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Significant change of spin transport property in Cu/Nb bilayer due to superconducting transition.

Ohnishi K, Ono Y, Nomura T, Kimura T - Sci Rep (2014)

Bottom Line: To observe such SC spin transports, the suppression of the extrinsic effects originating from the heating and Oersted field due to the electric current is a crucial role.Pure spin current without accompanying the charge current is known as a powerful mean for preventing such extrinsic effects.By using this ideal platform, we found that the spin absorption is strongly suppressed by the SC transition of Nb.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Physics, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan [2] Research Center for Quantum Nano-Spin Sciences, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan.

ABSTRACT
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 be able to open a new avenue in the field of spintronics, namely superconducting spintronics because a superconductor itself has great potential for future nanoelectronic applications. To observe such SC spin transports, the suppression of the extrinsic effects originating from the heating and Oersted field due to the electric 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 even in a conventional pure spin current device based on laterally-configured spin valve because of the heating around 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 that the super-conducting Nb is an insulator for the pure spin current.

No MeSH data available.


Related in: MedlinePlus

Schematic illustrations of the spatial distribution of the electro-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.
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f4: Schematic illustrations of the spatial distribution of the electro-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.

Mentions: We then show the results for the spin transports in the Cu/Nb bilayer film. Figure 3(b) shows the nonlocal spin valve curve measured at 10 K with the current amplitude of 180 μA. Although the clear resistance change between the parallel and anti-parallel state was observed, the obtained spin signal is 0.17 mΩ. 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-hand side of Fig. 461634. The nonlocal spin valve curve measured at 2.3 K is shown in Fig. 3(c). Here, the bias current is also 180 μA. The spin signal 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 spins1135. However, the Cooper pairs cannot be formed from the pure spin current, in which the up-spin and down-spin electrons flow oppositely each other, as schematically shown in the right-hand side of Fig. 4. This is a clear demonstration that the SC gap suppresses the spin current. It also should be noted that the background resistance of the nonlocal signal at 2.3 K was almost zero while the large background around 40 mΩ was observed at 10 K. The large background at 10 K is caused by the spreading of the electric flux line originating from the quasi-two dimensional nonmagnetic Cu channel. But, it can be 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 μA. This is a strong evidence that the nanopillar structure developed here strongly reduces the heat generation31.


Significant change of spin transport property in Cu/Nb bilayer due to superconducting transition.

Ohnishi K, Ono Y, Nomura T, Kimura T - Sci Rep (2014)

Schematic illustrations of the spatial distribution of the electro-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.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4151101&req=5

f4: Schematic illustrations of the spatial distribution of the electro-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.
Mentions: We then show the results for the spin transports in the Cu/Nb bilayer film. Figure 3(b) shows the nonlocal spin valve curve measured at 10 K with the current amplitude of 180 μA. Although the clear resistance change between the parallel and anti-parallel state was observed, the obtained spin signal is 0.17 mΩ. 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-hand side of Fig. 461634. The nonlocal spin valve curve measured at 2.3 K is shown in Fig. 3(c). Here, the bias current is also 180 μA. The spin signal 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 spins1135. However, the Cooper pairs cannot be formed from the pure spin current, in which the up-spin and down-spin electrons flow oppositely each other, as schematically shown in the right-hand side of Fig. 4. This is a clear demonstration that the SC gap suppresses the spin current. It also should be noted that the background resistance of the nonlocal signal at 2.3 K was almost zero while the large background around 40 mΩ was observed at 10 K. The large background at 10 K is caused by the spreading of the electric flux line originating from the quasi-two dimensional nonmagnetic Cu channel. But, it can be 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 μA. This is a strong evidence that the nanopillar structure developed here strongly reduces the heat generation31.

Bottom Line: To observe such SC spin transports, the suppression of the extrinsic effects originating from the heating and Oersted field due to the electric current is a crucial role.Pure spin current without accompanying the charge current is known as a powerful mean for preventing such extrinsic effects.By using this ideal platform, we found that the spin absorption is strongly suppressed by the SC transition of Nb.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Physics, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan [2] Research Center for Quantum Nano-Spin Sciences, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan.

ABSTRACT
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 be able to open a new avenue in the field of spintronics, namely superconducting spintronics because a superconductor itself has great potential for future nanoelectronic applications. To observe such SC spin transports, the suppression of the extrinsic effects originating from the heating and Oersted field due to the electric 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 even in a conventional pure spin current device based on laterally-configured spin valve because of the heating around 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 that the super-conducting Nb is an insulator for the pure spin current.

No MeSH data available.


Related in: MedlinePlus