OPENSUBJECT AREAS:
SPINTRONICS
SUPERCONDUCTING DEVICES

Significant change of spin transport
property in Cu/Nb bilayer due tosuperconducting transition
Kohei Ohnishi1,2, Yuma Ono1, Tatsuya Nomura1 & Takashi Kimura1,2,3

Received
17 June 2014
Accepted
11 August 2014
Published
2 September 2014

Correspondence andrequests for materials
should be addressed toT.K. (t-kimu@phys.
kyushu-u.ac.jp)

1

Department of Physics, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan, 2Research Center for Quantum Nano-Spin
Sciences, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan, 3CREST, Japan Science and Technology Agency,
Sanbancho, Tokyo 102-0075, Japan.

The combination between the spin-dependent and super-conducting (SC) transports is expected to provide
intriguing properties such as crossed Andreev reflection and spin-triplet superconductivity. This may beable to open a new avenue in the field of spintronics, namely superconducting spintronics because asuperconductor itself has great potential for future nanoelectronic applications. To observe such SC spintransports, the suppression of the extrinsic effects originating from the heating and Oersted field due to theelectric current is a crucial role. Pure spin current without accompanying the charge current is known as a
powerful mean for preventing such extrinsic effects. However, non-negligible heat flow is found to exist evenin a conventional pure spin current device based on laterally-configured spin valve because of the heatingaround the spin injector. Here, we develop a nanopillar-based lateral spin valve, which significantly reduces
the heat generation, on a superconducting Nb film. By using this ideal platform, we found that the spin
absorption is strongly suppressed by the SC transition of Nb. This demonstration is the clear evidence thatthe super-conducting Nb is an insulator for the pure spin current.

M

anifestation of the conduction electrons in a solid state is one of the most important aspects for theelectrical transports in the condensed matter physics. The strong correlation between the electrontransport properties and the electron spin, namely spin-dependent transports, has received the considerable attentions after the discovery of giant magneto-resistance (GMR) effect1–3. Owing to the developments ofthe theoretical understanding and experimental techniques, the spin current, which is a flow of the spin angularmomentum, is found to play the central role for the electron transports in ferromagnetic/nonmagnetic hybridnanostructures. Especially, a pure spin current, which is a spin current without accompanying the charge current,
enables the precise and sensitive detection of the spin-related phenomena such as spin Hall4–6 and spin Seebeck7,8effects.
Spin currents, so far, have been investigated mainly in normal-conductor/ and semiconductor/ferromagnet
hybrid systems intensively. The combination between the spin and super-conducting (SC) transports is also
expected to provide intriguing transport properties, leading to an emerging field in spintronics because a superconductor itself has great potential for future nanoelectronic applications such as Josephson circuit and quantum
computing9,10. Although a number of experimental studies have been performed11–21, several intriguing phenomena suggested in theoretical study have not been demonstrated22–24. Moreover, the experimental study on subgap
transports such as a crossed Andreev reflection and a spin triplet proximity effect have still exciting topics insuperconducting spintronics because of their fantastic properties25–29.
To investigate aforementioned phenomena in more detail, use of the pure spin currents yields several advantages compared with the spin-polarized current because the charge current induces spin-independent extrinsic
effects such as the Joule heating and Oersted field15,16. The pure spin current is, in general, generated by nonlocal
electrical spin injection using the laterally configured ferromagnetic/nonmagnetic hybrid structures30. However,even in such structures, the influence of the heat around the injecting junction significantly affects the superconducting properties. In fact, the bias current in the lateral spin valves is found to induce the strong suppressionof the superconducting gap21. As a result, the spin polarized electron can enter the superconductor as quasiparticles. Moreover, the geometrical disorders in laterally configured multi-terminal devices may produce theextrinsic spin scattering. These deteriorations make it difficult to observe the drastic change of the spin transport
by the superconducting transition. Therefore, an improved device structure is required for further developments
SCIENTIFIC REPORTS | 4 : 6260 | DOI: 10.1038/srep06260

1www.nature.com/scientificreports

Figure 1 | Scanning electron microscope image together with a schematic illustration of the prepared nano-pillar-based lateral spin valve. Two Py
nanopillars are placed on the Cu/Nb bilayer film and are connected to the top Cu electrodes.

