Enhancement of the Superconducting Transition Temperature inNb/PermalloyBilayers by Controlling the Domain State of the Ferromagnet

Abstract

In (S/F) hybrids the suppression of superconductivity by the exchange field h(ex) of the ferromagnet can be partially lifted when different directions of h(ex) are sampled simultaneously by the Cooper pair. In F/S/F trilayers where the magnetization directions of the F layers can be controlled separately, this leads to the so-called spin switch. Here we show that domain walls in a single F layer yield a similar effect. We study the transport properties of Ni(0.80)Fe(0.20)/Nb bilayers structured in strips of different sizes. For large samples a clear enhancement of superconductivity takes place in the resistive transition, in the very narrow field range (order of 0.5 mT) where the magnetization of the Py layer switches and many domains are present. This effect is absent in microstructured samples.

Magnetic coupling in superconducting spin valves with strong ferromagnets

“…A special remark concerns the point that no traces are found of an enhancement of the superconductivity by domain averaging from Cooper pairs, as found before both in CuNi-and Py-based devices, 6,7 and also more recently in a different set of Py-based devices. 20 In particular, for the large-area bilayer sample BL20, enhanced superconductivity in the domain state might have been expected.…”

“…6 This is basically due to the fact that Cooper pairs break less easily when they sample an inhomogeneous exchange field, which can be considered a lateral variant of the spin-valve effect. 7 For spin-valve systems with strong ferromagnets, the situation is more complicated. In a number of reports, with magnets such as Ni ͑Ref.…”

Theory of domain-wall superconductivity in superconductor/ferromagnet bilayers

“…Naturally, it will be smaller near the domain wall and we may expect that superconductivity would be more robust near them. The local increase of the critical temperature in presence of magnetic domains was observed experimentally in Ni 0.80 Fe 0.20 /Nb bilayers (with Nb thickness around 20 nm) 7 , and it was attributed to DWS formation.…”

Domain-wall enhancement of superconductivity in superconductor/ferromagnet hybrids: Case of weak ferromagnets

“…It is interesting to remark that this difference would not be observable in Nb/Py, since in that case the switching was in such a small field range ͑order 10 −4 T͒ that a possible averaging difference would go unnoticed. 22 That leaves an explanation for the kink feature in R s ͑H͒. Looking again at Fig.…”

“…Changes in T c are mostly less than 10 mK, and in bilayer experiments involving Py it was demonstrated that the relative enhancement of superconductivity over domains walls resulted in a similarsized increase in T c . 22 For the weak ferromagnets such as Cu 1−x Ni x ͑with x in the range 0.5-0.6͒, which played such an important role both in demonstrating junctions and ͑appar-ent͒ spin switching, it was not yet investigated whether domains in the F layer can enhance superconductivity in a similar way.…”