Magnetic Measurements of Critical Current Density, Pinning, and Flux Creep

Abstract

This chapter introduces standard and advanced magnetic characterization techniques, discusses their proper evaluation and point out experimental pitfalls. It focuses on the experimental methods, whereas the underlying physics will be explained only to the extent needed to understand experimental issues. The chapter also focuses on the critical current density in the mixed state, which is determined by the pinning strength of the defect structure, and, in some cases, by current limitation at grain boundaries. The movement is driven by the Lorentz force acting on the vortices when a macroscopic current density is applied, but can be impeded by vortex pinning by defects in the microstructure of the superconductor. The evaluation based on the critical state model assumes homogeneous properties of the material so that the geometry of the current loops is given by the sample geometry. A variety of magnetic techniques allows assessing much more than the critical current but details of flux pinning and dynamics or granularity effects.