Room-temperature high-efficiency spin injection via van der Waals tunnel contact

Domain: physics
Published: 2026-01-10
Editorial outcome: major_revision

Abstract
Efficient and reliable spin injection at room temperature with scalability is crucial for spintronic applications but remains challenging. Direct ferromagnetic metal deposition on two-dimensional materials often leads to inefficient transparent contacts. Here, we introduce an indium buffer layer between ferromagnetic cobalt (Co) and graphene to establish high-efficiency van der Waals (vdW) tunnel contacts. This buffer layer facilitates a physisorption interface between Co and graphene with a well-defined vdW gap, which functions as an effective spin tunnel barrier. Through buffer layer thickness optimization, we achieved a room-temperature spin injection efficiency of approximately 25% in graphene, comparable to the best single-crystalline oxide-tunnel-barrier-based devices, alongside explicit nonlocal spin valve signals and Hanle spin precession. We further demonstrate the scalability of our approach through uniform performance across multi-channel graphene spin valves and its versatility by achieving efficient spin injection in semiconducting MoS2 with an average efficiency of about 19.7%. Our strategy offers a simple, cost-efficient, and industry-compatible method for future large-scale and efficient spintronic applications.

Introduction
Graphene has emerged as an ideal spin transport platform due to its extremely small spin-orbital coupling, negligible hyperfine interaction as well as ultra-high charge mobility1–3. Electrical spin injection by direct deposition of ferromagnetic (FM) metal on graphene mostly forms a transparent contact with small contact resistance, which results in extremely low spin injection efficiency4,5. To maximize the potential of graphene for spintronic applications, various dielectric materials, such as metal oxides and van der Waals (vdW) hexagonal boron nitride (hBN)6–11, have been exploited and inserted between the FM electrode and graphene to form a tunnel contact in order to improve the injection efficiency12–14. In fact, the growth of a uniform oxide barrier is non-trivial and ubiquitous pinholes form due to high surface diffusivity on the dangling-bond-free graphene surface. By using TiO2 as a seed layer, an atomically smooth MgO barrier has been grown on graphene, so far with the widely accepted highest spin injection efficiency of 26–30%15. However, reaching such high efficiency relies on complex and delicate growth control based on ultra-high vacuum molecular beam epitaxy (MBE) technique, which hinders its widespread applications. Two-dimensional (2D) vdW hBN has been predicted as an ideal pinhole-free tunnel barrier16. Nevertheless, the precise control of its layer thickness and avoidance of transfer-process-induced contamination and bubbles/wrinkles are challenging, making the reported efficiency vary widely from device to device17–21.

A promising way to achieve reliable spin injection with simple processing is based on dielectric-barrier-free vdW contact between FM electrode and graphene, where naturally formed vdW gap serves as the tunnel barrier22,23. Recent studies using transfer techniques to stack 2D ferromagnet on graphene to form vdW contacts have claimed successful spin injection, but most without the explicit proof of Hanle precession signal24–27. Moreover, the efficiency remains low and scalability has yet to be addressed, as large-scale growth and transfer of 2D ferromagnets are immature. To advance toward scalable applications, making vdW contact based on three-dimensional (3D) FM metal instead of 2D ferromagnet is more promising28,29. Yet, directly evaporating 3D FM metal on 2D materials hardly forms vdW contact. Theoretical calculations have shown that FM metal, such as cobalt (Co), generates a strongly hybridized chemisorption interface when adsorbed on graphene, being the reason for forming a transparent contact with a negligible barrier and extremely small contact resistance30,31. A viable solution is to insert a buffer metal layer, which forms a physisorption interface with graphene, before depositing the FM metal layer so as to mitigate their direct hybridization. Inspiring results have been shown32,33, whereas, unambiguous demonstration of room-temperature spin injection via combinational observation of nonlocal spin valve signal and Hanle spin precession as well as achieving high spin injection efficiency, has yet to be realized.

In this work, by growing an indium (In) buffer layer between ferromagnetic Co electrode and graphene, we show that vdW contact with a gap of ~2.83 Å and an effective tunnel barrier height of ~62.8 meV has been successfully established. Analysis based on a one-dimensional drift-diffusion model of spin transport further corroborates the vdW-gap-mediated efficient spin tunnelling and transport in our Co/In-contacted devices. Both the nonlocal spin valve signal and Hanle spin precession have been explicitly observed at room temperature, which unambiguously proves successful spin injection via the vdW tunnel contact. Upon optimizing the In buffer layer thickness and gate control, a spin injection efficiency of ~25% at room temperature and ~35% at low temperature has been achieved, comparable to the best values reported in oxide-tunnel-barrier-based graphene lateral spin vales (LSV). By making a multi-channel LSV device on an ultra-long graphene sample, both high device yield and performance uniformity were demonstrated, further supporting the scalability of our approach. Finally, we show that our strategy is readily applicable to other 2D materials such as semiconducting MoS2, where an efficiency as high as ~19.7% was obtained.

