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Sandwich structures

Superconducting Spin-Valve-Effects in Nanoscale Ferromagnet-Superconductor-Multilayers

Fulde-Ferrell-Larkin-Ovchinikov (FFLO) State

Singlet superconductivity and ferromagnetism are usually mutually exclusive, because of superconductivity requires the electron spins to be aligned antiparallel in order to create Cooper pairs, whereas the ferromagnetic exchange field tends to align them parallel. Nevertheless, Fulde-Ferell [1] and Larkin-Ovchinikov [2]  predicted superconductivity on a ferromagnetic background, however with non-vanishing momentum of the Cooper pairs. A FFLO-like pairing can be generated in superconductor (S) / ferromagnet (F) nanolayers, which results in plenty of new physical effects, such as the oscillation of the superconducting transition temperature as a function of the ferromagnetic layer thickness, or even a complete extinction and recovery of superconductivity as a result of the interference of the superconducting pairing wave function (Fig. 1). An experimental proof of these effects is shown in Fig. 2 [3]. Such phenomena also appear in F/S/F trilayers, which represent the core of a superconducting spin valve structure [4].

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Fig. 1: a), b) and c) Oscillation of the pairing wave function Φ, d) Oszillation of the superconducting transition temperature Tc in a superconductor (S) – ferromagnet (F) bilayer. Due to the reflection of the pairing wave function at the surface of the ferromagnet it interferes with itself leading to destructive (a) and c)) and constructive (b)) interference, depending on the thickness of the ferromagnetic layer dF. This leads to the oscillatory dependence of the superconducting transition temperature on the thickness of the ferromagnetic layer (d)).

2

Fig. 2: First experimental proof of reentrant superconductivity in a S/F bilayer, i.e. the recovery of superconductivity at higher ferromagnetic layer thickness after complete extinction. The solid and dashed lines show theoretical fits for the „clean“ and „dirty“ case of the ferromagnetic layer, respectively. [3].

 

Singlet Spin-Valve

Based on these effects, it is in principle possible to realize a spin-valve. For this purpose, an antiferromagnet-ferromagnet-superconductor-ferromagnet (AF-FSF) system (Fig. 3) is investigated, with the antiferromagnet pinning the magnetization direction of one ferromagnetic layer. Thus, different relative magnetic orientations of the ferromagnetic layers can be realized by making use of an external magnetic field. The superconducting transition temperature varies for parallel and antiparallel orientation of the magnetizations of the ferromagnetic layers (Fig. 4). For a given working temperature, the system can be switched by an external magnetic field from a superconducting into a normalconducting state and vice versa [5].

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Fig. 3: Schematic design of a AF-FSF sample series. The individual samples were cut along the dotted lines.

4

Fig. 4: Theoretical calculation of the superconducting transition temperature Tc in a FSF-type spin valve. The dashed and dotted lines show the behavior of Tc as a function of the ferromagnetic layer thickness for the parallel and antiparallel alignment of the magnetizations, respectively, while the red solid line shows the difference between both and, thus, the magnitude of the spin valve effect [5].

 

Triplet Spin-Valve

Beside the effects described above, the interplay between an inhomogenous magnetization and a FFLO-like superconducting state can generate s-wave triplet superconductivity. The electron spins of the Cooper pairs of this triplet state are parallel, so that the Cooper pairs are not broken due to the concurring spin ordering mechanisms. This leads to a long-range triplet component of superconductivity. This triplet component, generated at non-collinear orientations of the magnetizations of two ferromagnetic layers, can lead to the formation of a minimum in the superconducting transition temperature at angles between the antiparallel and parallel orientation (triplet spin-valve effect) [6]. This effect is investigated in a superconductor-ferromagnet-normalconductor (N) -ferromagnet-antiferromagnet (SF1-N-F2-AF) system (Fig. 5). The normalconducting (nc) layer is necessary to decouple the two ferromagnetic layers (F1, F2). A comparison of experimental results with the theoretical prediction is shown in Fig. 6. [7].

 

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Fig. 5: Schematic design of a SF1NF2-AF sample series. The individual samples were cut along the dotted lines.

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Fig. 6: (a) Angular dependence of the reduced superconducting transition temperature Tc/Tc0 according to the model in [6]; here Tc0 is the superconducting transition temperature of an isolated S film. (b) Experimental data on the dependence of the superconducting transition temperature Tc on the applied magnetic field. The correspondent resulting magnetic configurations of both ferromagnetic layers are indicated by pictograms and numbers in red circles. The arrows show the direction of the magnetic field sweep. [7]

 

Current Investigations

Currently, the appearance of triplet superconductivity, as well as memory effects (i.e. the resistance of the sample depends on the magnetic history), arising from specific magnetic configurations, are investigated in AF/FSF type spin valve structures. [8]. Furthermore, the basic understanding of S/F systems is deepened. For this purpose e.g. the upper critical field, as well as thermal activated flux flow and the influence of an applied magnetic field on the oscillations of the critical temperature are investigated in S/F samples.

