IS MAGNETIC PINNING A DOMINANT MECHANISM IN NB-TI

We have compared the pinning behavior of an artificial pinning center (APC) composite and a nanometer-filament Nb 46.5 wt% Ti composite to that of a conventional Nb 48 wt% Ti composite. The microstructure of the APC composite resembles the conventional composite, where ribbons of normal metal form the pinning centers, whereas the nanometer-filament composite has no internal normal metal but pins instead at the filament surface. The APC composite exhibits much stronger pinning relative to B(c2) than the conventional composite (21.4 GN/m3, 7 T vs. 18.9 GN/m3, 11 T), which is possibly due to the increased amount of pinning center (50 vol.% vs. 25 vol.%), however the proximity effect reduces the B(c2) unfavorably. In all three composites, F(p) was proportional to (1-b)3/2, which suggests that the magnetic interaction, rather than core pinning, dominates. Moreover, F(p) obeyed a scaling relation as T tends to T(c) in the conventional composite, and over a wide range of temperature in the APC composite. In these regimes, one pinning body is also dominant, where pinning done by clusters of pinning centers, rather than the individual pins, is suggested.