Contact induced spin relaxation in graphene spin valves with Al2O3 and MgO tunnel barriers

Amamou, Walid; Lin, Zhisheng; Baren, Jeremiah van; Turkyilmaz, Serol; Shi, Jing; Kawakami, Roland

doi:10.1063/1.4943681

Cited by 36 publications

(42 citation statements)

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“…Graphene is a promising material for lateral spin transport due to its low spin orbit coupling and high carrier mobility, leading to long spin diffusion lengths at room temperature 1,2 . The graphene/ferromagnet (FM) interface has proven to be the bottleneck for achieving high spin lifetimes and high spin injection efficiencies, due to spin absorption by the ferromagnetic contacts 3-13 , the possibility of contact-induced spin relaxation mechanisms other than spin absorption 14 , and the challenge of separating these effects from the spin injection and detection efficiencies of the ferromagnet contacts. Understanding spin relaxation and spin absorption at graphene/FM junctions is also important for technological applications such as all-spin logic, in which the magnetization of a nanomagnet is switched by spin-transfer torque when a pure spin current is absorbed 15 .…”

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Spin Absorption by In Situ Deposited Nanoscale Magnets on Graphene Spin Valves

et al. 2018

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An in situ measurement of spin transport in a graphene nonlocal spin valve is used to quantify the spin current absorbed by a small (250 nm x 750 nm) metallic island. The experiment allows for successive depositions of either Fe or Cu without breaking vacuum, so that the thickness of the island is the only parameter that is varied. Furthermore, by measuring the effect of the island using separate contacts for injection and detection, we isolate the effect of spin absorption from any change in the spin injection and detection mechanisms. As inferred from the thickness dependence, the effective spin current = 2 ℏ absorbed by Fe is as large as 10 8 A/m 2 . The maximum value of is limited by the resistance-area product of the graphene/Fe interface, which is as small as 3 Ωµm 2 . The spin current absorbed by the same thickness of Cu is smaller than for Fe, as expected given the longer spin diffusion length and larger spin resistance of Cu compared to Fe. These results allow for a quantitative assessment of the prospects for achieving spin transfer torque switching of a nanomagnet using a graphene-based nonlocal spin valve.Graphene is a promising material for lateral spin transport due to its low spin orbit coupling and high carrier mobility, leading to long spin diffusion lengths at room temperature 1,2 . The graphene/ferromagnet (FM) interface has proven to be the bottleneck for achieving high spin lifetimes and high spin injection efficiencies, due to spin absorption by the ferromagnetic contacts 3-13 , the possibility of contact-induced spin relaxation mechanisms other than spin absorption 14 , and the challenge of separating these effects from the spin injection and detection efficiencies of the ferromagnet contacts. Understanding spin relaxation and spin absorption at graphene/FM junctions is also important for technological applications such as all-spin logic, in which the magnetization of a nanomagnet is switched by spin-transfer torque when a pure spin current is absorbed 15 . Despite apparent progress [16][17] , this goal has been challenging to achieve in graphene, which is why an experiment observing the evolution of the absorbed spin current while varying the thickness of the nanomagnet is valuable.In this Letter, we quantify the spin current absorbed by a nanomagnetic island deposited on a nonlocal graphene spin valve. Spin transport measurements are completed in situ while growing the Fe island and the results are interpreted using a 2-D finite-element model. We determine that the effective spin current

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Spin Absorption by In Situ Deposited Nanoscale Magnets on Graphene Spin Valves

et al. 2018

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“…In particular, graphene is attractive for both propagating and manipulating spin information over long distances because of its long spin-diffusion length λ s [20][21][22]. Studies of spin Spin precession and spin Hall effect in monolayer graphene/Pt nanostructures 2 relaxation in graphene has been explored using standard Hanle measurements [23][24][25][26][27][28][29] and out-of-plane precession to determine the spin relaxation anisotropy [30]. Improvements in the graphene quality have led to λ s in the range of tens of micrometers, which is already suitable for a number of applications [31,32].…”

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Spin precession and spin Hall effect in monolayer graphene/Pt nanostructures

