Strong increase in critical field and current in magnet-superconductor hybrids

Superconducting properties of 60 nm thick lead bismuth (Pb82Bi18) films in a spatially varying magnetic field created by a hybrid structure of soft magnetic (25 μm wide iron) and nonmagnetic (250 μm wide brass) layers perpendicular to the superconducting film surface are reported. The applied homogeneous external magnetic field is redistributed in the superconducting film due to the high magnetic permeability of iron. This results in alternating parallel stripes of regions of low and high magnetic field. Thus the values of second critical field HC2 and critical current density are also much higher for current parallel to the magnetic stripes than in a control superconducting film without any magnetic field modulation. For the current perpendicular to the magnetic stripes, the superconductor behaves like an inhomogeneous resistor with regions of low and higher TC in series. Thus, the critical current IC for the perpendicular case is much less than either the control film or the case for current parallel to...