Influence of the magnetic configuration on the vortex-lattice instability in Nb/permalloy bilayers

We studied the dynamic instability of the vortex lattice in Nb/permalloy $(\mathrm{permalloy}={\mathrm{Ni}}_0.8{\mathrm{Fe}}_0.2)$ bilayers with different Py thickness ${d}_{\mathrm{Py}}$. When ${d}_{\mathrm{Py}}$ is larger than some critical value, ${d}_{\mathrm{cr}}^{\mathrm{ESD}}$, which in our samples has been evaluated to be $\ensuremath{\simeq}125$ nm, an inhomogeneous magnetic configuration is induced in the samples as a precursor to a fully developed stripe domain state. We found that this inhomogeneous magnetic configuration of the Py layer promotes larger values of the critical velocity for the vortex lattice instability, ${v}^*$, with respect to the homogeneous case.