AC susceptibility and magnetization of Nb-tube processed Nb3Al composite wires

AC susceptibility in dc-background fields of 0 - 6 T and M - H hysteresis loop at 4.2 - 16 K have been measured on Nb-tube processed Nb3Al wires. The wires consist of triple stacking of elementary composites and contain 1.8 million Al-alloy cores before reaction. It has been revealed that the superconductivity measured involves contributions from 3 different areas of the wires, (1) the outermost Nb tube, (2) the middle stack Nb tubes, and (3) the innermost stack Nb tubes containing the Nb3Al filaments. The onset temperature of the area 3 corresponds to the resistive transition of Nb3Al. The abrupt decrease in the effective diameter from 5 - 15 μm to 2 - 3 μm around 6 T and at 4.2 K may be due to the transition occurring in this area. Proximity effect-induced dc coupling and/or micro-bridging seem to be responsible for this transition