Artificial pinning center Nb-Ti superconductors with alloyed Nb pins

A magnetic interaction has been proposed as the dominant fluxon-defect interaction in conventional and artificial pinning center (APC) Nb-Ti, emphasizing the proximity length (/spl xi//sub N/) of the pinning center as the important parameter for optimizing the high field critical current density. We have characterized APC composites containing 25 vol.% of Nb pins alloyed with 7.5 wt.% Ta and 10 wt.% W to deliberately test the predictions of this model. We found that the bulk flux pinning force of the Nb10W-pin composite (/spl xi//sub N/(Nb10W)/spl sim/32 nm) exhibited a magnetic field (H) and pin size (d/sub p/) dependency that is more consistent with conventionally processed Nb47Ti (/spl xi//sub N/(/spl alpha/-Ti)<32 nn) than with existing pure Nb-pin APC composites (/spl xi//sub N/(Nb)/spl sim/83 nm). The Nb7Ta-pin composite (/spl xi//sub N//spl sim/59 nm) had intermediate F/sub p/(H,d/sub p/) behavior, thus qualitatively supporting the model.