Interaction between vortices and ferromagnetic microstructures in twinned cuprate/manganite bilayers

Abstract

The role of magnetic microstructures and correlated defects on magnetic field and supercurrent distributions in cuprate/manganite bilayers was studied using the magneto-optical imaging technique. For this purpose, bilayer structures were deposited on twinned $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ substrates. Experimental results indicate that different magnetic patterns could be induced in the ferromagnetic ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ film deposited on twinned substrate. Depending on the induced magnetic microstructure of the ferromagnetic layer, the vortex pinning due to the twin-boundary network in the $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ layer becomes strongly modulated. In particular, the force acting on vortices due to the interaction with macroscopic magnetic domains, having out-of-plane tilted magnetization, was locally measured. The measurements at low temperature show that the magnetic interaction slightly contributes to the total pinning force density over the domains. It turns out that the alternating out-of-plane magnetization induces spontaneous counterflowing supercurrent loops in the superconducting layer. Finally, reference measurements made on a bilayer with nonferromagnetic manganite $({\mathrm{Nd}}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3})$ show that twin boundaries without localized magnetic moments produce homogeneous and strong vortex pinning in the twinned bilayers.