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Jan Bärenfänger, Klaus Zollner, Lukas Cvitkovich, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Stefan Hartl, Jaroslav Fabian, Jonathan Eroms, Dieter Weiss, Mariusz Ciorga

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[Highly efficient lateral spin valve device based on graphene/hBN/Fe<sub>3</sub>GeTe<sub>2</sub>](https://mdr.nims.go.jp/datasets/adaf038a-3c0c-4867-ad57-965d92b895a7)

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Highly efficient lateral spin valve device based on graphene/hBN/Fe3GeTe22D Materials     PAPER • OPEN ACCESSHighly efficient lateral spin valve device based ongraphene/hBN/Fe3GeTe2To cite this article: Jan Bärenfänger et al 2025 2D Mater. 12 045008 View the article online for updates and enhancements.You may also likeLight-matter interactions in layeredmaterials and heterostructures: from moiréphysics and magneto-optical effects toultrafast dynamics and hybrid meta-photonicsLuca Sortino, Marcos H D Guimarães,Alejandro Molina-Sánchez et al.-Ionically gated transistors based on two-dimensional materials for neuromorphiccomputingKe Xu and Susan K Fullerton-Shirey-Roadmap on quantum magnetic materialsAntonija Grubiši-abo, Marcos H DGuimarães, Dmytro Afanasiev et al.-This content was downloaded from IP address 202.208.135.13 on 12/08/2025 at 23:25https://doi.org/10.1088/2053-1583/adf453https://iopscience.iop.org/article/10.1088/2053-1583/adc4f5https://iopscience.iop.org/article/10.1088/2053-1583/adc4f5https://iopscience.iop.org/article/10.1088/2053-1583/adc4f5https://iopscience.iop.org/article/10.1088/2053-1583/adc4f5https://iopscience.iop.org/article/10.1088/2053-1583/adc4f5https://iopscience.iop.org/article/10.1088/2053-1583/adb8c3https://iopscience.iop.org/article/10.1088/2053-1583/adb8c3https://iopscience.iop.org/article/10.1088/2053-1583/adb8c3https://iopscience.iop.org/article/10.1088/2053-1583/adbe892D Mater. 12 (2025) 045008 https://doi.org/10.1088/2053-1583/adf453OPEN ACCESSRECEIVED24 April 2025REVISED21 July 2025ACCEPTED FOR PUBLICATION25 July 2025PUBLISHED6 August 2025Original content fromthis work may be usedunder the terms of theCreative CommonsAttribution 4.0 licence.Any further distributionof this work mustmaintain attribution tothe author(s) and the titleof the work, journalcitation and DOI.PAPERHighly efficient lateral spin valve device based ongraphene/hBN/Fe3GeTe2Jan Bärenfänger1, Klaus Zollner2, Lukas Cvitkovich2, Kenji Watanabe4, Takashi Taniguchi5,Stefan Hartl1, Jaroslav Fabian2, Jonathan Eroms1, Dieter Weiss1 and Mariusz Ciorga1,3,∗1 Institute for Experimental and Applied Physics, University of Regensburg, 93040 Regensburg, Germany2 Institute for Theoretical Physics, University of Regensburg, 93040 Regensburg, Germany3 Department of Experimental Physics, Faculty of Fundamentals Problems of Technology,WrocławUniversity of Science and Technology,50-370 Wrocław, Poland4 Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044,Japan5 Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044,Japan∗ Author to whom any correspondence should be addressed.E-mail: mariusz.ciorga@pwr.edu.plKeywords: spin injection, 2D magnets, van der Waals heterostructuresSupplementary material for this article is available onlineAbstractIn this work we report efficient out-of-plane spin injection and detection in an all-van der Waalsbased heterostructure using only exfoliated 2D materials. We demonstrate spin injection bymeasuring spin-valve and Hanle signals in non-local transport in a stack of Fe3GeTe2 (FGT),hexagonal boron nitride (hBN) and graphene layers. FGT flakes form the spin aligning electrodesnecessary to inject and detect spins in the graphene channel. The hBN tunnel barrier provides ahigh-quality interface between the ferromagnetic electrodes and graphene, eliminating theconductivity mismatch problem, thus ensuring efficient spin injection and detection with spininjection efficiencies of up to P= 40%. Our results demonstrate that FGT/hBN/grapheneheterostructures form a promising platform for realizing 2D van der Waals spintronic devices.1. IntroductionCombining two-dimensional (2D) materials into vander Waals (vdW) heterostructures opens up unpre-cedented possibilities to study novel physical phe-nomena and to develop new device concepts [1].Adding magnets to the rich library of vdW mater-ials, comprising metals, insulators, semiconductors,and topological insulators, has invigorated the fieldof spintronics [2–6]. One of the key issues in spin-tronics is the efficient generation of spin polariza-tion in non-magnetic materials [7]. Electrical spininjection from ferromagnetic materials has emergedas a highly effective method to achieve this goal. Thefirst reports on electrical