Irreversibility fields of superconducting niobium alloys

The irreversibility line of superconductors is most usually established fro m magnetization curves. However, many low-T-c materials show extremely reve rsible magnetization curves, while still having a finite critical current. Confirmation of a reversibility line requires other measurements. We have m ade measurements of de magnetization, ac susceptibility, and magnetoresisti vity as a function of applied held and temperature on Nb ahoy samples in or der to investigate the irreversibility line in low-T-c superconductors. The results show that there exists an observable field region below the mean-f ield critical field B-c2 where the magnetization is reversible during a cyc le of increasing and decreasing field, which is in agreement with a previou s report by Suenaga et al. In addition to de magnetization, ac susceptibili ty and magnetoresistivity measurements were also carried out on the same sa mple as alternative techniques to probe the irreversibility line to determi ne the best way of distinguishing a genuine thermally activated reversibili ty from a finite, but low, critical current density. The results showed tha t the collapse of the de magnetic hysteresis, the onset of the diamagnetic ac susceptibility (or the peak of the ac loss) and the zero resistance occu r at nearly the same field, namely, the irreversibility field Bi,. These ex perimental observations indicate that the irreversibility line is not uniqu e to high-ir, oxides but also exists in conventional superconducting metall ic alloys although much closer to B-c2. However, it is difficult to reconci le these results with measurements on other low-T-c materials which show ze ro resistance up to the surface critical field B-c3.