| The unexpectedly low hysteretic energy loss of the high temperature A-15 superconductor niobium-tin was investigated with three generations of flux penetration measurement apparatus. The samples were a variety of electron-beam coevaporated films on Hastelloy tubes. Many of the samples were multilayer composites of niobium-tin with normal metal barrier layers from 9 to 250 nm thick.; An inductive ac loss measurement system was developed first, which used a self-resonant field magnet and probe mounted sample in a dual-dewar for temperature control. A balanced pair of detection coils and digital signal averaging gave unambiguous ac loss measurements. Those samples with very high ac losses were explained by the critical state model and volume critical currents consistent with dc measurements. Most of the low loss samples involved strong surface effects, however, which were not consistent with known mechanisms. The measured high-resolution electric field waveforms of these low loss samples were analyzed in a form readily comparable with previous model calculations.; The apparatus was then modified to measure ac waveforms in a dc bias field, allowing additional comparisons by separating surface and volume currents. Finely layered super-conductors were found to have greatly enhanced volume pinning of vortices. The large columnar grains in thick single layers were found to have homogeneous pinning.; To further separate the surface-effect current into superheating and supercooling components, an apparatus was developed to detect the response of a single initial field change. Results of several samples showed: (1) the flux exit field was equal to the surface induction, as expected, (2) the magnitude and field dependence of the low-field flux entry barrier were at least a factor of three smaller than expected for an ideal surface, (3) the Meissner effect was directly observable below about 200 Oe, (4) the effective superheating surface barrier of internal layers was small, but enhancements of desirable morphology and bulk pinning force were enormous, and (5) the entry barrier was severely degraded by trapped reverse-sign vortices. The latter is consistent with published tunneling and microwave resistivity measurements, and is qualitatively consistent with L. Kramer's (1973) theoretical study of the stability limit of the superheated state. |
