Asymmetric flux pinning in laterally nanostructured fer - romagnetic / superconducting bilayers

– We investigated the pinning of flux lines in a superconducting film by a regular array of magnetic antidots. The sample consists of a Co/Pt multilayer with perpendicular magnetic anisotropy in which a regular pattern of submicron holes is introduced and which is covered by a type-II superconducting Pb film. The resulting ferromagnetic/superconducting heterostructure shows a pronounced asymmetric magnetization curve with respect to the field polarity. This asymmetry clearly demonstrates that the magnetic contribution dominates the pinning potential imposed by the magnetic antidots on the superconducting film. Strong pinning of flux lines (FLs) is a prerequisite for a superconductor to achieve a high critical current density jc, a condition that for instance can be realized by artificially introducing regularly distributed pinning centres (e.g. antidots) in a superconducting film. The resulting commensurability between the flux line lattice (FLL) and the periodic pinning potential created by the regular array of antidots in the superconductor is clearly observed as peaks or shoulders in the curves of the magnetization M(H) or jc(H) [1–5], with H the perpendicularly applied magnetic field. Recently, the interest shifted to the pinning behaviour of submicron magnetic pinning arrays in contact with a superconductor [6–14]. In this letter we present an experimental study of the flux pinning in a superconducting Pb film in contact with a Co/Pt multilayer with an array of submicron holes (magnetic antidots). The correct choice of preparation conditions and film thicknesses produces a Co/Pt multilayer with perpendicular magnetic anisotropy [15]. Several terms can contribute to the pinning mechanism in this hybrid ferromagnetic/superconducting system, e.g., the corrugated surface of the Pb film due to the deposition on top of the Co/Pt antidot array, the high magnetic permeability of the ferromagnet [6,7], the direction and magnitude of the magnetic moment [9, 10], and the local stray field of the ferromagnet [10–12]. We show that the M(H) curve of the above described heterostructure is strongly asymmetric, (∗) E-Mail: [email protected]