Trapped magnetic flux density and influence of crossed fields in structures combining large grain, bulk (RE)BCO superconductors and soft ferromagnetic discs

Bulk (RE)BCO superconductors are able to trap record magnetic fields and can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. We combine soft ferromagnetic pieces with bulk, large grain (RE)BCO high temperature superconductors to form superconductor/ferromagnet (SC/FM) hybrid structures. We study how a ferromagnetic disc influences the trapped flux density at the surface of a SC/FM structure at 77 K and the magnetic moment of the whole structure. The flux density generated by the superconductor is shielded above the ferromagnet. The saturation of the ferromagnet by the flux of the superconductor is investigated with the help of a finite element model and a simple analytical formula is suggested to determine the saturation threshold. Finally, we show that the ferromagnetic disc lowers the relative demagnetization effect caused by the application of magnetic field cycles orthogonal to the main magnetization (crossed field configuration).