of superconducting spintronics. We have recently developed an
innovative device structure, nanopillar-based lateral spin valve, forgenerating the nonlocal spin current efficiently31,32. In this structure,
the heat from the spin injector can be suppressed significantly
because of the minimized channel length of the spin injector. In
the present letter, we investigate the transport properties of the purespin current in a nonmagnetic/superconducting interface by
employing the nanopillar-based lateral spin valve structure.
Figure 1 shows a scanning electron microscope image of the fabricated device together with the schematic illustration of the device.
First, we have prepared the Py/Cu/Nb trilayer film by using a highvacuum sputtering and evaporation system with the base pressureless than 2 3 1025 Pa. Here, Nb and Py are 45-nm thick and 30-nm
thick, respectively, and were deposited by the magnetron sputterings.
The Cu spacer whose thickness is 100 nm was deposited by the Jouleevaporation. Then, the trilayer film was patterned into the nanopillar
structure by using electron-beam lithography and Ar ion milling.
The detail of the fabrication procedure is described in our previous
paper31. The lateral dimensions for two ferromagnetic nanopillars are
270 3 220 nm2 and 340 3 240 nm2, and the center-center distance
between two ferromagnetic nanopillars is approximately 500 nm.
Since the trilayer was prepared by in-situ multi-layered film grownsystem, the interfaces between each metal have ideal conditions.
To evaluate the transport property of the prepared film, we firstmeasured the temperature dependence of the resistance for the Cu/
Nb bilayer by using the probe configuration shown in Fig. 2(b). Here,the measurement was performed by the current-bias lock-in technique with the current amplitude of 180 mA. As shown in Fig. 2(a),the resistance shows a typical normal conducting (NC) behavior withthe residual resistance ratio of 3.2 above 7.1 K and becomes the zeroresistive superconducting state below 7.1 K. These characteristics
indicate that the current flows mainly in the Cu film above 7.1 K
and the current flows in the superconducting (SC) Nb film below
7.1 K, as schematically shown in Fig. 2(b). The transition temperature higher than the typical values in the patterned Nb thin films
and the abrupt resistance change at 7.1 K indicate that the Nb layer
has great performances as the superconductor33.
We then evaluated the spin transport by mean of the nonlocal spin
valve measurement using the Py nanopillars. Here, the injection of
the spin-polarized electrons from the Py nanopillar (Py1) into the
Cu/Nb bilayer produces the spin current both in the left-hand andright-hand side. The pure spin current is produced in the right-handside, and can be detected by measuring the electrical voltage using
another Py nanopillar (Py2)32. The voltage difference between parallel and anti-parallel states divided by the injecting current is known
as a spin signal. The detected spin signal reflects the magnitude of thespin current at the Py2/Cu interface. Before showing the results in thebilayer film, we show a typical nonlocal spin valve signal in the Py/Cu
nanopillar lateral spin valve without Nb layer in Fig. 3(a), as a reference. A spin signal with the magnitude of 0.85 mV was clearlyobserved at 10 K. The spatial homogeneity of the spin injection using
SCIENTIFIC REPORTS | 4 : 6260 | DOI: 10.1038/srep06260

the Py nanopillar was evaluated by changing the current- and voltage-probe positions in the nonmagnetic channels. We confirmedthat the obtained spin signals in two configurations are almost same.
This indicates that the Py nanopillar produce the homogeneous spin
accumulation underneath the injecting electrode. In the Cu/Nbbilayer film, since the spin relaxation process in the Cu channel isaffected by the electron conducting state in the Nb, the spin signal
should be modified from that in the reference sample34. In addition,
the significant change should be observed in the spin signal by thetransition from the NC to the SC in the Nb layer.
We then show the results for the spin transports in the Cu/Nbbilayer film. Figure 3(b) shows the nonlocal spin valve curve measured at 10 K with the current amplitude of 180 mA. Although theclear resistance change between the parallel and anti-parallel state
was observed, the obtained spin signal is 0.17 mV. This strong reduction of spin signal clearly indicates that the NC Nb layer strongly
absorbs the spin current flowing in the Cu layer because of the strong
spin relaxation of the Nb, as schematically shown in the left-handside of Fig. 46,16,34. The nonlocal spin valve curve measured at 2.3 K isshown in Fig. 3(c). Here, the bias current is also 180 mA. The spinsignal is enhanced by a factor of 5 with respect to that at 10 K,
indicating that the spin absorption is strongly suppressed by the
SC transition of the Nb layer. This can be understood as follows.
At the NC Cu/SC Nb interface, in order to inject the electrons into
the superconductor, the electrons have to be transformed into a
Cooper pair consisting of two electrons with opposite spins11,35.
However, the Cooper pairs cannot be formed from the pure spincurrent, in which the up-spin and down-spin electrons flow oppositely each other, as schematically shown in the right-hand side ofFig. 4. This is a clear demonstration that the SC gap suppresses thespin current. It also should be noted that the background resistanceof the nonlocal signal at 2.3 K was almost zero while the large background around 40 mV was observed at 10 K. The large backgroundFigure 2 | (a) Temperature dependence of the bilayer resistance. At 7.1 K,
the sharp transition was observed as in the inset. (b) Schematic illustrations
for the current flowing condition above 7.1 K (top) and below 7.1 K(bottom).
2

