Influence of Co Layer Thickness on the Superconducting Properties of Nanolayered Nb/Pt/Co Heterostructures: Implications for Ultrafast Sensors, Spin Valves, and Quantum Circuits

High Resolution Image Download MS PowerPoint Slide The ability to control and manipulate the vortex state in type-II superconductors (SC) is important for applications of superconducting devices. Planar SC/ferromagnetic (FM) bilayers are alternative systems where this manipulation can be done. In this work, we studied the influence of thin FM Co layers on the SC Nb state in sputtered Nb(20 nm)/Pt(4 nm)/Co( t Co = 0–20 nm)/Pt(4 nm) nanohybrids. Oscillations in critical superconducting temperature T C and critical current density J C with changing of t Co values were observed, and their occurrences were confirmed by replicated samples. The T C oscillations do not decay in amplitude with the increase of FM thickness, differently to what has been observed in Nb/FM nanohybrids and Nb/Pt systems. These findings suggest that the Nb superconducting properties ( T C and J C ) are strongly dependent on two main contributions: the proximity effect and the morphology of the FM Co layer that goes from small out-of-plane isolated magnetic domains nucleated in granular-like Co layer ( t Co ≈ 1 nm) to a continuous Co layer regime in thicker Co layers. Using our results and some data from the literature on magnetism of Pt/Co, a high density (low density) of smaller (larger) magnetic domains formed for 2 nm ≤ t Co ≤ 8 nm ( t Co > 8 nm) is assumed to be responsible for the vortex pinning centers at the Nb films; i.e., the size and density of Co magnetic domain structures determine the pinning centers that influence the SC properties of Nb(20 nm)/Pt(4 nm)/Co nanohybrids.