Abstract
The flux-pinning landscape in type-II superconductors determines the response of the flux line lattice to changing magnetic fields. Typically, the flux vortex behaviour is hysteretic and well described within the framework of the Bean critical-state model and its extensions. However, if the changing magnetic field does not move the flux vortices from their pinning sites, their response remains linear and reversible. The vortex displacement, then, is characterised by the Campbell penetration depth, which itself is related directly to the effective size of the pinning potential. Here, we present measurements of the Campbell penetration depth (and the effective size of the pinning potential) as a function of magnetic field in a single-grain bulk GdBa2Cu3O superconductor using a pick-up coil method. Hence, the hysteretic losses, which take into account the reversible vortex movement, are established.
Highlights
The critical-state model proposed by Bean [1] represents the foundational framework which has been used to predict successfully most macroscopic observables of hard typeII superconductors in the mixed state
In this paper we present a simple method of measuring the Campbell penetration depth and, with it, the effective size of the pinning centres, based on Campbell’s original pick-up method
If the applied field is reversed the density gradient will be established in the opposite direction, and the vortices will move from one edge of the pinning potential to the other before reestablishing the critical state
Summary
The critical-state model proposed by Bean [1] represents the foundational framework which has been used to predict successfully most macroscopic observables of hard typeII superconductors in the mixed state. A sufficiently small force, acting on a pinned vortex, will not displace the vortex from its potential well; instead the vortex will move from its equilibrium position within the potential well itself, and will return to equilibrium once the force is removed. This movement within the potential well is reversible and is neglected within the Bean model framework
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