Specimen geometry effects on the irreversible magnetization in the low field regime for specimens of bulk Nb3Sn

Abstract

Irreversible magnetization curves in particular in self field conditions were investigated for specimens of bulk Nb3Sn as an isotropic and non-granular alternative to high Tc materials. The specimens were cut into squares with side length to thickness ratios of about 0.8 to almost 20, leading to diamagnetic slopes |χd| ranging from slightly more than 1 (slab in parallel field) to 13 (plate in perpendicular field). The peak in the low field magnetization (caused by the field dependence of jc) moves from negative field for specimens with small |χd| (where it should be according to the standard Bean model) to zero or even positive fields for specimens with large |χd| . Finite element self consistent calculations with field dependent and thus spacially varying jc were used to fit the measured hysteresis curves. A strong current peak in the center plane of the specimens — caused by the demagnetization field — probably causes the shift of the magnetization peak to positive field. Best agreement between the experimental and calculated magnetizations have been obtained for the extreme geometries: either thin disk in perpendicular (χd −1.2).