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 (permalloy=Ni0.8Fe0.2) bilayers with different Py thickness dPy. When dPy is larger than some critical value, dcrESD, which in our samples has been evaluated to be ?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.