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

Abstract

We propose that a magnetic pinning mechanism is the dominant flux-pinning mechanism of proximitycoupled, 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 ␣-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 N have comparable dimensions͒, 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/ N ͑e.g., different Nb-Ti wire diameters͒ 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 tϳ N /3, while for varying defect composition the peak F p gets higher when N is made shorter. Artificial pinning center Nb-Ti wires having short 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.

High critical-current density with less anisotropy in BaFe2(As,P)2 epitaxial thin films: Effect of intentionally grown c-axis vortex-pinning centers

“…These values are higher than the previously reported ones, including those for 122-type compounds [e.g., BaFe 2 As 2 :P films with artificial pinning centers created by BaZrO 3 nanoparticles [26] and oxygen-rich BaFe 2 As 2 /BaFe 2 As 2 :Co superlattice (SL) films [28] ] and conventional alloy superconducting wires (Nb-Ti, [33] Nb 3 Sn, [34] and MgB 2 [35] ). other 122-type films [7,26,28,29] in similar ranges of H and T. The BaFe 2 As 2 :P epitaxial film obtained in this study exhibited a lower anisotropy than those of BaFe 2 As 2 :P [7] and BaFe 2 As 2 :Co/Fe buffer.…”

Pinning Properties of Commercial Nb-Ti Wires Described by a 2-Components Model

“…In fact, the explored temperature range is such that the BCS gap, [12], vary in the range 0.98-0.87, and the non-scaling effects could be neglected. The pinning regimes and mechanisms in NbTi are complex and varied [13], so that it is difficult to provide an accurate quantitative description of the pinning mechanisms in such systems. However, bearing in mind that the 2-components formulation should be viewed as a simple model system, in the following we will try to give an explanation about the sources of these different contributions.…”

Increased critical current density in Nb–Ti wires having Nb artificial pinning centers

“…As we could detect no difference in the average pin thickness or the pin thickness distribution at the optimum flux pinning size ͑d p ϭ40 nm͒ for either composite, we attributed the different J c (d p ) behavior to pin/matrix interdiffusion caused by the fourth extrusion. Since interdiffusion should reduce the pin proximity length ( N ) by alloying the Nb, this should lead to the elementary pinning force ( f p ) reaching its peak for thinner pins, 14 thus raising the density of pins at maximum f p , and enhancing the bulk pinning force (F p ) at higher fields where higher J c is most desirable. However, such interdiffusion should also diminish the composition gradient at the pin/ matrix interface, thereby decreasing the strength of f p .…”