Enhanced spin pumping into superconductors provides evidence for superconducting pure spin currents

Jeon, Kun-Rok; Ciccarelli, Chiara; Ferguson, A. J.; Kurebayashi, Hidekazu; Cohen, L. F.; Montiel, X.; Eschrig, Matthias; Robinson, Jason W. A.; Blamire, M. G.

doi:10.1038/s41563-018-0058-9

Cited by 138 publications

(204 citation statements)

References 33 publications

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“…Interestingly, almost a decade ago, theoretical studies [15,16] suggested a time-varying magnetization M(t) of the FM as a reciprocal equivalent to M(x) for the generation of spin-polarized triplet supercurrents in a diffusive metallic FJJ [15] and also in a FM/NM/SC structure (NM: normal metal) [9]. However, subsequent ferromagnetic resonance (FMR) studies on Nb/Ni80Fe20 bilayers [17,18] and Nb/Ni80Fe20/Nb trilayers [19] have shown that spin angular momentum transfer in such structures is predominantly mediated by quasiparticles (QPs) for the superconducting state and thus largely suppressed at a lower temperature T by the development of singlet superconductivity and the freeze-out of available QP states [17,20,21]. This is likely because the magnitude of M(t) inhomogeneity or noncollinearity, parameterized by the magnetization precession angle θM, is too small (a few degrees at 10-20 GHz) [17][18][19] to yield the measurable effect of M(t)-induced triplet supercurrents [15,16].…”

Section: Introductionmentioning

confidence: 99%

Effect of Meissner Screening and Trapped Magnetic Flux on Magnetization Dynamics in Thick Nb/Ni80Fe20/Nb Trilayers

et al. 2019

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We investigate the influence of Meissner screening and trapped magnetic flux on magnetization dynamics for a Ni80Fe20 film sandwiched between two thick Nb layers (100 nm) using broadband (5-20 GHz) ferromagnetic resonance (FMR) spectroscopy. Below the superconducting transition Tc of Nb, significant zerofrequency line broadening (5-6 mT) and DC resonance field shift (50 mT) to a low field are both observed if the Nb thickness is comparable to the London penetration depth of Nb films (≥ 100 nm). We attribute the observed peculiar behaviors to the increased incoherent precession near the Ni80Fe20/Nb interfaces and the effectively focused magnetic flux in the middle Ni80Fe20 caused by strong Meissner screening and (defect-)trapped flux of the thick adjacent Nb layers. This explanation is supported by static magnetic properties of the samples and comparison with FMR data on thick Nb/Ni80Fe20 bilayers. Great care should therefore be taken in the analysis of FMR response in ferromagnetic Josephson structures with thick superconductors, a fundamental property for high-frequency device applications of spin-polarized supercurrents.

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Section: Introductionmentioning

confidence: 99%

Effect of Meissner Screening and Trapped Magnetic Flux on Magnetization Dynamics in Thick Nb/Ni80Fe20/Nb Trilayers

et al. 2019

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“…The above fitting is consistent with the H=0 limit. A common requirement to realize SOC-driven emergent phenomena, from topological insulators, Majorana fermions, and LRT is typically SOC that is already inherently strong in the normal state, for example, implemented with heavy elements and in narrow-band semiconductors [8,9,16,34]. In contrast, the platform we have studied reveals a peculiar superconducting behavior even when a rather weak SOC in the normal state leads to a negligible magnetic anisotropy.…”

