Influence of Co Layer\nThickness on the Superconducting\nProperties of Nanolayered Nb/Pt/Co Heterostructures: Implications\nfor Ultrafast Sensors, Spin Valves, and Quantum Circuits

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