High field flux pinning and the upper critical field of Nb-Ti superconductors

Nb-Ti wires of different composition and metallurgical state have been studied near the upper critical field (H/sub c2/) in an attempt to separate H/sub c2/ the irreversibility field (H*) and the resistively measured transition field (H/sub r/). For optimized multifilamentary Nb-47wt.%Ti wire, we find that H*=10.2 T, H/sub c2/=10.8 T and H/sub r/=11.8 T at 4.2 K. The transitions were studied by extended electric field vs. current density curves, magnetization and small current transport measurements. We found a strong correlation between magnetization, bulk flux pinning (F/sub p/) extrapolation and extended E-J curves for the value of H*, all giving H*=10.2 T. We have also shown that the maximum in H/sub r/ occurs for Nb-44wt.% Ti.