Abstract Body

Ba_1-xK_xFe₂As₂ superconductors have strong potential for magnet applications through their very high upper critical field, relatively high superconducting transition temperature and manufacturability through the powder-in-tube (PIT) route. However, the critical current density in PIT tapes is still low compared to the incumbent technologies, so a greater understanding of the limiting factors is required.

We have measured in-field critical currents (I_c) of stainless steel and silver double-sheathed monofilament Ba_0.6K_0.4Fe₂As₂ superconductor tapes at elevated temperatures from 15 K to 35 K. At 20 K the critical current density is up to 140 kA/cm² in low (optimal) field and 22 kA/cm² in 8 T.

In the low-field region we observe an anomalous and sharp suppression of I_c centred at zero field. This feature is non-hysteretic for lower temperatures and perpendicular field, but becomes hysteretic for higher temperatures in perpendicular field and all temperatures in parallel field. The low-field suppression is reflected also in the n-values which can otherwise be very high, in excess of 100, in optimal field.

Magnetic-field hysteresis of I_c is generally attributed to flux exclusion / flux trapping in granular superconductors and this is likely to be the case also in the present conductors. The origin of the low-field I_c anomaly is not yet clear, though it may relate to magnetism in the sheath or superconducting material.

Acknowledgment

This work was supported by the New Zealand Ministry of Business, Innovation and Employment (MBIE) Strategic Science Investment Fund “Advanced Energy Technology Platforms” under contract No. RTVU2004.