Results
SpinsignalsinCo/In-contactedgraphenelateralspinvalves
In ordertomake adirectcomparison,LSVdeviceswithbothCo/IncontactsandCocontactswerefabricatedonasinglegraphenesample side by side as shown in Fig. 1a and Supplementary Fig. 1a. Experi- mentaldetailscanbefoundinMethods.First,two-terminalcurrent- voltage(I-V)curvesatdifferenttemperaturesfortheCo/In-contacted channelandtheCo-contactedchannelweremeasuredandshownin Fig.1b.ItcanbeseenthatwithanInbufferlayer,theCo/In-contacted I-V shows nonlinear behaviour with weak temperature dependence, whichischaracteristicoftunnelcontact.Thebias-dependentdiffer- entialresistanceofthecontact(i.e.,dV/dI)iscalculatedandshownin Supplementary Fig. 1b. The range of our two-terminal resistance (100~300kΩ)iscomparabletothatreportedintheMBE-grownpin- hole-freeCo/MgO/graphenetunnelcontacts(100~200kΩ)15,indicat- ing the potential for efficient spin injection. In contrast, the Co- contacted I-V shows linear behaviour with much smaller resistance, indicating a transparent contact was formed between Co and graphene5.

Next,thespinvalvesignalwasmeasuredinthestandardnonlocal four-terminal geometrywith AClock-intechnique (see Methods for details).Bysweepingthemagneticfieldalongtheeasyaxisofmagnetic electrodes (B), the nonlocal voltage V was recorded with a fixed y NL currentIinjectinginthelocalcircuitloop.Figure1cshowsthestandard spinvalvesignalmeasuredfortheCo/In-contacteddevice,whereR is NL definedasthenonlocalV dividedbythelocalI.WhenB isswept NL y backandforth,R jumpsfromparallelstatetoanti-parallelstateand NL then back to parallel state, due to the difference of coercive fields betweentheinjectoranddetectorelectrodesasdeterminedbytheir respective widths34. The difference of R between the parallel and NL antiparallelstates,markedasΔR ,isindicativeofthestrengthofspin- NL polarizedcurrentinjectedandtransportedinthegraphenechannel.As canbeseen,forCo/In-contacteddevice,ΔR isaslargeas~30Ω,while NL forthe Co-contacteddevice,itisnegligiblysmall(~60mΩ,referto SupplementaryFig.1c)aspreviouslyreported4.Thelargespinsignal observedinCo/In-contacteddeviceallowsustomeasureitsHanlespin precessionunderperpendicularmagneticfieldB,asshowninFig.1d. z The standard Hanle curves showing maximized/minimized signal at zerofieldanddiminishingsignalatincreasedfieldforbothparalleland anti-parallelstatesarecharacteristicofspindephasingduetoLarmor precession,whichfurtherconfirmsthatspin polarized current has beeninjectedintographenethroughtheCo/Intunnelcontacts.Spin parameterscanbeobtainedbyfittingthedatatothesolutionofone- dimensionalBlochequations5:

[Equation omitted for brevity as per instructions]

where ± is for the parallel (antiparallel) magnetization state, L is thechannellengthbetweeninjectorelectrodeanddetectorelectrode, Disthespindiffusionconstant(approximatelyequaltochargediffu- sionconstant),τ isthespinlifetime,andω =gμ B =ℏisLarmorfre- s L B z quency with g-factor of ~ 2.0 for graphene6. D can be calculated independentlybyusingtheEinsteinrelationwithparametersextrac- tedfromthetransfercurveshowninSupplementaryFig.1d,soτ is s theonlyfittingparameterandavalueof~8p0pffiffiffisffiffiffiwffiffiasobtained,yielding aspindiffusionlengthλ of~1.55μm(λ (cid:3) Dτ ).Wedidnotobserve s s s anyHanlespinprecessionsignalfortheCo-contacteddevice,probably duetotheextremelysmallspinsignalandlowsignal-to-noiseratio.
WecanfurthercalculatethespinpolarizationP,i.e.theaveragedspin injection/detectionefficiency,bythefollowingequation5

[Equation omitted for brevity as per instructions]

whereWandσarethewidthandconductivityofthegraphenechannel respectively.BysubstitutingtheΔR andλ obtainedaboveintothe NL s equation,wegetalargespinpolarizationof~25%atroomtempera- ture.Thisvalueiscomparabletothebestonereportedinoxidetunnel barrier-basedgrapheneLSVsandsurpassesmosthBNtunnelbarrier- basedgrapheneLSVs(refertoSupplementaryTable1).

