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International audience; The flux-line-lattice response to AC magnetic fields in type-II superconductors is determined by the shape of the flux pinning potential in the material. This response is normally well described within the Bean critical state model, provided the amplitude of the AC magnetic field is sufficient to unpin and move the vortices from their pinning sites, leading to hysteresis. Conversely, if the vortices remain within their potential wells, their movement remains reversible, and cannot be described by the Bean critical state model. Instead, the mixed state can be viewed as an ensemble of coupled linear harmonic oscillators, each oscillating within its potential well. This is explained by the Campbell model, which extends upon the Bean model to include the finite size of the pinning potential, and a corresponding linear and elastic pinning force. Here, the dynamic equations for flux movement – given a linear pinning force – are described, linearized, and solved analytically. Subsequently, the solution is used to extract the value of the Campbell penetration depth of an applied AC magnetic field from experimental data obtained with a pick-up method.

Invited talk at the International Workshop on Numerical Modelling of High Temperature Superconductors in Caparica, Portugal.

Superconducting bulks, acting as high-field permanent magnets, are promising for many applications. An important effect in bulk permanent magnets is crossed-field demagnetization, which can reduce the magnetic field in superconductors due to relatively small transverse fields. Crossed-field demagnetization has not been studied in sample shapes such as rectangular prisms or cubes. This contribution presents a study based on both three-dimensional (3-D) numerical modeling and experiments. We study a cubic Gd-Ba-Cu-O bulk superconductor sample of size 6 mm magnetized by field cooling in an external field of around 1.3 T, which is later submitted to crossed-field magnetic fields of up to 164 mT. Modeling results agree with experiments, except at transverse fields 50% or above of the initial trapped field. The current paths present a strong 3-D nature. For instance, at the midplane perpendicular to the initial magnetizing field, the current density in this direction changes smoothly from the critical magnitude, Jc, at the lateral sides to zero at a certain penetration depth. This indicates a rotation of the current density with magnitude Jc, and hence force free effects like flux cutting are expected to play a significant role., The work of M. Kapolka was supported in part by the Research and Development Operational Programme funded by the ERDF under Project SIVVP, ITMS 26230120002 for the use of computing resources, and in part by the Grant Agency of the Ministry of Education of the Slovak Republic and the Slovak Academy of Sciences (VEGA) under Contract 2/0126/15. The work of M. Ainslie was supported in part by the Royal Academy of Engineering Research Fellowship and in part by the Engineering and Physical Sciences Research Council Early Career Fellowship EP/P020313/1.