simply by cutting the first $10--100\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ in the magnetic relaxation. On the other hand, at very long times, the magnetic states relax toward those corresponding to field profiles with only shielded flux or only entrapped flux, depending on the amplitude of the field change with respect to the full penetration field of the considered superconducting samples. In addition, we have performed numerical simulations in order to reproduce the relaxation curves measured on the BSCCO(2223) tapes; this allowed us to interpret correctly also the first seconds of the $M(t)$ curves.