Visualizing Vortex Dynamics in Py/Nb Thin Film Hybrids by Low Temperature Magnetic Force Microscopy

We have analyzed the vortex dynamics in Py(1 ?m)/SiO2(10 nm)/Nb(360 nm) thin film heterostructures as resulted from Magnetic Force Microscopy (MFM) frequency shift maps at low temperatures. The Nb film thickness has been chosen larger than the superconducting London penetration depth of about ? L=68 nm at the measuring temperatures. Above the Nb T c, the stripe-like Py stray field is visualized with half-period w Py=520 nm. Below the Nb T c, we have found that in a zero applied field, the supercurrents established in the Nb layer completely screen the out-of-plane component of the Py stray field. However, when the samples are cooled in a uniform external magnetic field, vortices are formed in chain-like configurations along the stripes with the same polarity. By decreasing and reversing the applied field, for low intensities, we have found a rigidity of the vortex array that remains "frozen" in the configuration as determined after the first field cooling run. The observed symmetry is then broken for higher values of the applied field, when an antivortex "avalanche" enters the Ferromagnetic/Superconducting (FM/SC) system.