Superconducting proximity effect in a mesoscopic ferromagnetic wire

Abstract

We present an experimental study of the transport properties of a ferromagnetic metallic wire (Co) in metallic contact with a superconductor (Al). As the temperature is decreased below the Al superconducting transition, the Co resistance exhibits a significant dependence on both temperature and voltage. The differential resistance data show that the decay length for the proximity effect is much larger than we would simply expect from the exchange field of the ferromagnet. 74.50.+r, 74.80.Fp, 85.30St Superconducting proximity effect consists in inducing superconductive properties in a non-superconducting metal. Although this effect has been studied for a long time 1 , it has gained some renewed interest due to recent experiments performed on samples of mesoscopic size. In such samples, the electron phase-breaking length L ϕ is larger than the sample length L. One can thus probe experimentally the characteristic energy scale of the proximity effect ǫ c =hD/L 2 , which is the Thouless energy related to the sample length. This has led for instance to the observation of large magnetoresistance oscillations in normal metal (N) loops in contact with a superconducting (S) island 2-4 . These oscillations provide a direct evidence for the long-range (up to L ϕ ) nature of the proximity effect. Another recent and striking result is the reentrant behaviour. The excess conductance induced by proximity effect is maximum at a temperature or a bias voltage equivalent to the sample Thouless energy 5 , but the normal state conductance reappears at lower energy.Most experiments were performed in noble metals or semiconductor 2D electron gas, where electron interactions are negligible. In a free electron model, the zerotemperature, zero-bias resistance of a mesoscopic metallic wire is predicted to recover the normal state value 6-8 . In the presence of interactions, theoretical studies 7,9 predict a severe modification of the transport properties. Attractive (respectively repulsive) electron-electron interactions are believed to result in a resistance lower (respectively higher) than the normal-state one 7 . This could provide a probe for interactions in normal metals like Au, Ag, etc 10 . In this communication, we present an experimental study of the superconducting proximity effect in a ferromagnetic metal (F). Magnetic metals are in the strong interaction limit. Exchange interactions between electrons in a ferromagnet usually lead to efficient Cooperpair breaking in F-S structures. However, it is worthwhile re-examining the actual proximity effect in a small ferromagnetic wire 11 . Some experiments 12,13 suggested long-range coherence effects, but without any clear conclusion.

Transfer-matrix description of heterostructures involving superconductors and ferromagnets

“…Therefore, the above simple picture has to be modified when the normal metal is replaced by a ferromagnet with two different Fermi surfaces for the two spins, resulting in new and interesting physical phenomena. In recent years, interplay between superconductivity and ferromagnetism has attracted considerable theoretical 2,3,4,5 and experimental 6,7,8,9 attention -both out of fundamental scientific interest and in view of the possibility of novel applications and devices. One important consequence of the spin splitting between the two bands in the ferromagnet is that the phase coherence between the particle-hole pair in the clean (dirty) limit is destroyed over a characteristic distance of v f /h ( D/h), where v f is the Fermi velocity, D the diffusion constant, and h an effective exchange energy which describes the spin splitting.…”

Long-Range Proximity Effects in Superconductor-Ferromagnet Structures

“…Although the singlet component of the condensate penetrates into the ferromagnet over a short length ξ h = D/h (h is the exchange field in the ferromagnet and D the diffusion coefficient), the triplet component, being of the order of the singlet one at the S/F interface, penetrates over a long length D/ǫ (ǫ is the energy). This long-range penetration leads to a significant increase of the ferromagnet conductance below the superconducting critical temperature Tc.In recent experiments on S/F structures a considerable increase of the conductance below the superconducting critical temperature T c was observed [1][2][3]. Although in a recent work [4] it was suggested that such an increase may be due to scattering at the S/F interface, a careful measurement of the conductance demonstrated that the entire change of the conductance was due to an increase of the conductivity of the ferromagnet [1,2].…”

“…In recent experiments on S/F structures a considerable increase of the conductance below the superconducting critical temperature T c was observed [1][2][3]. Although in a recent work [4] it was suggested that such an increase may be due to scattering at the S/F interface, a careful measurement of the conductance demonstrated that the entire change of the conductance was due to an increase of the conductivity of the ferromagnet [1,2].…”

“…Although in a recent work [4] it was suggested that such an increase may be due to scattering at the S/F interface, a careful measurement of the conductance demonstrated that the entire change of the conductance was due to an increase of the conductivity of the ferromagnet [1,2].…”

Microscopic theory for a ferromagnetic nanowire/superconductor heterostructure: Transport, fluctuations, and topological superconductivity

“…16 However, a number of recent experimental studies have demonstrated an unexpectedly long-ranged proximity effect in mesoscopic superconductor-ferromagnet hybrid structures. [17][18][19][20][21][22] There are a number of theoretical scenarios which can explain this phenomenon. In most of the scenarios pair correlations inside the ferromagnet are attributed to some type of equal-spin triplet pairing.…”