Static and dynamic properties of the vortex lattice in superconductor/weak ferromagnet bilayers

Abstract

By measuring I–V characteristics as a function of the temperature and the external magnetic field, we have analyzed the static and dynamic properties of the vortex lattice in Nb/Pd0.84Ni0.16 bilayers. In particular, the critical current density Jc for the onset of the vortex motion and the dynamic instability of the moving vortex lattice at high driving currents have been studied and compared to the results obtained in a single Nb film. We find that Jc is smaller in the bilayers than in the single superconducting film due to the smaller value of the superconducting order parameter in the bilayers. The critical velocity v* for the occurrence of the instability is larger in the S/F bilayers than in the single S layer. However, the quasiparticle energy relaxation rate extracted from v* is almost temperature-independent, implying that a different relaxation mechanism plays a role in the Nb/Pd0.84Ni0.16 bilayers.

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

“…The anomalous increase of v * at very low fields (up to ∼0.05 T) followed by a decrease at higher fields obtained for the Nb film confirms what was already observed in thin films of superconducting materials in the dirty limit and in the nonmesoscopic regime [34][35][36][37][38]40]. The value v * 600 m/s at high field is larger with respect to what was previously measured in Nb single films at similar fields and temperatures [2,6,7]. This result can be ascribed to FIG.…”

“…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 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].…”

Robustness of the0−πtransition against compositional and structural ageing in superconductor/ferromagnetic/superconductor heterostructures

“…More details concerning the sample preparation are described in Ref. 9. Pd 1−x Ni x is a weak ferromagnetic alloy widely employed in the framework of the research of the coexistence of S and F orderings, due to the tunability of its exchange energy E ex by a small amount of the Ni content.…”

“…In particular, the magnetic and electrical properties of Pd 84 Ni 16 (hereafter PdNi) were widely investigated in Ref. 9, resulting in an exchange energy E ex 14 meV, a ferromagnetic coherence length ξ F 3 nm and a Curie temperature T c = 200 K. Moreover, it is also worth noticing that a number of works present in the literature suggest that Nb/PdNibased systems show relatively high transparency of the S/F interface. Microwave measurements are a useful tool to investigate the fundamental properties of superconducting materials, since they give information on both quasiparticles and superfluid density.…”