Superconductors in realistic geometries: Geometric edge barrier versus pinning

Abstract

The magnetic response of type-II superconductors can be irreversible due to two different reasons: vortex pinning and barriers for flux penetration. Even without bulk pinning and in absence of a microscopic Bean-Lingston surface barrier for vortex penetration, superconductors of nonellipsoidal shape can exhibit a large geometric barrier for flux penetration. This edge barrier and the resulting irreversible magnetization loops and flux-density profiles are computed from continuum electrodynamics for superconductor strips and disks with constant thickness, both without and with bulk pinning. Expressions for the field of first flux entry H_en and for the reversibility field H_rev above which the pin-free magnetization becomes reversible are given. Both fields are proportional to the lower critical field H_c1 and else depend only on the specimen shape. These realistic results are compared with the reversible magnetic behavior of ideal superconductor ellipsoids.