spin injection and detec-tion in graphene, by Tombros et al in 2007, util-ized conventional ferromagnetic Co electrodes within-plane magnetization and insulating oxide tunnelbarriers [8]. This work not only confirmed graphene’spotential as an excellent spin transport medium withspin relaxation lengths up to 2µm, but also revealedthe critical role of tunnel barriers in overcoming theconductivity mismatch problem between the highlyconductive ferromagnet and graphene. Despite theinitial successes, yielding spin injection efficiencies ofapproximately P≈ 30% and non-local spin valve sig-nals of up to 130Ω [9], the fabrication of high-quality,uniform oxide tunnel barriers on graphene provedchallenging [8]. The non-uniform growth of thesebarriers often results in pinholes, effectively short-circuiting the barrier and diminishing spin injec-tion efficiency [8, 10]. Since then, the foundations ofspin transport in graphene have been established [11]including also the role of proximity effects [12–16]in order to enhance and manipulate the spin signal[3]. Furthermore, the potential of crystalline hBN aspinhole-free tunnel barriers for spin injection intographene has been demonstrated [17–24].© 2025 The Author(s). Published by IOP Publishing Ltdhttps://doi.org/10.1088/2053-1583/adf453https://crossmark.crossref.org/dialog/?doi=10.1088/2053-1583/adf453&domain=pdf&date_stamp=2025-8-6https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://orcid.org/0009-0006-1900-8047https://orcid.org/0000-0002-6239-3271https://orcid.org/0000-0003-2453-507Xhttps://orcid.org/0000-0003-3701-8119https://orcid.org/0000-0002-1467-3105https://orcid.org/0000-0002-3009-4525https://orcid.org/0000-0003-2212-9537https://orcid.org/0000-0002-9630-9787https://orcid.org/0000-0003-2441-7874mailto:mariusz.ciorga@pwr.edu.plhttps://doi.org/10.1088/2053-1583/adf4532D Mater. 12 (2025) 045008 J Bärenfänger et alThe challenges with conventional materials havemotivated the exploration of all-vdW heterostruc-tures for graphene spintronics. The unique propertiesof vdW materials, such as their atomically flat sur-faces and perfect crystallinity, as well as their abil-ity to be stacked independently of lattice matching,provide a robust solution to the limitations of con-ventional material growth techniques, enabling fullycrystalline spin transport devices. The discovery ofmetallic 2D ferromagnets [25] enables the creationof spin injection and detection devices made entirelyof vdW materials. Although significant progress hasbeen made in 2D spintronics, existing spin injec-tion and detection platforms often adopt a hybridapproach, combining spin-selective contacts basedon 2D ferromagnets with those based on conven-tional ferromagnets, using an oxide tunnel barriersolely between graphene and the latter [26–28]. Toour knowledge, platformswith spin-selective contactsbased solely on 2D ferromagnets have not utilizeda tunnel barrier so far [29–31]. In both scenarios,the absence of a tunnel barrier between the graphenechannel and the 2D ferromagnet leads to low spininjection or detection efficiencies as a result of theconductivity mismatch between both materials. Thisunderscores the necessity of a well-defined tunnelbarrier in all-vdW spintronic devices. Furthermore,spin injection and detection have been demonstratedthrough proximity effects induced by Cr2Ge2Te6 ongraphene, yielding relatively small non-local signalsof up to 0.18Ω [5].In this paper, we report on highly efficient spininjection and detection in all-vdW spin transportdevices with a hexagonal boron nitride (hBN) tun-nel barrier between Fe3GeTe2 (FGT) and monolayergraphene. We observe clear spin valve and Hanlesignals in non-local transport measurements, fromwhich we determine the spin injection efficiency,spin relaxation times, and spin diffusion constants.To complement our experimental findings, we per-formed density functional theory (DFT) calculations,providing further insights into the spin transportmechanisms in this promising vdW platform.2. Experimental detailsWe observed spin signals in two very similar spininjection devices, sample A and sample B. Here wepresent the measurement results for sample A, whilethe measurements for sample B are summarized inthe supplementary information (figures S1 and S2). Amicroscope image of sample A is shown in figure 1(a).The device consists of a monolayer graphene channelwith two ferromagnetic contacts on top, composedof an FGT/hBN structure. hBN, FGT and graphenewere exfoliated onto p++ doped silicon (Si) chipswith a 90 nm SiO2 capping layer [32]. However, theFGT flakes were exfoliated in a glovebox with anO2 concentration below 0.1 ppm. The widths of thetwo FGT flakes are 2.6µm and 1.6µm for sampleA and 2.3µm and 1.6µm for sample