Magnetic Relaxation, Flux Pinning and Critical Currents in Superconductors.

A systematic study of the magnetic flux pinning properties in superconductors has been undertaken in an attempt to understand the differences between the flux creep behavior of classical superconductors and high-temperature superconductors (HTSC's). In HTSC's, the ratio of the effective flux pinning energy to the thermal energy, U _0/{kT}, is much smaller than that of conventional superconductors, often approaching unity. This results in much larger creep rates in HTSC's than in conventional superconductors. It is necessary to find suitable models that describe flux creep in both classical superconductors (i.e., Nb-Ti) and HTSC's (i.e., Tl_2Ba_2Ca _2Cu_3O _{10}). Results show that while these two classes of materials are quantitatively very different, a single pinning barrier model adequately describes both, within the proper region of the H-T plane. The model is applied to a variety of superconductors and the results are contrasted. Although the H-T plane appears to be very different for HTSC's than for conventional superconductors, qualitatively the same physics describes both. In HTSC's, near the upper critical field there exists a relatively wide region of superconducting fluctuations, followed successively by regions of thermodynamic reversibility, thermally assisted flux flow (TAFF), flux creep, and finally a rigid flux lattice where little, if any, motion of the flux lattice occurs. All of these regions are also present in conventional superconductors, but are often much more difficult, especially the irreversibility transition and the fluctuation region. The central finding of the flux creep analysis is that the region of flux creep is defined as a band in the H-T plane in which 2 <= U_0/ {kT} <= 100, and that the flux creep model applies best within this band. Neither the single barrier model or a model utilizing a distribution of pinning energies is found to be adequate the entire H-T plane, which is not surprising because the physics of each region are very different.