Flux-pinning mechanism of proximity-coupled planar defects in conventional superconductors: Evidence that magnetic pinning is the dominant pinning mechanism in niobium-titanium alloy
- Electromagnetic Technology Division, National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80303 (United States)
- Applied Superconductivity Center, University of Wisconsin--Madison, 1500 Johnson Drive, Madison, Wisconsin 53706 (United States)
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 {ital f}{sub {ital p}}({ital H}) is {ital nonmonotonic} when it is optimum (i.e., when the defect thickness {ital t} and the proximity length {xi}{sub {ital N}} have comparable dimensions), the total pinning force {ital F}{sub {ital p}}({ital H}) generally does {ital not} show temperature scaling. Characteristic changes in the magnitude and shape of {ital F}{sub {ital p}}({ital H}) at constant {ital T} but at different {ital t}/{xi}{sub {ital 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 {ital f}{sub {ital p}} and getting the highest number density of pins. For a given defect composition this state is reached when {ital t}{approximately}{xi}{sub {ital N}}/3, while for varying defect composition the peak {ital F}{sub {ital p}} gets higher when {xi}{sub {ital N}} is made shorter. Artificial pinning center Nb-Ti wires having short {xi}{sub {ital N}} pins appear to be vital for obtaining high {ital J}{sub {ital 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 {ital F}{sub {ital p}}({ital H},{ital T},{ital t}) data obtained on special laboratory-scale artificial pinning-center Nb-Ti wires. {copyright} {ital 1996 The American Physical Society.}
- OSTI ID:
- 279494
- Journal Information:
- Physical Review, B: Condensed Matter, Vol. 53, Issue 10; Other Information: PBD: Mar 1996
- Country of Publication:
- United States
- Language:
- English
Similar Records
Quantitative description of a very high critical current density Nb-Ti superconductor during its final optimization strain. II. Flux pinning mechanisms
Proximity effect in flux pinning