 

The AG Nanoscale Layered Structures

The AG Nanonscale Layered Structures investigates the effects described above in superconductor-ferromagnet layered structures. The samples are prepared, characterized electrically at low temperatures in a He4 or He3 evaporation cryostat or in a He3-He4 dilution refrigerator, as well as structurally, with Rutherford-Backscattering-Spectrometry (RBS) (Fig. 7) and transmission electron microscopy (TEM) (Fig. 8), and magnetically with a superconducting quantum interference device (SQUID) magnetometer (Fig. 9).

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Fig. 7: RBS spectrum of a sample of a FSF sample series. The peaks are related to the correspondent layers. [4]

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Fig. 8: Cross-sectional TEM picture of a sample of a FSF sample series. [4]

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Fig. 9: Magnetic moment of a sample of a SF1NF2-AF sample series (Nb/Cu41Ni59/nc-Nb/Co/CoOx). While (a) shows the hysteresis of the system as a whole, (b) shows the contribution of both F layers seperately. The shift of the blue curve (Co) due to the exchange biasing by the antiferromagnet is obvious. Due to the easy magnetization direction of the Cu41Ni59 layer (red curve) being perpendicular to the film plane (which can be concluded from measurements with different orientations of the magnetic field (see insert)), the magnetic orientations shown as pictograms can be concluded. [7]

Literature:

[1] P. Fulde and R. A. Ferrell, Superconductivity in a Strong Spin-Exchange Field, Phys. Rev. A 135, A550 (1964).

[2] A. I. Larkin and Y. N. Ovchinnikov, Inhomogenous State of Superconductors, ZhETF 47, 1136 (1964) [Sov. Phys. JETP 20, 762 (1965)].

[3] V. I. Zdravkov, A. Sidorenko, G. Obermeier, S. Gsell, M. Schreck, C. Müller, S. Horn, R. Tidecks, and L. R. Tagirov, Reentrant Superconductivity in Nb/Cu1-xNix Bilayers, Phys. Rev. Lett 97, 057004 (2006).

[4] J. Kehrle, V.I. Zdravkov, G. Obermeier, J. Garcia-Garcia, A. Ullrich, C. Müller, R. Morari, A. S. Sidorenko, S. Horn, L. R. Tagirov, and R. Tidecks, Critical Temperature Oscillations and Reentrant Superconductivity Due to the FFLO Like State in F/S/F Trilayers, Ann. Phys. (Berlin) 524/1, 37 (2012).

[5] V.I. Zdravkov, J. Kehrle, G. Obermeier, S. Gsell, M. Schreck, C. Müller,   H. A. Krug Von Nidda, J. Lindner, J. Moosburger-Will, E. Nold, R. Morari, V. V. Ryazanov, A. S. Sidorenko, S. Horn, R. Tidecks, and L. R. Tagirov, Reentrant Superconductivity in Superconductor/Ferromagnetic-Alloy Bilayers, Phys. Rev. B 82, 054517 (2010).

[6] Y.V. Fominov, A.A. Golubov, T.Y. Karminskaya, M.Yu. Kupriyanov, R.G. Deminov and L.R. Tagirov, Superconducting Triplet Spin Valve, Pis'ma v ZhETF 91, 329 (2010) [JETP Lett. 91, 308 (2010)].

[7] V.I. Zdravkov, J. Kehrle, G. Obermeier, D. Lenk, H.-A. Krug von Nidda, C. Müller, M.Yu. Kupriyanov, A.S. Sidorenko, S. Horn, R. Tidecks, L.R. Tagirov, Experimental Observation of the Triplet Spin-Valve Effect in a Superconductor-Ferromagnet Heterostructure, Phys. Rev. B 87, 144507 (2012)

[8] V.I. Zdravkov, D. Lenk, R. Morari, A. Ullrich, G. Obermeier, C. Müller, H.­A. Krug von Nidda, A.S. Sidorenko, S. Horn, R. Tidecks, and L.R. Tagirov, Memory Effect and Triplet Pairing Generation in the Superconducting Exchange Biased Co/CoOx/Cu41Ni59/Nb/Cu41Ni59 Layered Heterostructure, Appl. Phys. Lett. 103, 062604 (2013)

 

Contact person:

Reinhard Tidecks

Group members:

Daniel Lenk

Claus Müller

Günter Obermeier

Aladin Ullrich

Vladimir Zdravkov

Alumni:

Mamoun Hemmida

Jan Kehrle

Thomas Mairoser

Cooperations:

Experimentalphysik IV:

Wolfgang Reiber

Birgit Knoblich

Experimentalphysik V:

Hans-Albrecht Krug von Nidda

Dana Vieweg

Experimentalphysik VI:

Alexander Herrnberger

External cooperations:

Solid State Physics Department, Kazan Federal University, Kazan, Russia:

Lenar R. Tagirov

D. Ghitsu Institute of Electronic Engineering and Nanotechnologies ASM, Kishinev, Moldova:

Anatoli S. Sidorenko

Roman Morari