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 ͑S͒/ferromagnetic ͑F͒ 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.

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

“…I c was determined by choosing a 5 μV criterion. In agreement with the results previously reported for the same system [6], at low fields J c is of the order of 10 10 values are almost independent of d Py and, in the case of Nb/Py200 and Nb/Py350, they are not strongly influenced by the inhomogeneity of the magnetic phase of the Py layers, as also shown in panel (c) of Fig. 2 where, for voltage values lower than V * , the two curves overlap for the three values of the magnetic field.…”

“…In accordance with Ref. [6] the values of v * are higher with respect to the Nb film at the same reduced temperature. Similarly, μ 0 H is lower for the bilayers and scales both as a function of d Py and t. These experimental data confirm what was previously observed [2,6]; i.e., when the critical velocity is larger the instability typically disappears at smaller field and/or higher temperatures because in these cases the critical voltage V * is close to the normal state voltage V n .…”

“…When the bias current is further increased, the linear flux-flow regime is present until a certain value I * , where a sudden jump of the voltage V * takes place in agreement with the Larkin-Ovchinnikov theory [9]. Such instabilities were previously observed in a number of different superconducting systems [1,6,7,[23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. V * is related to the so-called critical velocity v * by the relation [9] At high fields the jump is replaced by a more continuous transition towards the normal state, as is clearly seen looking at the curves corresponding to the fields of 0.4 and 0.5 T in the inset of Fig.…”

“…In particular, from the analysis of the critical velocity v * for the occurrence of the instability of the moving vortex lattice [9], information on the quasiparticle relaxation time τ E was obtained, and it was shown that in order to reduce the values of τ E it is useful to couple the superconductor to a strong ferromagnet, such as permalloy [2]. This conclusion was based on measurements performed on Nb/Py bilayers (Py ≡ permalloy = Ni 0.8 Fe 0.2 ) which showed values of the relaxation time much smaller than that for the single Nb film [6]. That study was performed on samples with the thickness of the Py layer, d Py , well below the critical thickness d ESD cr (ESD stands for emerging stripe-domain phase) where an inhomogeneous magnetic regime can be formed [10].…”

“…In contrast, in the case of S/F hybrids a thin F layer will be completely proximized so that different superconducting properties appear at a distance of the order of α. Moreover, by studying the dynamic properties of the vortex lattice in S/F structures, it was found that the performance of the detecting devices in terms of the quasiparticle relaxation rates seems to be strongly enhanced by the presence of the F layer [1,2,[6][7][8]. In particular, from the analysis of the critical velocity v * for the occurrence of the instability of the moving vortex lattice [9], information on the quasiparticle relaxation time τ E was obtained, and it was shown that in order to reduce the values of τ E it is useful to couple the superconductor to a strong ferromagnet, such as permalloy [2].…”