et al. 2017

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IntroductionSpin Hall effects (SHE) are a family of relativistic phenomena in which electrical currents generate transverse spin currents and vice versa as a consequence of spin-orbit coupling (SOC) [1]. In non-magnetic materials, they provide a versatile tool for the generation and detection of spin currents without the insertion of ferromagnets (FMs). Early theoretical investigations showed that the SHE can be of extrinsic [2,3] or intrinsic [1,4] origin. Experimental observations have been carried out in semiconductor and metallic systems by means of optical [5,6], electrical [7] and magnetization dynamics [8] methods, triggering many fundamental studies and potential applications [9].The exploration of the SHE using nonlocal (NL) measurements [7,10] have been frequently performed in metals using Pt as the large SOC material [11,12]. In semiconductor systems, detection and modulation of the inverse SHE (ISHE) has been achieved in n-GaAs by means of spin precession in combination with external electric fields [13]. These studies have led to a better comprehension of the phenomena and more accurate determination and control of the spinto-charge conversion efficiency, which is quantified by the spin Hall angle (α SHE ). For instance, α SHE can be enhanced by using metallic alloys [14,15], or tuned by changing the electrical resistivity [16]. Moreover, recent experiments have demonstrated that spin currents generated by the SHE can be large enough to switch the magnetization of nanomagnets [17,18] and induce fast domain wall motion [19], highlighting the potential use of the SHE in conventional spintronic devices [18] and pointing the path towards future spintronics without FMs.Additionally, 2D materials have emerged as promising system for spintronics owing to their tuneable electronic properties. In particular, graphene is attractive for both propagating and manipulating spin information over long distances because of its long spin-diffusion length λ s [20][21][22] AbstractSpin Hall effects have surged as promising phenomena for spin logics operations without ferromagnets. However, the magnitude of the detected electric signals at room temperature in metallic systems has been so far underwhelming. Here, we demonstrate a two-order of magnitude enhancement of the signal in monolayer graphene/Pt devices when compared to their fully metallic counterparts. The enhancement stems in part from efficient spin injection and the large spin resistance of graphene but we also observe 100% spin absorption in Pt and find an unusually large effective spin Hall angle of up to 0.15. The large spin-to-charge conversion allows us to characterise spin precession in graphene under the presence of a magnetic field. Furthermore, by developing an analytical model based on the 1D diffusive spin-transport, we demonstrate that the effective spinrelaxation time in graphene can be accurately determined using the (inverse) spin Hall effect as a means of detection. This is a necessary step to gather full understanding of the consequences of ...

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“…8 , high quality tunnel barriers are critical for obtaining higher spin relaxation times (τ s ) in graphene because barriers with pinholes or rough surface morphology can cause additional contact-induced spin relaxation, which has received a great deal of interest recently. [10][11][12][13][14] As opposed to growing oxide tunnel barriers on graphene, a thin insulating twodimensional (2D) van der Waals material can also be used as a tunnel barrier. A particular material of interest is single (or few) layer h-BN because of its various suitable properties 15 : large energy band gap ~5.97 eV, high crystallinity, spin filtering 16 , absence of pinholes and dangling bonds, atomic lattice similar to graphene, and chemical stability at ambient conditions.…”

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Nanosecond spin relaxation times in single layer graphene spin valves with hexagonal boron nitride tunnel barriers

Singh

Katoch

et al. 2016

Applied Physics Letters

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Abstract:We present an experimental study of spin transport in single layer graphene using atomic sheets of hexagonal boron nitride (h-BN) as a tunnel barrier for spin injection. While h-BN is expected to be favorable for spin injection, previous experimental studies have been unable to achieve spin relaxation times in the nanosecond regime, suggesting potential problems originating from the contacts. Here, we investigate spin relaxation in graphene spin valves with h-BN barriers and observe room temperature spin lifetimes in excess of a nanosecond, which provides experimental confirmation that h-BN is indeed a good barrier material for spin injection into graphene. By carrying out measurements with different thicknesses of h-BN, we show that few layer h-BN is a better choice than monolayer for achieving high non-local spin signals and longer spin relaxation times in graphene.* Author to whom correspondence should be addressed: kawakami.15@osu.edu 2 Graphene is a promising spin channel material for next generation spintronic devices due to the experimental demonstration of long spin diffusion lengths at room temperature 1-3 and theoretical predictions of long spin relaxation times 4,5 arising from the weak spin-orbit and hyperfine couplings 5,6 .However, experimentally measured spin relaxation times [1][2][3]7,8 in graphene are orders of magnitude shorter than theoretically predicted 4,5 . In graphene spin valves, the tunnel barrier plays a crucial role for spin injection by circumventing the problem of impedance mismatch 9 between graphene and the ferromagnetic electrodes. As demonstrated by Han et. al. 8 , high quality tunnel barriers are critical for obtaining higher spin relaxation times (τ s ) in graphene because barriers with pinholes or rough surface morphology can cause additional contact-induced spin relaxation, which has received a great deal of interest recently. [10][11][12][13][14] As opposed to growing oxide tunnel barriers on graphene, a thin insulating twodimensional (2D) van der Waals material can also be used as a tunnel barrier. A particular material of interest is single (or few) layer h-BN because of its various suitable properties 15 : large energy band gap ~5.97 eV, high crystallinity, spin filtering 16 , absence of pinholes and dangling bonds, atomic lattice similar to graphene, and chemical stability at ambient conditions. In addition, atomically clean vertical heterostructures of h-BN/graphene can be mechanically assembled using polymer-based transfer techniques 17,18 . The first experimental report demonstrating spin injection into graphene using a monolayer h-BN tunnel barrier showed τ s less than 100 ps 19 . This was followed by the work of Kamalakar et. al. 20,21 and Fu et. al. 22 , which used chemically grown h-BN barriers, yielding τ s ~ 500 ps. Another recent study using an encapsulated geometry 23 with graphene sandwiched between a thick bottom layer of h-BN and a monolayer of h-BN on top showed τ s less than 200 ps. As evident from these studies, graphene spin valve devices ...

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Contact induced spin relaxation in graphene spin valves with Al2O3 and MgO tunnel barriers

Cited by 36 publications

References 36 publications

Spin Absorption by In Situ Deposited Nanoscale Magnets on Graphene Spin Valves

Spin Absorption by In Situ Deposited Nanoscale Magnets on Graphene Spin Valves

Spin precession and spin Hall effect in monolayer graphene/Pt nanostructures

Nanosecond spin relaxation times in single layer graphene spin valves with hexagonal boron nitride tunnel barriers

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