Upper frequency limits for vortex guiding and ratchet effects

Abstract

Guided and rectified motion of magnetic flux quanta are important effects governing the magneto-resistive response of nanostructured superconductors. While at low ac frequencies these effects are rather well understood, their manifestation at higher ac frequencies remains poorly investigated. Here, we explore the upper frequency limits for guided and rectified net motion of superconducting vortices in epitaxial Nb films decorated with ferromagnetic nanostripes. By combining broadband electrical spectroscopy with resistance measurements we reveal that the rectified voltage vanishes at a geometrically defined frequency of about 700 MHz. By contrast, vortex guiding-related low-ac-loss response persists up to about 2 GHz. This value corresponds to the depinning frequency $f_\mathrm{d}^\mathrm{s}$ associated with the washboard pinning potential induced by the nanostripes and exhibiting peaks for the commensurate vortex lattice configurations. Applying a sum of dc and microwave ac currents at an angle $\alpha$ with respect to the nanostripes, the angle dependence of $f_\mathrm{d}^\mathrm{s}(\alpha)$ has been found to correlate with the angle dependence of the depinning current. In all, our findings suggest that superconductors with higher $f_\mathrm{d}^\mathrm{s}$ should be favored for an efficient vortex manipulation in the GHz ac frequency range.