www.nature.com/scientificreports(b) w/ NC Nb(c) w/ SC Nb

Figure 3 | (a) Nonlocal spin valve curve measured at 10 K in the nanopillar based Py/Cu lateral spin valve without the Nb layer together with the probe
configuration for the measurement (right). Nonlocal spin valve curves measured at 10 K (NC state) (b) and 2.3 K (SC state) (c) together with therespective probe configurations.

at 10 K is caused by the spreading of the electric flux line originatingfrom the quasi-two dimensional nonmagnetic Cu channel. But, it canbe eliminated by the superconducting Nb channel. Thus, the SC Nb
layer efficiently transforms the charge current into the super current.
We also mention that the magnitude of the spin signals both at 2.3 K
and 10 K was almost constant under the bias current below 300 mA.
This is a strong evidence that the nanopillar structure developed here
strongly reduces the heat generation31.
Finally, we focus on the spin transport in the Cu layer in which the
Cooper pairs exist because of the proximity effect. Influence of the
Cooper pair on the non-equilibrium spin accumulation was studied.
Figure 5 shows the magnetic field dependence of the contact resistance for the Py1/Cu junction. The probe configuration for this measurement is shown in the inset. The small abrupt resistance changes
in low magnetic field are due to the anisotropic magnetoresistance of

Figure 4 | Schematic illustrations of the spatial distribution of theelectro-chemical potential at the Nb/Cu interface. The NC Nb strongly
absorbs the spin current, resulting in the large reduction of the spin
accumulation in the Cu. The SC Nb cannot absorbs the spin current
because of the superconducting gap.
SCIENTIFIC REPORTS | 4 : 6260 | DOI: 10.1038/srep06260

the Py nanopillar. The gradual reversible resistance change below
100 mT can be understood by the field dependence of the resistancefor the Cu layer. Owing to the proximity effect from the Nb, thecurrent flowing around the Cu/Nb interface becomes the super current. As a result, the resistance of the Cu around the interface
becomes zero in the absence of the magnetic field. However, thesuper current induced by the proximity effect is easily spoiled byapplying the magnetic field. Thus, the gradual field dependence ofthe resistance is the evidence that Cooper pairs exist in the Cu chan-

Figure 5 | Magnetic field dependence of the contact resistance of the Py1/
Cu junction together with the probe configuration for this measurement.
The insets show the schematic illustration of the conducting state in the
Cu/Nb layer.
3www.nature.com/scientificreports
nel. It should be noted that the field dependence of the nonlocal spin
valve signal does not show such a gradual change. This indicates thatthe spin relaxation process in the Cu layer is not affected by theproximity effect from the superconductor.
In summary, we have developed an innovative device structure forinvestigating the transport of the pure spin currents in a superconductor/ferromagnet hybrid structure. We experimentally demonstrated that the pure spin current is effectively reflected by thesuperconducting Nb although the normal-conducting Nb effectively
absorbs the pure spin current. Using the proximity effect, we wereable to stabilize the Cooper pairs with the non-equilibrium spin
accumulation in the Cu. We found that the spin relaxation process
in the Cu layer is not affected by the Cooper pairs.
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Acknowledgments
The authors (KO and TK) would like to thank Dr. S. Maekawa and S. Takahashi for theirvaluable comments. This work is partially supported by Grant-in-Aid for Scientific
Research (S) (25220605), CANON Foundation and CREST.

Author contributions
Y.O. and T.N. carried out the experimental work, including the preparation of the sample.
K.O. carried out the analysis of the experimental results. K.O. and T.K. supervised theexperimental research and wrote the main manuscript text including the figure
preparations.

Additional information
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Ohnishi, K., Ono, Y., Nomura, T. & Kimura, T. Significant changeof spin transport property in Cu/Nb bilayer due to superconducting transition. Sci. Rep. 4,6260; DOI:10.1038/srep06260 (2014).
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