Magnetic memory effect in type-II superconductor/ferromagnet bilayers

Abstract

We study the temperature dependence of the critical current density, JcS/F(T ), of a Nb/PdNi (PdNi = Pd84Ni16) bilayer before and after the application of a magnetic field oriented either out-of-the-plane or in-the-plane of the substrate. Nb and PdNi layers interact through both electromagnetic and proximity coupling. The values of JcS/F strongly depend on the magnetic history of the samples. Indeed, the JcS/F values measured when the PdNi is in the out-of-plane remanent state are reduced by a factor of two, in the whole investigated temperature range, compared to the case when the PdNi is in the demagnetized state. This behavior can be accounted for by the out-of-plane magnetocrystalline anisotropy of the PdNi layer producing stray fields which, in turn, can induce a spontaneous vortex phase in the Nb layer. The topology of these vortices is strongly modified by the proximity coupling as confirmed by theoretical calculations. The JcS/F values are only weakly affected by the in-plane remanence of the PdNi layer.