B. The thick-nesses of the injecting and detecting electrodes forsample A are 145 nm and 85 nm, respectively, whilefor sample B they are 66 nm and 113 nm. The dis-tance between the two FGT flakes, which defines thelength of the spin transport channel, is d= 5µm forsample A and 5.6µmfor sample B,measured betweenthe centers of the flakes. The stack was assembledinside the glovebox on a p++ doped Si chip with a285 nm thick layer of dielectric SiO2 using a stand-ard dry transfer technique employing polycarbonate[33]. The highly doped silicon is used as a globalback gate. The graphene was then patterned intoa Hall bar using electron beam lithography (EBL)and reactive ion etching . The width of the Hallbar is 3.5µm. Subsequently, the contacts to the Hallbar and to the ferromagnetic electrodes were pre-pared using EBL and standard thermal evaporation ofTi(5 nm)/Au(150 nm). A schematic of the completedsample is shown in figure 1(b). The layer sequence ofthe device thus consists of a mono-layer of grapheneon top of the SiO2 substrate followed by a 0.9 –1.3 nm thick layer of hBN (measured with atomicforce microscopy (AFM)) just below the FGT flakes,which are then covered by another hBN flake as a cap-ping layer. The thin hBN flake acts as a tunnel bar-rier to ensure good spin injection efficiency [34]. It isworth noting that the samples without a tunnel bar-rier did not show any spin signal.All experiments were carried out in a cryostatcapable of reaching temperatures as low as 1.5 K,with the sample mounted on a rotating holder thatallowed varying the angle between the sample andthe applied external magnetic field. Spin injectionexperiments were performed in a standard non-localconfiguration (see figure 1(c)), with the charge cur-rent flowing between one of the FM contacts anda reference non-magnetic contact at the end of themesa [35]. The charge current flowing through theFGT/hBN/graphene structure generates a spin accu-mulation in graphene, which diffuses away fromthe junction in all directions (red shaded region infigure 1(c)). The spin accumulation can then bedetected by the second FGT/hBN contact, placed ata distance d from the injecting contact, outside thecharge current path. The non-local voltage meas-ured between the detector and the reference con-tact serves as a measure of the spin accumulationbeneath the detector. The electronic measurementswere carried out using a Yokogawa 7651 as the DCcurrent source and a Keithley 2400 as a back gatevoltage source. The measured non-local voltage wasamplified by a FEMTO DLPVA-101 voltage amp-lifier that was connected to a SynkTek MCL1-54022D Mater. 12 (2025) 045008 J Bärenfänger et alFigure 1. (a) Optical micrograph of sample A. (b) Schematic of the samples. The encapsulating hBN is omitted for clarity. (c)Schematic of the non-local measurement setup. For the spin valve (Hanle) measurements the external magnetic field was sweptout-of-plane (in-plane) along (perpendicular to) the easy-axis of the FGT electrodes.multi-channel data acquisition system. Voltages atother voltage probes, for example the three-terminalvoltage V3T, were measured with the SynkTek dataacquisition system alone, without an amplifier. SinceFGT has itsmagnetic easy-axis out-of-plane, the non-local spin valve experiments were all performed bysweeping the external magnetic field in this direction.For the Hanle measurements, the external magneticfield was swept in-plane, along the long axis of thespin contacts, perpendicular to the transport channel.3. Results and discussion3.1. Electrical characterizationBefore describing the results of the spin measure-ments, we first discuss the electrical characteriza-tion of the device components. We characterized thegraphene channel bymeasuring its sheet resistanceRSas a function of the back gate voltage, determiningthe charge neutrality point at Vg =−4 V, in the Hallbar section of the device (see figure 2(a)). From thismeasurement, mobilities of up to 11 000 cm2Vs−1were extracted, consistent with the results of theHall measurements (see figure S3 in the supplement-ary information). In figure 2(b) we show the I–V-curve of the injection electrode, as a function ofthe three-terminal voltage. The zero-bias resistance-area product R3T, 0VA characterizes the tunnel bar-rier, according to Britnell etal [36]. The measuredR3T,0VA∼ 95 kΩ ·µm2 corresponds to the hBN flakebeing two layers thick, which is consistent with theAFM measurements within the measurement accur-acy. Furthermore, the switching behavior of theinjecting FGT electrode was monitored by measur-ing the transverse voltage across the FGT flake, whilea constant current was sent from the injecting FGTelectrode into the graphene (see figure 2(c)). In aferromagnet, the transverse voltage