Magnetization rotation in a superconductor/ferromagnet bilayer ring structure

The magnetic-flux distribution in a bilayer ring consisting of superconducting Nb and ferromagnetic amorphous Gd19 Ni81 is studied at low temperature by magneto-optical imaging. The ring is macroscopic with an outer diameter of 500 μm and an inner diameter of 300 μm. Below the superconducting transition and with a small magnetic field applied along the ring axis, we observe a field enhancement at the inner edge and a field reduction at the outer edge, opposite to what is observed for a bare superconducting ring. With help from numerical simulations we show this to be due to an in-plane rotation of the magnetization of the Gd19 Ni81 layer toward the radial direction, under the influence of shielding currents in the superconductor. In the superconductor, the resulting stray field of the magnetic layer is in the same direction as the field due to the superconducting shielding currents. As a consequence, these shielding currents are reduced. Alternatively, for a fixed critical current, the superconductor will be able to withstand a larger external field if coated by the magnetic material; it is hardened by it. © 2009 The American Physical Society.