Flux-pinning mechanism of proximity-coupled planar defects in conventional superconductors: Evidence that magnetic pinning is the dominant pinning mechanism in niobium-titanium alloy

We propose that a magnetic pinning mechanism is the dominant flux-pinning mechanism of proximity-coupled, planar defects when the field is parallel to the defect. We find compelling evidence that this pinning mechanism is responsible for the strong flux-pinning force exerted by ribbon-shaped {α}-Ti precipitates and artificial pins in Nb-Ti superconductors, instead of the core pinning mechanism as has been hitherto widely believed. Because the elementary pinning force fₚ(H) is nonmonotonic when it is optimum (i.e., when the defect thickness t and the proximity length ξN have comparable dimensions), the total pinning force Fₚ(H) generally does not show temperature scaling. Characteristic changes in the magnitude and shape of Fₚ(H) at constant T but at different t/ξN (e.g., different Nb-Ti wire diameters) are also direct consequences of the pinning mechanism. The optimum flux-pinning state is a compromise between maximizing fₚ and getting the highest number density of pins. For a given defect composition this state is reached when t{~}ξN/3, while for varying defect composition the peak Fₚ gets higher when ξN is made shorter. Artificial pinning center Nb-Ti wires having short ξN pins appear to be vital for obtaining high Jc at high fields because only then is the elementary pinning force optimized at small pin thicknesses which permit a high number density of vortex-pin interactions and a large bulk pinning force. We find verification of our predictions in experimental Fₚ(H,T,t) data obtained on special laboratory-scale artificial pinning-center Nb-Ti wires. {} 1996 The American Physical Society.