High-velocity instabilities in the vortex lattice of Nb/permalloy bilayers

Abstract

We have studied the critical current density ${J}_{c}$ for onset of vortex motion and the dynamic instability of the moving vortex lattice at high driving currents in superconducting (S)/ferromagnetic (F) $\mathrm{Nb}∕{\mathrm{Ni}}_{0.80}{\mathrm{Fe}}_{0.20}$ bilayers and in a single Nb film with the same thickness. The samples are all characterized by relatively high values of the pinning strength. The measured current-voltage characteristics are successfully described in the framework of the Larkin-Ovchinnikov model, modified in order to take into account the effect of the pinning close to the instability. We find that ${J}_{c}$ is smaller in the S/F bilayers than in the single film and argue that this is due to the strongly inhomogeneous order parameter in the bilayers. Also, the critical velocity ${v}^{*}$ for the occurrence of the instability is found to be significantly larger in the S/F bilayers than in the single S layer. By extracting the quasiparticle energy relaxation rate from ${v}^{*}$, we show that this effect is due to the same inhomogeneous order parameter and the resulting lower average value of the superconducting gap.