AtomicSTEMcharacterizationsofinterfacialstructure
To understand the large spin signalobserved in our Co/In-contacted grapheneLSVdevice,Co(35nm)/InelectrodeswithdifferentInthick- nesses were deposited on multilayer graphene (MLG), followed by a high-resolution scanning transmission electron microscopy (STEM) study.Forcomparison,acontrolsamplewithdirectdepositionofCo electrodeswasalsofabricated.Bright-field(BF)imagesoftheinterfaces betweenmetalandMLGwithoutandwiththeinsertionofInbufferlayer areshowninFig.2a–c,respectively.Apparently,withoutInbufferlayer, thelayeredstructureofMLGisdisrupted,asevidencedbytheforma- tionofadisorderedtransitionslabwithdepthof~4nm(brighterregion betweentheyellowdashedlinesinFig.2a,detailedanalysisinSupple- mentaryFig.2).Bothenergy-dispersiveX-rayspectroscopy(EDS)map- pingsandelectronenergylossspectroscopy(EELS)profileshowthatCo diffuses into the disordered graphene region, further confirming the chemisorption nature of the interface between Co and graphene as predicted by theoretical calculations30,31. In stark contrast, with an In bufferlayer,thesurfaceoftheMLGisprotectedfromanydamageas manifestedbyitsperfectlayeredpatterns,forminganintactinterface between Co/Inandgraphene(yellowdashed linesinFig.2band 2c). Specifically,forthe4nm-Insample(Fig.2b),EDSmappingsshowthatIn andCointer-diffusesintoeachother,formingauniformCo/Inmixture with very sparse In embedded in Co. Such Co/In inter-diffusion pre- sumablypreventsfurtherdiffusionofCointoMLGandaphysisorption interfaceisthusestablished.Zoomed-inannular-bright-field(ABF)and BFimagesshowninSupplementaryFig.3clearlyresolveawell-defined vdWgapof~2.83ÅbetweenCo/InlatticeandMLG.ForthethickerIn sample(6nm,Fig.2c),similarresultswereobservedexceptthattheCo/ InmixturestartedtobedelaminatedwithmoreInaggregatingnearthe interface.Therefore,4nmInsetsacriticalthicknessforpreventingCo from destroying the interface and diffusing into graphene. Raman spectroscopy shown in Supplementary Fig. 4 further confirms that 4 nm-In buffered graphene is intact without any emergence of defect peak, while direct Co-contacted graphene becomes highly defective. The formation of a vdW interface in the Co/In-contacted graphene samples underscores the efficacy of the In buffer layer in mitigating physicaldamageandchemicalhybridizationattheFMmetal/graphene interface.

UnderstandingthevdW-gap-mediatedtunnellingandspin transport
IthasbeenshownthatafinitevdWgapinthetop-contactinterfacecan actasa tunnel barrier for carriers22,23, which is also the case in our devices.TheeffectivebarrierheightofthevdWtunnelcontactcanbe estimatedusingtheBrinkman,DynesandRowell(BDR)model35,which describesthetunnelconductanceofasymmetricbarriersas

[Equation omitted for brevity as per instructions]

whereG/G isthenormalizedchannelconductancewithG beingthe 0 0 zero-biasconductance,φistheeffectivebarrierheightasschemati- callydepictedinSupplementaryFig.5a,Δφisbarrierheightdifference between the two sides of the trapezoidal tunnel barrier, A0 =4(2m)1/2d/3ℏ,mistheelectronmassanddisthebarrierthickness (assumed to be equal to the vdW gap of ~ 2.83Å). By fitting the experimentaldatainSupplementaryFig.5btotheaboveequation,we obtainedφ=62:8meV withΔφ=18:9meV.

