Upper Frequency Limits for Vortex Guiding and Ratchet Effects

The guided and rectified motion of magnetic flux quanta are important effects governing the magnetoresistive response of nanostructured superconductors. While at low ac frequencies these effects are rather well understood, their manifestations at higher ac frequencies remain 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, a vortex-guiding-related low-ac-loss response persists up to about 2 GHz. This value corresponds to the depinning frequency f(d)(s) associated with the washboard pinning potential induced by the nanostripes, which exhibits peaks for commensurate vortex-lattice configurations. By applying a sum of dc and microwave ac currents at an angle alpha with respect to the nanostripes, the angle dependence of f(d)(s) (alpha) is found to be correlated with the angle dependence of the depinning current. In all, our findings suggest that superconductors with higher f(d)(s) should be favorable for efficient vortex manipulation in the gigahertz ac frequency range.