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

Superconducting properties of 60 nm thick lead bismuth (Pb(82)Bi(18)) films in a spatially varying magnetic field created by a hybrid structure of soft magnetic (25 mu m wide iron) and nonmagnetic (250 mu m 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 H(C2) 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 T(C) in series. Thus, the critical current I(C) for the perpendicular case is much less than either the control film or the case for current parallel to the magnetic sheets. These results demonstrate that it is possible to strongly increase I(C) and H(C2) by simple redirection of magnetic flux with a soft magnetic microstructure. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3076517]