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 alpha-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 f(p)(H) is nonmonotonic when it is optimum (i.e., when the defect thickness t and the proximity length xi(N) have comparable dimensions), the total pinning force F-p(H) generally does not show temperature scaling. Characteristic changes in the magnitude and shape of F-p(H) at constant T but at different t/xi(N) (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 f(p) and getting the highest number density of pins. For a given defect composition this state is reached when t similar to xi(N)/3, while for varying defect composition the peak F-p gets higher when xi(N) is made shorter. Artificial pinning center Nb-Ti wires having short xi(N) pins appear to be vital for obtaining high J(c) 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 F-p(H, T,t) data obtained on special laboratory-scale artificial pinning-center Nb-Ti wires.