Is magnetic pinning a dominant mechanism in Nb-Ti?

Abstract

The authors 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 that of the conventional composite, where ribbons of normal metal form the pinning centers, whereas the nanometer-filament composite has no internal normal metal but pins at the filament surface instead. The APC composite exhibits much stronger pinning relative to B/sub c2/ than the conventional composite (21.4 GN/m/sup 3/. 7 T vs. 18.9 GN/m/sup 3/, 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/sub c2/ unfavorably. In all three composites, F/sub p/ was proportional to (1-b)/sup 3/2/, which suggests that the magnetic interaction, rather than core pinning, dominates. F/sub p/ obeys a scaling relation as T tends to T/sub 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, and it is suggested that pinning is done by clusters of pinning centers rather than by the individual pins.