Influence of soft ferromagnetic materials on the magnetic flux density above large grain, bulk high temperature (RE)BCO superconductors: measurements and modelling

Due to their ability to trap high magnetic fields, bulk (RE)BCO superconductors 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 the ferromagnetic pieces influence (i) the profile shape of the trapped magnetic flux density at the surface of each structure and (ii) the decrease of the trapped field under an applied field that is orthogonal to the main magnetization (crossed field configuration). Hall probe mappings of the trapped magnetic flux density profile above the hybrid structures at 77 K are compared to modelled profiles using a 2D finite element method. Modelling results are in excellent qualitative and quantitative agreement with the measurements. The model is then used to study the magnetic flux distribution and predict the behaviour for other constitutive laws and ferromagnet geometries. For the modelled configurations, the magnetic flux density is enhanced on the face opposite to the ferromagnet. Both thickness and saturation magnetization of the ferromagnetic material are found to be important parameters. The saturation regime of the ferromagnet can be predicted using a simple analytical model. We show that thin ferromagnetic discs can be driven to full saturation even though the superconductor magnetic field is much smaller than their saturation magnetization. Remarkably, the beneficial influence of the ferromagnet on the trapped field can be observed even if the trapped flux density above the superconductor is much larger than the ferromagnet saturation magnetization. Finally, we show that the ferromagnetic material acts as a magnetic shield and lowers the relative demagnetization effect caused by the application of transverse (crossed) magnetic field cycles.