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confidence: 99%

Interfacial Spin-Orbit Coupling: A Platform for Superconducting Spintronics

et al. 2020

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Spin-orbit coupling (SOC) is a key interaction in spintronics, allowing an electrical control of spin or magnetization and, vice versa, a magnetic control of electrical current. However, recent advances have revealed much broader implications of SOC that is also central to the design of topological states, including topological insulators, skyrmions, and Majorana fermions, or to overcome the exclusion of two-dimensional ferro-magnetism expected from the Mermin-Wagner theorem. SOC and the resulting emergent interfacial spin-orbit fields are simply realized in junctions through structural inversion asymmetry, while the anisotropy in magnetoresistance (MR) allows for their experimental detection. Surprisingly, we demonstrate that an all-epitaxial ferromagnet/ MgO/metal junction with only a negligible MR anisotropy undergoes a remarkable transformation below the superconducting transition temperature of the metal. The superconducting junction has a three orders of magnitude higher MR anisotropy and supports the formation of spin-triplet superconductivity, crucial for superconducting spintronics, and topologically-protected quantum computing. Our findings call for revisiting the role of SOC in other systems which, even when it seems negligible in the normal state, could have a profound influence on the superconducting response.For over 150 years magnetoresistive effects have provided attractive platforms to study spin-dependent phenomena and enable key spintronic applications [1]. Primarily, spintronics relies on junctions with at least two ferromagnetic layers to provide sufficiently large magnetoresistance (MR). Record room-temperature MR and commercial applications employ junctions of common ferromagnets, such as Co and Fe with MgO tunnel barrier [2,3]. Alternatively, MR occurs in single ferromagnetic layers with an interplay of interfacial spin-orbit coupling (SOC). However, in metallic systems this phenomenon, known as the tunneling anisotropic MR (TAMR) [4], is typically < 1% and precludes practical applications. Here we show experimentally that a negligible MR in an all-epitaxial ferromagnet/MgO/metal junction is drastically enhanced below the superconducting transition temperature of the metal. We explain this peculiar behavior with the role of the interfacial SOC in the formation of spin-triplet superconductivity which can enable low-power superconducting spintronics [5-7] and topologically-protected quantum computing [8,9].

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“…The development of spintronics had already benefited from the use of superconducting materials, resulting in longer spin lifetimes and energy-efficient components [4,5]. Now, triplet supercurrents formed by spin-polarized Cooper pairs add the possibility of transporting a net spin component at zero resistance and thus pave the way for spintronic devices that are less liable to overheat [6][7][8][9][10][11][12][13][14][15][16]. The key challenge in the field is the nonequilibrium and on-demand generation of equal-spin Cooper pairs in a viable fashion [17][18][19][20][21], desirably avoiding the complicated manipulation of magnetic components.…”

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On-demand thermoelectric generation of equal-spin Cooper pairs

Keidel

Hwang

Trauzettel

et al. 2020

Phys. Rev. Research

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Superconducting spintronics is based on the creation of spin-triplet Cooper pairs in ferromagnetsuperconductor (F-S) hybrid junctions. Previous proposals to manipulate spin-polarized supercurrents on-demand typically require the ability to carefully control magnetic materials. We, instead, propose a quantum heat engine that drives equal-spin supercurrents on-demand without the need for manipulating magnetic components. We consider a S-F-S junction, connecting two leads at different temperatures, on top of the helical edge of a two-dimensional topological insulator. The heat and charge currents generated by the thermal bias are caused by different transport processes, where electron cotunneling is responsible for the heat flow to the cold lead and, strikingly, only crossed Andreev reflections contribute to the charge current. Such a purely nonlocal Andreev thermoelectric effect generates a spin-polarized supercurrent between the superconductors that can be switched on/off by tuning their relative phase. We further demonstrate that the detection of the spin-triplet supercurrent is facilitated by rather low fluctuations of the thermoelectric current for temperature gradients comparable to the superconducting gap. arXiv:1907.00965v2 [cond-mat.mes-hall]

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Enhanced spin pumping into superconductors provides evidence for superconducting pure spin currents

Cited by 138 publications

References 33 publications

Effect of Meissner Screening and Trapped Magnetic Flux on Magnetization Dynamics in Thick Nb/Ni80Fe20/Nb Trilayers

Effect of Meissner Screening and Trapped Magnetic Flux on Magnetization Dynamics in Thick Nb/Ni80Fe20/Nb Trilayers

Interfacial Spin-Orbit Coupling: A Platform for Superconducting Spintronics

On-demand thermoelectric generation of equal-spin Cooper pairs

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