Flux profiles of superconductors in high magnetic fields

Abstract

A probe has been designed and built for measuring the magnetisation critical current density J(_c)(B,T) and magnetic field profiles inside superconducting samples. This is the first flux penetration probe capable of making measurements as a function of temperature, in the range 2-30 K (accurate to 100 mK), which combines high ac. fields of up to 100 mT (at 19.7 Hz) with very large dc. fields of up to 17 T.A critical state model has been used to calculate the magnetic response of a superconductor to low frequency, high amplitude ac. fields. The analysis has been performed for both a cylindrical and slab geometry and calculated up to the lO(^th) harmonic. The overshoot commonly observed in magnetic field profiles is found to be an artifact of the analysis and is not indicative of sample granularity. Flux penetration measurements have been made on commercial, multifilamentary NbTi wire from 4.2 K up to T(_c) in magnetic fields up to 10 T. The J(_c)(B,T) is in good agreement with transport data on the same wire and the functional form of the volume pinning force obeys the Fietz-Webb scaling law of the form F(_p)(_x)h(l-h), where h is the reduced field. The spatial variation of J(_c)(B,T) is small and the harmonic response in a transverse field is consistent with the analysis for a non-granular sample. This is expected for a bulk pinning, homogeneous superconductor such as NbTi. Measurements have also been made on high quality, bulk PbMo(_6)S(_8) (PMS). After hot isostatic pressing the J(_c)(B,T) increased by a factor of 20. Doping PMS with gadolinium provides a rare opportunity to study the coexistence of superconductivity and magnetism. Although T(_c) increases, both J(_c)(B,T) and H(_in)(T) decrease dramatically. For all of the PMS samples, Fp(_x)h(^1/2)(1`-h)(^2) which suggests that J(_c)(B,T) is limited by gram boundary pinning.