Polarized neutron imaging and three-dimensional calculation of magnetic flux trapping in bulk of superconductors

Abstract

Polarized neutron radiography was used to study the three-dimensional magnetic flux distribution inside of single-crystal and polycrystalline Pb cylinders with large (cm${}^{3}$) volume and virtually zero demagnetization. Experiments with single crystals being in the Meissner phase ($T<{T}_{c}$) showed the expected expulsion of magnetic field. 99.9999 wt $%$ pure polycrystalline samples were exposed to the same homogeneous magnetic field (6.4 mT) and only a portion of the applied field was expelled. The trapped field in the sample ($T<{T}_{c}$, ${B}_{\mathrm{ext}}=0$ T) showed a nearly Gaussian spatial distribution, centered on the cylinder axis and decreasing towards the surface of the cylinder. In the direction along the cylinder axis the trapped flux was nearly constant. The expelled field outside of the samples followed $1/R$ dependence. These measurements provided a unique and detailed picture of macroscopic superconducting samples, confirming the existence of both uniform bulk Meissner expulsion in single crystals and bulk flux trapping with nearly-Bean-model profiles due to flux pinning in polycrystalline samples.