Apparent anisotropy effects of upper critical field in high- textured superconducting Nb-Ti tapes

Inhomogeneity and irreversibility field of superconducting Nb-Ti tapes

“…Thus, in a range of sufficiently high EPJ Web of Conferences 185, 08004 2018https://doi.org/10.1051/epjconf/201818508004 MISM 2017 magnetic fields, it is to be expected that the grain bodies will transform into a normal state, while the grain boundary will still be superconducting. Indeed, this division into two superconducting components with different H c2 has been observed experimentally earlier: through the features of the appearance of the satellite electric field [9], and through the apparent anisotropy of the upper critical field [10].…”

“…The fact that the irreversibility field has a clear correlation with the extrapolation of the volume pinning force, what was also observed in [6], hints at the fact that both characteristic fields are a manifestation of a single physical phenomenon. It was shown in [9,10] that in the fields above H F only the grain boundaries remain in superconducting state, while the grain bodies become normal. This mechanism gives a qualitative explanation for such dramatic decrease in the critical current and the appearance of reversible magnetization, since superconducting currents are mainly caused not by pinning, but by reversible surface currents.…”

“…The irreversibility field has been investigated on a coldrolled Nb-Ti tape. Electrodynamics and the structure of this tape have been studied in details by our group earlier [8][9][10]. This is a single-phase tape of the Nb-50%wtTi alloy, with the pinning due to the reduction of the mean free path of the electron at the grain boundaries.…”

“…Transport measurements were carried out in the same superconducting solenoid. Flat samples, adhered to dielectric corundum substrate, with bridge geometry along the rolling direction were made by laser cutting (the bridge was 1.0 mm in width and 6.7 mm in length) by analogy with [10]. In all experiments, the field was perpendicular to the plane of the tapes, so the pinning force was determined as a simple product of the field and the critical current density, determined by the criterion 1 μV/cm.…”

“…The upper critical field (H c2 = 11.6T) has been determined from measurements of the resistivity with an exciting current of 5 mA. It was shown in [10] that a decrease in the current below does not lead to a significant shift in H c2 determined in the middle of the transition. An alternative way to determine upper critical field [6] by the intersection of the linear extrapolation of the resistive transition through the baseline voltage gives value H c2 =11.4 T. Thus, the difference between H* and H c2 is about 0.9 T.…”