Crossover between different regimes of inhomogeneous superconductivity in planar superconductor-ferromagnet hybrids

Abstract

We studied experimentally the effect of a stripelike domain structure in a ferromagnetic BaFe${}_{12}$O${}_{19}$ substrate on the magnetoresistance of a superconducting Pb microbridge. The system was designed in such a way that the bridge is oriented perpendicular to the domain walls. It is demonstrated that depending on the ratio between the amplitude of the nonuniform magnetic field ${B}_{0}$, induced by the ferromagnet, and the upper critical field ${H}_{c2}$ of the superconducting material, the regions of the reverse-domain superconductivity in the $H\ensuremath{-}T$ plane can be isolated or can overlap ($H$ is the external magnetic field, $T$ is temperature). The latter case corresponds to the condition ${B}_{0}/{H}_{c2}<1$ and results in the formation of superconductivity above the magnetic domains of both polarities. We discovered the regime of edge-assisted reverse-domain superconductivity, corresponding to localized superconductivity near the edges of the bridge above the compensated magnetic domains. Direct verification of the formation of inhomogeneous superconducting states and external-field-controlled switching between the normal state and inhomogeneous superconductivity were obtained by low-temperature scanning laser microscopy.