TheroleofthevdWtunnelbarriercanbefurtherexplainedbased on the one-dimensional drift-diffusion model ofspin transportpro- posed by Takahashi and Maekawa36. The non-localspinvalve signal ΔR isgivenby
NL

[Equation omitted for brevity as per instructions]

wherePisthespinpolarizationoftheinjectedcurrent,P isthespin F polarizationoftheFMelectrode,R isthecontactresistancebetween C thegrapheneandFMelectrode,Listhespacingbetweentheinjector anddetectorelectrodes,R G = λ ρ /σ G G W andR F = λ ρ F F /A F arethespinresistances ofthegrapheneandFMelectrode,respectively,Wisthewidthofthe graphene channel, A is the contactarea between graphene and FM electrode, λ (λ ) is the spin diffusion length in the graphene (FM G F electrode),σ istheconductivityofgraphene,ρ istheresistivityof G F the FM electrode. We take the typical parameters from the device shownin Fig. 1 and from literatures as follows:W=1um, L=2:6um, λ =1:55um, σ =0:6mS, P =0:437, ρ =6×10 (cid:2)8Ωm38 and G G F F λ =0:06um39.Thenonlocalspinvalvesignal(ΔR )asafunctionof F NL theR iscalculatedforthreedifferentPandshowninSupplementary C Fig.6.FortheexperimentallydeterminedspinpolarizationofP=25%, thecalculatedΔR saturatesat~31ΩwhenthecontactresistanceR NL C becomes large enough, agreeing almost exactly with the measured valueasshowninFig.1c.Thisperfectagreementfurthercorroborates thevdW-gap-mediatedefficientspintunnellingandtransportinour Co/In-contacteddevices.

In-thickness-dependentandgate-tunablespinsignal
WenowturnourattentiontooptimizingthethicknessoftheInbuffer layerforefficientspininjection.AseriesofCo/In-contactedgraphene LSV devices with varied In thickness (up to 15nm) and fixed Co thickness(35nm)weremeasured,andselectedresultsareshownin Fig.3a,wherealldevicesshowastandardspinvalvesignal,butwith differentΔR .ThecorrespondingHanlespinprecessioncurvescanbe NL foundinSupplementaryFig.7.Two-terminalI-Vcurvesofthesedevi- ces were measured and plotted in Fig. 3b, showing nonlinear char- acteristicswith distinct differential resistances.Figure 3c shows the zero-bias resistance-area product, ΔR and spin polarization NL asafunctionoftheInthicknessinFig.3dand3e,respectively.Itcanbe clearlyseenthatthereisanoptimalwindowaround4~5nmforgen- eratingmaximalP,coincidingwiththecriticalthicknessoftheInbuffer layer for effective interface protection as evidenced by the STEM analyses.ForeitherthinnerorthickerInbufferlayer,thespinpolar- izationdiminishesrapidly.ThisisunderstandablebecauseifInistoo thin,itisnon-continuous(refertoSupplementaryFig.8),thusweak- eningthebufferingeffectandapartiallychemisorbedinterfacecan stillbeformed;ifInistoothick,itstartstoaggregateattheinterface (refertoFig.2c)sothatspindephasingbecomesnon-negligibledueto thepresenceoftheInlayer.Therefore,inthefollowingexperiments,4 nm-thickInbufferlayerisalwaysdepositedifotherwisenoted.

Next,weshowthatthespinsignalishighlygate-tuneableinour Co/In-contactedgrapheneLSVdevice.Thespinvalvesignalsmeasured atdifferentgatevoltagesforatypicaldeviceatlowtemperatureare showninFig.4a,withthemagnitudebeingtunedbynearlytenfoldin ourmeasuredrange.ThiscanbeseenmoreclearlyinFig.4b,where ΔR takenfromFig.4aasafunctionofV areplottedtogetherwith NL g thetransfercurveofthedevice.Overall,ΔR changesproportionally NL (inversely) with the trans-resistance (trans-conductance) of the gra- phenechannelandexhibitsapeakaroundtheDiracpoint(V =1.7V), DP whichagreeswellwiththebehaviourofLSVwithtunnelcontactsas predicted by the drift-diffusion theory of spin transport15,40,41. This againprovesthatthespinsignalisgovernedbythevdW-gap-mediated spintunnellingandtransportinourCo/In-contacteddevice.Byfitting Hanlecurves(refertoSupplementaryFig.9)toEq.(1),bothspinlife- timesandspindiffusionlengthscanbeobtained,basedonwhichthe spin polarizations are calculated according to Eq. (2) and shown in Fig.4c,whereapeakvalueashighas~35%wasachievedbytuninggate voltages.

