Superconducting Spin Switch with Infinite Magnetoresistance Induced by an Internal Exchange Field

Li, Bin; Roschewsky, Niklas; Assaf, Badih A.; Eich, Marius; Epstein-Martin, Marguerite; Heiman, D.; Münzenberg, Markus; Moodera, Jagadeesh S.

doi:10.1103/physrevlett.110.097001

Cited by 118 publications

(102 citation statements)

References 38 publications

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“…Such a behavior has been confirmed by analysis of the inverse proximity effect (i.e., suppression of superconductivity in an S-layer in contact with a ferromagnet) for F/S/F [16,17] and S/F/F [18] structures.…”

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confidence: 66%

Controllable generation of a spin-triplet supercurrent in a Josephson spin valve

2014

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It has been predicted theoretically that an unconventional odd-frequency spin-triplet component of superconducting order parameter can be induced in multilayered ferromagnetic structures with non-collinear magnetization. In this work we study experimentally nano-scale devices, in which a ferromagnetic spin valve is embedded into a Josephson junction. We demonstrate two ways of in-situ analysis of such Josephson spin valves: via magnetoresistance measurements and via in-situ magnetometry based on flux quantization in the junction. We observe that supercurrent through the device depends on the relative orientation of magnetization of the two ferromagnetic layers and is enhanced in the non-collinear state of the spin valve. This provides a direct prove of controllable generation of the spin-triplet superconducting component in a ferromagnet.An interplay of superconductivity (S) and ferromagnetism (F) in hybrid S/F heterostructures leads to a variety of unusual physical phenomena [1][2][3][4][5][6][7][8][9][10]. Of particular interest is a possibility of generation of an unconventional odd-frequency spin-triplet component of the superconducting condensate [2,7]. The ferromagnetic exchange energy is usually much larger than the superconducting energy gap. Consequently, a conventional spin-singlet superconducting order parameter decays at a short range ∼ 1 nm in a spatially uniform, mono-domain ferromagnet. Experimental observations of a long-range proximity effect through strong ferromagnets [11,12] and, in particular, through almost fully spin-polarized half-metals [13][14][15] is consistent with appearance of the spin-triplet component, which is insensitive to strong magnetic and exchange fields. However, it may also be due to various types of artifacts and, at certain circumstances, a longrange spin-singlet component can be realized in clean S/F heterostructures [9]. Therefore, unambiguous confirmation for existence of the spin-triplet superconductivity in S/F heterostructures requires controllable tunability of the phenomenon. This is also prerequisite for potential applications of S/F heterostructures in spintronics.The spin-triplet order parameter in S/F heterostructures is generated in presence of an active spin-mixing interface [5,7] or in case of a spatially non-uniform distribution of magnetization [2]. The latter can be achieved in spin valve structures with several F-layers [1,3,6,[8][9][10]. Both the spin-singlet and the spin-triplet components depend on the angle between magnetization of F-layers in such superconducting spin valves. The spin-singlet component is at maximum for the antiparallel (AP) and minimum at the parallel (P) state of the spin valve [9]. The spin-triplet component is maximum at the non-collinear state with 90• misalignment between magnetic moments and zero both in P-and AP-states [3,8]. Such a behavior has been confirmed by analysis of the inverse proximity effect (i.e

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confidence: 66%

Controllable generation of a spin-triplet supercurrent in a Josephson spin valve

2014

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“…Good candidates for the tunneling barriers are europium chalcogenides (EuO and EuS) for which values of P ranging from 80 up to almost 100% have been reported. [13][14][15][16][18][19][20][21] Also very high spin-filtering has been reported in GdN barriers [22][23][24] with polarizations as large as 97% at 15 K (even larger values are expected as T decreases).…”

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confidence: 99%

“…Values from 0.1 meV up to few meV for the Zeeman splitting have been reported. 16,21,25,26 We emphasize that the amplitude of the thermoelectric effect also depends on the length of the N wire. We assumed that L is smaller than the superconducting coherence length [see Eq.…”

