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

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) Nb/Ni(0.80)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.