Model for the response of superconducting/ferromagnetic structures subjected to crossed fields

Bulk (RE)BCO samples can be used as efficient permanent magnets with trapped magnetic flux densities of a few teslas at 77 K. However, their magnetization may decay over time when subjected to time-varying parasitic fields perpendicular to the initial trapped flux density. In this work, we consider hybrid structures made of superconducting pellets and soft ferromagnetic rings or disks and investigate numerically how their magnetization varies under such crossed-field excitations. We consider a finite-element model in the A-V formulation, which is developed for both the full 3D configuration of the system and for a computationally lighter 2D approximation. The models provide the volumic distribution of current densities, the field lines, and the magnetization of the system, and the latter is compared to experimental measurements obtained with a bespoke magnetometer. A critical assessment is made outlining the advantages and limitations of the 2D approach. It is demonstrated that the ferromagnetic material helps in increasing the initial trapped field and in reducing the crossed-field effects. For the case of a single ferromagnetic disk put on top of the superconducting pellet, we study the dependence of the initial magnetization with respect to the disk thickness and the saturation field of the ferromagnetic material, and discuss the resulting optimal thickness. We also suggest different hybrid structures made of one superconducting pellet and one or several ferromagnetic disks or rings, and show numerically that a compromise is to be found between the maximum level of induction which can be reached and the improved protection against crossed-field effects.