Irreversibility fields of superconducting niobium alloys

Abstract

The irreversibility line of superconductors is most usually established from magnetization curves. However, many low-Tc materials show extremely reversible magnetization curves, while still having a finite critical current. Confirmation of a reversibility line requires other measurements. We have made measurements of dc magnetization, ac susceptibility, and magnetoresistivity as a function of applied field and temperature on Nb alloy samples in order to investigate the irreversibility line in low-Tc superconductors. The results show that there exists an observable field region below the mean-field critical field Bc2, where the magnetization is reversible during a cycle of increasing and decreasing field, which is in agreement with a previous report by Suenaga et al. In addition to dc magnetization, ac susceptibility and magnetoresistivity measurements were also carried out on the same sample as alternative techniques to probe the irreversibility line to determine the best way of distinguishing a genuine thermally activated reversibility from a finite, but low, critical current density. The results showed that the collapse of the dc magnetic hysteresis, the onset of the diamagnetic ac susceptibility (or the peak of the ac loss) and the zero resistance occur at nearly the same field, namely, the irreversibility field Birr. These experimental observations indicate that the irreversibility line is not unique to high-Tc oxides but also exists in conventional superconducting metallic alloys although much closer to Bc2. However, it is difficult to reconcile these results with measurements on other low-Tc materials which show zero resistance up to the surface critical field Bc3.

Source-reported materials — not catalogue approval

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