ScalabilityandversatilityofCo/InvdWtunnelcontact
Finally,wedemonstratetwoadvantagesofourapproach–scalability andgeneralapplicabilitytoothermaterialsystems.Toshowthefirst merit, we fabricated two LSV devices based on chemical vapour deposited (CVD) graphene (refer to Supplementary Fig. 10). The detailsoftheproton-assistedCVDgrowthofultra-flatgraphenecanbe foundinourpreviouswork42.ThefirstLSVdevicehasatotalofseven identicalchannelsasshowninFig.5a.Inspiringly,weobtainedspin valvesignalsandcorrespondingHanlespinprecessionsignalsinall thesechannelsatroomtemperature(refertoSupplementaryFig.11 andSupplementaryTable2).Thecalculatedspinpolarization(Fig.5c) liesinanarrowrangeof14%~18.5%withanaveragevalueof~16.5%, whichshowsgooduniformity.Itshouldbenotedthatthepotential spinsinkeffectduetononidealcontactinLSVwithmultipleelectrodes couldlimittheobservationoftheentirespinsignal;therefore,thespin injectionefficiencyweextractedactuallysetsthelowerboundofthe realvalue.Theseconddevicehasanultra-longchannelof15μmas shown in Fig. 5d. Room-temperature spin valve signals and corre- spondingHanlespinprecessionweremeasuredandshowninFig.5e and5f,respectively.ByfittingtheHanlecurvestoEq.(1),weobtaineda spinlifetimeof~0.9nsandaspindiffusionlengthof~5.9μm,which arecomparabletoapreviousreportonlong-channelgrapheneLSV withoxidetunnelbarriers43.WewanttopointoutthattheΔR of~3Ω NL obtainedinourdeviceisonetotwoordersofmagnitudelargerthan thatobservedinLSVdeviceswithsimilarchannellength10,44,45.Thespin polarizationPinourdevicewasfurthercalculatedtobe~13%.Ahigher spinpolarizationof~20%wasachievedatalowertemperature(refer toSupplementaryFig.12).Alltheseresultsaresignificantconsidering that the etching procedure in the fabrication process may induce defectsandextracontaminantsinourCVD-growngraphenesamples. Withfastdevelopmentofgraphenegrowthandtransfertechniques, wafer-scaledeviceapplicationsareanticipatable.Toshowthesecond merit,wemadetwoMoS LSVdevicesbasedontheCo/InvdWtunnel 2 contact as shown in Fig. 5g and Supplementary Figs. 13−15, where multilayersampleswereusedforourin-planespininjectionexperi- ment in order to avoid the D’yakonov-Perel type spin relaxation inducedbytheRashba-likespin-orbitcoupling46.Themeasuredspin valve signals for different channels of the first device are shown in Fig.5h.Thechannel-length-dependentΔR isfurtherplottedinFig.5i NL andfittedbasedonEq.(2),yieldingaspindiffusionlengthof~364nm, which is longer than that reported previously47. The average spin polarizationwascalculatedtobeashighas~19.7%(refertoSupple- mentaryNote1).Strikingly,wewereabletomeasurebothspinvalve signalandHanlespinprecessionintheseconddevice(refertoSup- plementaryFig.14)whichis,toourbestknowledge,thefirstunam- biguousobservationofsuchsignalsinMoS2.

Methods
Devicefabrication
Monolayer Graphene was prepared by mechanically peeling high- quality graphite (natural graphite from HQ Graphene) onto SiO2 (285nm)/Si substrate and identified by optical contrast as well as Ramanspectra.Strip-shapedgraphenewasspeciallychosenforspin valvedevicefabrication.Spininjectionanddetectionelectrodeswith differentwidthswereobtainedbystandarde-beamlithography,fol- lowed by e-beam evaporation and a lift-off process. In a vacuum chamberwithabasepressureof<10⁻⁷torr,anInlayerwithadifferent thicknesswasfirstdepositedatarateof0.2Å/s,followedbydeposit- inga35-nm-thickColayeratarateof0.4Å/s.Subsequently,a5-nm- thickAulayerwasdepositedontoptoprotectCofromoxidation.The samplethenunderwentastandardlift-offprocessinacetone,followed by rinsing with isopropanol and blowing drywith nitrogen. Forthe ultralong spin valve device, proton-assisted, CVD-grown ultra-flat graphene was used. The graphene film was first transferred onto SiO2 (285nm)/Si substrate and then patterned into a long strip by e-beam lithography, followed by oxygen plasma etching. The same devicefabricationprocedureapplies.

Electricalandspintransportmeasurements
TheelectricaltransportmeasurementswerecarriedoutinaCryofree superconducting magnet system (CFMS-12T-30VTI, Cryogenic Co.) usingtwosourcemetres(Model2400andModel2450,KeithleyInc.). Toapplybackgatevoltage,thesourceterminalwasconnectedtothe backgate,andtheleakagecurrentthroughtheSiO2 dielectriclayerwas monitored.Forthespintransportmeasurements,analternatingcur- rentwithafrequencyof13.171HzwasappliedbyaKeithley6221cur- rentsource,andthenonlocalvoltagewasmeasuredbyanSR830lock- inamplifierequippedwithahighimpedancepre-amplifierSR551.The spinvalvesignalwascollectedbyscanningthein-planemagneticfield along the easy axis of the ferromagnetic electrodes. Hanle measurementswereconductedbyfirstaligningtheinjectorelectrode and detector electrode in a parallel state or anti-parallel through applyinganin-planemagneticfield,followedbyrotatingthesample 90 degrees and scanning the magnetic field perpendicularly to the samplesurface.