is composed ofthe regular and the anomalous Hall voltage, withthe latter being proportional to the magnetizationof the ferromagnetic (Rxy = RRH +RAH = R0µ0 ·H+RS ·M, [37]). Therefore, we can attribute a sharpstep in the transverse anomalous Hall voltage to theabrupt switching of the magnetization in our inject-ing FGT electrode. This switching is consistent withthe switching of the non-local voltage in spin-valvemeasurements, as described later.3.2. Non-local spin valveNon-local spin-valve measurements are a standardway of detecting spin accumulation in lateral spininjection devices [8, 35, 38, 39]. Here, a magneticfield is swept along the easy-axis of the spin elec-trodes, which in our case is oriented out-of-plane, andthe non-local voltage Vnl is measured at the detector,with a current flowing in the injector circuit. Changesin Vnl are observed whenever the magnetization ofone of the contacts switches, leading to a transitionbetween parallel and anti-parallelmagnetization con-figurations in the two spin aligning electrodes. Infigure 3(a) we show a typical spin valve trace, wherewe plot Vnl normalized by the injection current I asa non-local resistance Rnl = Vnl/I. The amplitude ofthe switching ∆Rnl serves as a measure of the gener-ated spin accumulation and is given by [7, 40]∆Rnl =PinjPdetRsλswexp(− dλs). (1)In the above equation, λs is the spin diffusion length,w is the width of the channel, and Pinj and Pdet arethe spin injection and detection efficiency, respect-ively. These efficiencies are defined as the spin polar-ization of the injected current directly underneaththe given contact when the contact is used as aninjector. Assuming the same interfaces at the injectorand detector contacts and for low injection currents,one can take Pinj ≈ Pdet = P. In general, however, Pinjcan depend on the injection current, leading to a cur-rent dependence of the measured signal, as shown infigure 3(b). We plot here∆Rnl measured at T= 1.5 Kfor gate voltages Vg = 45V and Vg =−45V, corres-ponding to electron and hole transport in graphene,32D Mater. 12 (2025) 045008 J Bärenfänger et alFigure 2. Electronic characterization of the device. (a) Sheet resistance RS of the graphene channel in the reference area as afunction of the back gate voltage. The measurements have been carried out at T= 1.5 K and with I= 50µA. (b) Characterizationof the hBN tunnel barrier. The current density is plotted against the measured three-terminal voltage at the injecting electrode,which constitutes a voltage dropped across the tunnel barrier contact. The current density, shown on the left side, presents thecurrent normalized by the area of the FGT/hBN contact. (c) Transversal voltage measured across the FGT flake, while sweepingthe external magnetic field out-of-plane during the spin valve measurements shown in figure 3(a). Red (black) line corresponds tothe up (down) sweep. The observed switching corresponds to the switching of magnetization in the FGT flake.Figure 3. (a) Non-local spin valve measurement at T= 1.5 K, Vg = 45V and a current of I=−250µA. The dip of the non-localsignal at low magnetic fields is clearly discernible. In yellow, the height of the non-local signal∆Rnl is shown. (b) Current, (c)back gate voltage and (d) temperature dependence of the non-local signal height in the electron (Vg = 45V) and hole(Vg =−45V) regime.respectively. For both carrier polarities,∆Rnl is higherfor a negative bias, corresponding to injection ofspin-polarized electrons from FGT into graphene orextraction of spin-polarized holes, respectively, anddecreases almost monotonically, as the injection cur-rent is changed towards positive values. For very highpositive currents at T= 1.5 K we even observe aninversion of the spin signal in the electron regime.Such behavior is typically driven by a change inthe sign of Pinj with bias, indicating an inversionof spin polarization around the Fermi level of theferromagnetic material. This phenomenon has beenobserved previously in both conventional graphenespin valve devices [41, 42] and in III–Vmaterials [38].42D Mater. 