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confidence: 99%

Proposal for a phase-coherent thermoelectric transistor

Giazotto

Robinson

Moodera

et al. 2014

Applied Physics Letters

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Identifying materials and devices which offer efficient thermoelectric effects at low temperature is a major obstacle for the development of thermal management strategies for low-temperature electronic systems. Superconductors cannot offer a solution since their near perfect electron-hole symmetry leads to a negligible thermoelectric response; however, here we demonstrate theoretically a superconducting thermoelectric transistor which offers unparalleled figures of merit of up to ∼ 45 and Seebeck coefficients as large as a few mV/K at sub-Kelvin temperatures. The device is also phasetunable meaning its thermoelectric response for power generation can be precisely controlled with a small magnetic field. Our concept is based on a superconductor-normal metal-superconductor interferometer in which the normal metal weak-link is tunnel coupled to a ferromagnetic insulator and a Zeeman split superconductor . Upon application of an external magnetic flux, the interferometer enables phase-coherent manipulation of thermoelectric properties whilst offering efficiencies which approach the Carnot limit.It is known that electron-hole symmetry breaking is essential for a material to posses a finite thermoelectric figure of merit 1,2 . In principle, conventional superconductors have a near perfect symmetric spectrum and therefore are not suitable for thermoelectric devices. However, if the density of states is spin-split by a Zeeman field a superconductor-ferromagnet hybrid device can provide a thermoelectric effect 3-5 with a figure of merit close to 1. Here we propose a multifunctional phase-coherent superconducting transistor in which the thermoelectric efficiency is tunable through an externally applied magnetic flux. A giant Seebeck coefficient of several mV/K and a figure of merit close to ∼ 45 is predicted for realistic materials parameters and materials combinations.The phase-coherent thermoelectric transistor is based on two building blocks. The first one is sketched in Fig. 1(a) and consists of a superconducting film (S R ) tunnel-coupled to a normal metal (N) by a ferromagnetic insulator (FI). The latter induces an exchange field (h) in S R which leads to a Zeeman spin-split superconducting DoS. The spectrum for spin-up (↑) and spin-down (↓) electrons is given bywhere| is the conventional Bardeen-Cooper-Schrieffer DoS in a superconductor, E is the energy, ∆ R is the order parameter, and Γ accounts for broadening. Due to the presence of the spin-splitting field, ∆ R depends on temperature (T ) and h. While the total DoS of S R , ν S R (E) = ν S R↑ (E) + ν S R↓ (E), is electron-hole symmetric [ Fig. 1(b)], the spin-dependent a) Electronic mail: giazotto@sns.it b) Electronic mail: jjr33@cam.ac.uk c) Electronic mail: sebastian bergeret@ehu.es ν S R↑(↓) (E) components are no longer even functions of the energy. This means that electron-hole imbalance can, in principle, be achieved using a spin-filter contact with a normal metal. This would yield a finite thermoelectric effect 3,4 in the N/FI/S R heterostructure shown in F...

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“…[43][44][45][46] This type of barrier possesses very large spin-filter efficiencies (typically larger than 95%) and, therefore, they are ideal candidates for the creation of spin-polarized currents. In tunnel junctions made of superconducting electrodes and spin-filter barriers, measurements of the tunneling conductance have revealed that the interaction between conducting electrons in the leads and the localized magnetic moments of the barrier lead to a Zeeman splitting in the density of states of the superconducting electrodes, 43,47 as theoretically expected.…”

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confidence: 99%

Phase-dependent heat transport through magnetic Josephson tunnel junctions

Bergeret

Giazotto²

2013

Phys. Rev. B

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We present an exhaustive study of the coherent heat transport through superconductor-ferromagnet (SF) Josephson junctions including a spin-filter (I sf ) tunneling barrier. By using the quasiclassical Keldysh Green's function technique we derive a general expression for the heat current flowing through a SF-I sf -FS junction and analyze the dependence of the thermal conductance on the spin-filter efficiency, the phase difference between the superconductors and the magnetization direction of the ferromagnetic layers. In the case of noncollinear magnetizations we show explicitly the contributions to the heat current stemming from the singlet and triplet components of the superconducting condensate. We also demonstrate that the magnetothermal resistance ratio of a SF-I sf -FS heat valve can be increased by the spin-filter effect under suitable conditions.

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Superconducting Spin Switch with Infinite Magnetoresistance Induced by an Internal Exchange Field

Cited by 118 publications

References 38 publications

Controllable generation of a spin-triplet supercurrent in a Josephson spin valve

Controllable generation of a spin-triplet supercurrent in a Josephson spin valve

Proposal for a phase-coherent thermoelectric transistor

Phase-dependent heat transport through magnetic Josephson tunnel junctions

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