STEMcharacterization
The cross-sectional STEM specimens were prepared using a Cryo- focused Ion Beam (FIB) in high vacuum (<10−6 mbar) in a liquid- nitrogen temperature environment. STEM imaging, EDS, and EELS analysisofverticalheterojunctionsofCo/In/MLGwereperformedona FEITitanThemiswithanX-FEGelectrongunandaDCORaberration correctoroperatingat300kV.Theinnerandoutercollectionangles fortheABF-STEMimages(β1andβ2)were9and36mrad,respectively, andtheβ2forBF-STEMimageswas4mrad,withaconvergencesemi- angleof25mrad.Thebeamcurrentwasabout100pAforimagingand spectrumcollection.AllSTEMexperimentswereperformedatroom temperature.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors made a substantial effort to answer all the questions raised by the 4 reviewers, which were properly answered in most cases. However, I still have some concerns about the spin injection in MoS2: the authors show the Hanle precession, but it is not clear if the data reported is the subtraction of parallel and antiparallel configuration or simply the parallel case. They should provide the 2 curves (parallel and antiparallel) as they do for graphene, otherwise a simple Lorentzian curve with magnetic field like the one shown could arise from some magnetoresistance effect like weak localization in the channel.", "reviewer_2": "Dear Editor,\nPlease find below my comments on the responses provided by the authors of the paper entitled “Room-temperature high-efficiency spin injection via van der Waals tunnel contact”, regarding my comments for the revue in Nature Electronics Journal.\nThe authors present a methodical study, from sample fabrication to physical characterization and spin-dependent transport in multi-terminal lateral spin-valves (LSV), and report and focus on the performance of the spin injection into 2D materials (graphene and MoS2) using a Co/In polarizing electrode. I would like to thank the authors for the responses and the additional information and new experiments they have performed and included in the manuscript. Nevertheless, I still have main reserves concerning the novelty of the results the authors provide on graphene based LSV. However, the improved work carried out on MoS2-LSV deserves to be highlighted.\nThe authors have now discussed the pivotal point regarding the spin injection efficiency, namely the impedance mismatch issue with measures and calculations. The authors provide the calculated two-terminal resistance, from electrical measurements and geometrical parameters. The contact resistance is included in a range from “100 ~ 300 kΩ” which is as mentioned by the authors “comparable to that reported in the MBE-grown pinhole-free Co/MgO/graphene tunnel contacts”, I agree. The authors then extract the spin polarization of Co/In electrode using the Takahashi and Maek’s formula (S. Takahashi and S. Maekawa, Spin injection and detection in magnetic nanostructures. Phys. Rev. B 67, 052409 (2003)) by implementing all the parameters of the device, including the non-local resistivity measured in the Co/In/graphene in two-terminal configuration. They obtain a spin efficiency of 25%. Nevertheless, this value which I note, remains “comparable to the best values reported in oxide-tunnel-barrier based graphene lateral spin valve”, as claimed by the authors (Han, W. et al. Tunneling Spin Injection into Single Layer Graphene. Phys. Rev. Lett., 105, 167202 (2010)). Authors provide sketch for two-terminal non-local measurements. I still have fundamental concern on the multi-LSV geometry. When spin is injected into the graphene channel, even if the detection is monitored between two terminals, all others electrodes are electrically connected with the graphene channel. Therefore, if the contact resistance of the Co/In electrode which detects the spin signal in the two-terminal configuration is too high compared to another distant electrode connected on graphene, then this distant electrode acts as a spin well. The effective spin signal drops, and it does not allow determine the full picture, i.e value, of the spin polarization of the electrode to be assessed because not all of the signal is detected (Seneor et al., Spintronics with graphene, MRS Bulletin 37, 1245 (2012)). In summary, without considering recent publication form Sarkar et al., Nat. Electron. 8, 215–221 (2025), which report on results on a Co/In/graphene-based LSV and about which I would have had the same doubts regarding its novelty in light of the work reported in recent years on graphene-based spintronics LSV (Stephan Roche et al. 2015 2D Mater. 2 030202), I do not recommend the article for publication in Nature Communications.\nIn contrast, the authors have strengthened their argument by providing new data and measurements on MoS2-LSV study. The field of research of TMDCs based spintronics, as MoS2, is fairly recent and deserves further exploration. With this new augmented dataset (STEM of Co/In/MoS2, spin-dependent transport on several samples comforting the reproducibility of authors’ previous measurements, extraction of a 20% spin polarization), the study deserves a new perspective. I encourage the authors to rethink the article and focus it on MoS2 results, which is of interest to the community and further benefit the field."