FLUX-PINNING MECHANISM OF PROXIMITY-COUPLED PLANAR DEFECTS IN CONVENTIONAL SUPERCONDUCTORS - EVIDENCE THAT MAGNETIC PINNING IS THE DOMINANTPINNING MECHANISM IN NIOBIUM-TITANIUM ALLOY

We propose that a magnetic pinning mechanism is the dominant flux-pinn ing mechanism of proximity-coupled, planar defects when the field is p arallel to the defect. We find compelling evidence that this pinning m echanism is responsible for the strong flux-pinning force exerted by r ibbon-shaped alpha-Ti precipitates and artificial pins in Nb-Ti superc onductors, instead of the core pinning mechanism as has been hitherto widely believed. Because the elementary pinning force f(p)(H) is nonmo notonic 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. Characterist ic changes in the magnitude and shape of F-p(H) at constant T but at d ifferent t/xi(N) (e.g., different Nb-Ti wire diameters) are also direc t consequences of the pinning mechanism. The optimum flux-pinning stat e is a compromise between maximizing f(p) and getting the highest numb er density of pins. For a given defect composition this state is reach ed 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 c enter Nb-Ti wires having short xi(N) pins appear to be vital for obtai ning high J(c) at high fields because only then is the elementary pinn ing force optimized at small pin thicknesses which permit a high numbe r density of vortex-pin interactions and a large bulk pinning force. W e find verification of our predictions in experimental F-p(H, T,t) dat a obtained on special laboratory-scale artificial pinning-center Nb-Ti wires.