Sn-Pb solders have a phase-separated microstructure consisting of Sn and Pb. While both elements (T_c = 3.2 K for Sn and 7.2 K for Pb) are type-I superconductors, the alloy exhibits flux-trapping behavior. Recent studies have revealed this flux trapping occurs because magnetic flux is captured within the Sn regions by supercurrents in surrounding Pb regions [1]. This phenomenon affects the low-temperature physical property measurements [2].

In this study, we investigate the magnetic-field hysteresis effect on the transport critical current (I_c) of the Sn-Pb solder wire at T = 4.2 K. I_c was measured using a standard four-probe method under several magnetic fields of H = 300, 400, 500, and 600 Oe after zero-field cooling (ZFC) and field cooling (FC) at 3000 Oe. A clear enhancement of I_c and sharper V-I transitions were observed at H = 300 and 400 Oe after FC. For example, at H = 300 Oe, I_c for the FC sample reached 127.7 A, which is notably higher than I_c = 107.8 A for the ZFC sample. At H = 500 and 600 Oe, V-I curves converge with negligible differences in I_c.

Our results can be explained by the stability of fluxes strongly-pinned through the Sn regions and mobile fluxes present in intermediate states. These findings suggest the potential use of this phenomenon for achieving a superconducting device like a superconductor diode using phase-separated alloys.

[1] H. Arima et al., Commun. Mater. 5, 34 (2024).

[2] T. Ichikawa et al., Jpn. J. Appl. Phys. 63, 100906 (2024).