}, "author_response": "Response: We are glad that you think most of the questions have been properly answered. As to your question on the spin injection in MoS , the Hanle precession data shown in Supplementary Fig. 2 14 were obtained by subtraction of parallel and antiparallel configuration. The detailed data are shown below, where Fig. a is spin valve signal for L=300 nm channel (refer to Supplementary Fig. 14b), Fig. b is the raw data of Hanle curves measured under both parallel and antiparallel configurations for this channel, Fig. c is the Hanle data with subtraction of the inclined background signal (Phys. Rev. Applied. 18, 014028 (2022)), and Fig. d is the final Hanle data obtained by subtraction of the parallel and antiparallel configuration in Fig. c and divided by two (refer to Supplementary Fig. 14b). We believe the simultaneous observation of Hanle precession in parallel and antiparallel configurations justifies the successful spin injection in MoS . For this point, we 2 have added two sub-figures in the new Supplementary Fig. 14.\n\nResponse: Thank you very much for reviewing our work once again. We highly appreciate your positive comments on the MoS -LSV part of the work. However, as to the graphene-based LSV part, 2 we believe the importance and advance of our work have NOT been fully recognized and appreciated.\n\nResponse: It seems your main argument against the novelty of our work is that we only claim comparable spin injection efficiency with the best value reported in oxide-tunnel-barrier based graphene lateral spin valve, but not higher. We have the following responses:\nFirstly, it should be emphasized that the best spin injection efficiency value reported by Han, W. et al. [1] was based on MBE-grown single-crystalline MgO tunnel barrier, where the growth process was complex and the UHV-condition is highly demanding, making it cost-inefficient and unfavorable for scaling up. By contrast, the follow-up work using sputtered MgO barriers (polycrystalline or non-crystalline) typically exhibited low spin injection efficiency of only ~ 5% [8].\nSecondly, there is a misunderstanding of the key points of our work, where the high spin injection efficiency is NOT the only claim. To sum up, the key novelty and advance of our work as compared to previous graphene-based spintronics work is actually that we have developed an effective 3D vdW contact strategy which offers a combination of advantages over all the previous methods on spin injection efficiency, process simplicity, repeatability, scalability and CMOS-compatibility, as comparatively shown in the table below. As you can see, in view of all the five key aspects that determine the practical application potential, our strategy all stands out.\n\nResponse: We do NOT think our explanation of the multi-LSV geometry measurement has been properly understood. So, let’s elucidate the measurement mechanism once again based on the schematics shown below, where Fig. a shows the case without “spin sink” (“spin well” as you name) effect and Fig. b shows the case with “spin sink” effect.\nThe so-called spin signal is the nonlocal voltage drop (V , for parallel configuration) measured p between detector electrode and reference electrode as shown in Fig. a. Because we always use the outermost electrode as reference, where the spin chemical potential is close to zero (due to long distance diffusion), so V is nearly solely determined by the spin chemical potential at the detector p electrode. Note that monolayer graphene is an absolute 2D system, so there should be NO shorting problem across the detector electrode (i.e. no current can “bypass” it; if otherwise the channel material has finite thickness instead, then current may “bypass” the electrode by flowing through the bottom side of the channel to the sink electrode), namely all spins injected at the injector electrode must diffuse to the detector electrode first, then pass it, and diffuse to remote electrodes. When there is a “spin sink” electrode between detector electrode and reference electrode as shown in Fig. b, the spin chemical potential will quickly drop to zero at this electrode before reaching the reference electrode. However, this doesn’t affect the spin chemical potential detected at the detector electrode at all, so V remains almost unaffected. In the worst case, for somehow if “not all of the p signal is detected” as you assumed, then V should be underestimated NOT overestimated, which p means that the spin injection efficiency we extracted actually sets the lower bound of the real value.\n\nResponse: It’s a regret that you may have underrated the potential impact Sarkar’s work and our work could bring to this field. We want to point out that we (Deyi Fu group and Barbaros Özyilmaz Group) have been working on 2D LSV for more than a decade by mainly using MgO or h-BN as tunneling barriers [1-3], but now we are thoroughly migrating to the new contact strategy, due to the above-mentioned advantages. However, without demonstrating room-temperature high-efficiency spin injection explicitly, as done in our work, this strategy may NOT be widely appreciated by other researchers in this field. We believe our pioneering work together with Sarkar’s can accelerate the large-scale applications of graphene-based spintronic devices as well as deepened researches in MoS -based semiconductor spintronics. Therefore, we believe it worth publishing in 2 the present journal, which shows broader impacts as compared to most other journals.\n\nResponse: For the MoS LSV work, our purpose is to quickly demonstrate the versatility of our 2 contact strategy for efficient spin injection in different materials systems. We agree that this part of work on 2D semiconductors is very important and deserves further detailed study, but it is out of the scope of our current work. This can be another independent work and actually we currently working on this direction."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors answered my request by providing the 2 Hanle curves (parallel and antiparallel) of the MoS2 lateral spin valve. Although the quality of the raw data is not so good and some doubts remain, I will recommend publication (and leave it to the readers to judge for themselves).", "reviewer_2": "Dear Editor,\nI would like to thank the authors for responding to my questions and for the significant improvements they have made to their manuscript. I now have a clearer understanding of the added contribution of these results. The authors present detailed structural dataset and spin-dependent transport results at room temperature in spin valves multi-lateral devices or LSV based on graphene and 2D molybdenum disulphide. They investigate indium (In) metal spacer between the Co and the graphene. The authors highlight the presence of a van der Walls (vdW) gap between graphene and indium, of the order of 3 A, which drives tunnelling transport and spin injection and detection.\nMy main concern on the novelty of this work, which I had previously found questionable, was based on the significant progress that these results could bring to the graphene-based spintronics community, which has been active for a few years and includes the Deyi Fu group and Barbaros Özyilmaz group, and not on their quality. The signals observed by the authors are clear, well presented, and shed light on spin-dependent transport in graphene and MoS2. Until then, the novelty of these results had not fully dawned on me, but now seems relevant to me in the light of the detailed responses provided by the authors, the added dataset and the modifications made in the manuscript.\nRegarding the results on MoS2 LVS devices, the authors have now provided in the Supplementary Materials new dataset on structural characterization of Co/In/MoS2 interface and new spin-dependent transport strengthening the results of the article.\nI want to point out the efforts of the authors to improve clarity and I am convinced that these results could be published now in Nature Communications.\nNevertheless, minor modifications could be made at this stage.\nAs I mentioned, and as the authors pointed out, the LSV geometry can only underestimate the amplitude of the detected spin signals, which is unfortunate. Potential spin wells would limit the observation of the entire spin signal when spin detection is measured by a non-local, distant electrode. I note that the authors have understood this point. Indeed, “In the worst case [..] Vp should be underestimated NOT overestimated, which means that the spin injection efficiency we extracted actually sets the lower bound of the real value.” I agree with the authors and it was my point from the beginning. This type of geometry with multi-lateral electrodes is not fully consistent and only provides underestimated signals. I suggest that the authors mention this point in their manuscript.\nThe authors have addressed the well-known problem of resistance mismatch between the electrode and graphene for spin injection and detection. This is a pivotal parameter for both detection and injection of spin signal thus prevent quenching of the induced spin polarization. But a more specific point describing this issue could be highlighted in the manuscript.\nAs the authors developed an “effective 3D vdW contact strategy […] for spin injection efficiency” relying on the vdW gap at the interface between indium and cobalt, I suggest that the authors highlight this more prominently in the manuscript by adding the TEM cross-section provided in the supplementary material."}, "author_response": "Response: Thanks very much for recommending the publication of our work.\n\nResponse: Thanks very much for recommending the publication of our work.\n\nResponse: Thanks for the suggestion. We have added a sentence to mention this point as highlighted in the main text: “It should be noted that potential spin sink effect due to nonideal contact in LSV with multiple electrodes could limit the observation of the entire spin signal, therefore the spin injection efficiency we extracted actually sets the lower bound of the real value.”\n\nResponse: Thanks for the suggestion. Actually the discussion of resistance mismatch problem has been highlighted in the main text by adding a new section entitled “Understanding the vdW-gap-mediated tunnelling and spin transport”.\n\nResponse: Thanks for the suggestion. Considering the logic structure of our discussion and the figure size constraint, instead of adding the vdw gap TEM image into the main text figure, we highlighted this point by adding a sentence saying “Zoomed-in annular-bright-field (ABF) and BF images shown in Supplementary Fig. 3 clearly resolve a well-defined vdW gap of ~ 2.83 Å between Co/In lattice and MLG.”"}]

Decision letter