12 (2025) 045008 J Bärenfänger et alFigure 4. (a) Hanle signal for parallel (red) and anti-parallel (gray) magnetization alignment of the FGT electrodes. Fits to themeasurements are shown as black curves. The non-local Hanle signal height at B= 0 T is∆Rnl, Hanle = 0.88Ω. (b) Non-local spinvalve measurement for the same measurement parameters. The spin valve height is∆Rnl, spin valve = 1.13Ω. The magnetic field isswept perpendicular and parallel to the magnetization direction of FGT in (a) and (b), respectively.In recent experiments with Fe5GeTe2, it was shownthat Fe5GeTe2 had an opposite spin polarization com-pared to that observed in Co electrodes for the entirerange of bias currents used [26]. In our experiments,the sign reversal of the spin valve signal is a resultof the sign change in the tunneling density of states(TDOS) in our structure, as will be discussed later inmore detail.In the entire range of bias currents, the sig-nal is much stronger for electrons than for holes,which is confirmed by plotting ∆Rnl as a functionof gate voltage, see figure 3(c). Additionally, it can beobserved that ∆Rnl increases with the absolute valueof Vg. Similar behavior was also observed at highertemperatures, as can be seen in the supplementaryfigure S4. In figure 3(d) we plot ∆Rnl as a functionof T for I=−250µA, showing a general trend ofdecreasing spin signal with increasing T. Whereas thecurrent dependence of∆Rnl can be linked to the biasdependence of Pinj, explaining its gate and temperat-ure dependence requires information about gate andT-dependence of Pinj, λs, and Rs. To experimentallydetermine λs and Pinj, we performed Hanle meas-urements, investigating spin precession in an externaltransversalmagnetic field, whichwewill discuss in thenext section.Apart from a clear spin-valve pattern, we alsoobserved another feature in the spin-valve measure-ments, namely a dip in the non-local signal at lowmagnetic fields, see figure 3(a) and figure S5 in thesupplementary information. Such a dip is typicallyassociated with the presence of magnetic momentsin a graphene channel, which introduce relaxationof spin currents through exchange coupling [43–45].Given that our samples were fabricated in an inertatmosphere and capped with hBN, and were not sub-jected to any hydrogenation [43, 44] or annealing[45] processes, which are reported to induce mag-netic moments, we cannot provide an explanation forthe origin of these magnetic moments. However, theresults of the Hanle measurements, discussed below,are also consistent with the presence of magneticmoments in the channel.3.3. Hanle signalInHanlemeasurements, the externalmagnetic field isapplied transversely to the orientation of the injectedspins, inducing their precession as they travel fromthe injector to the detector [8]. As a result of diffus-ive motion and spin relaxation, the spins dephase anddepolarize, which is reflected in themeasuredVnl [7].In figure 4(a) we plot a Hanle signal for the injectioncurrent I=−250µA, at a temperature T= 1.5 K anda back-gate voltage of Vg = 60V for the anti-parallel(gray) and parallel (red)magnetization configurationof the two ferromagnetic electrodes of sample A. Thesimilar plot for sample B can be seen in figure S1(d).Since the spins injected from FGT are polarized outof plane, we applied an external in-plane magneticfield, parallel to the long axis of FGT flakes. The dif-ference of the signal measured for parallel and anti-parallel sweeps at B= 0 T gives ∆Rnl,Hanle = 0.88Ω,which is slightly lower than the corresponding spinvalve signal ∆Rnl, sv = 1.13Ω (see figure 4(b)). Thissmall discrepancy between the Hanle and spin valvesignals may be attributed to the presence of magneticmoments, which reduce the spin signal at low mag-netic fields, thereby reducing the height of the Hanlecurve. This observation is consistent with the find-ings of the non-local spin valve measurements, whichalso indicated the presence of magnetic moments.It is noteworthy that these magnetic moments arebelieved to be extrinsic and not associated with the52D Mater. 12 (2025) 045008 J Bärenfänger et alFigure 5. (a) Current, (b) temperature and (c) gate voltage dependence of the spin injection efficiency P obtained from fits to theHanle curves. Error bars, determined as the fitting errors, are in most cases smaller than the size of the symbols.FGT electrodes. However, we cannot rule out a hys-teresis of the signal due to the measurement proced-ure, as we first recorded the spin valves for all currentsand back gate voltages and afterwards performed theHanle sweeps. The small hysteresis with respect to thecurrent or gate voltage cannot be excluded and couldpotentially lead to a smaller signal in theHanle curves.There is also a small asymmetry between the signalin parallel and anti-parallel configuration, which wecannot account for at the moment.At finite transverse fields, we clearly observe oscil-lations of the signal as a result of spin precession andsimultaneous decay of the signal as a result of spindephasing. At a sufficiently large magnetic field B≳0.2 T, spins depolarize through dephasing, the spinsignal approaches zero, and the measured non-localresistance Rnl, offset constitutes the non-local baselineresistance [46, 47]. The solid lines in figure 4(a)are fitting curves based on the steady-state solu-tion of the spin drift-diffusion equation [7] ∂µs∂t =µs ×ωL +Ds∇2µs − µsτs, with the boundary condi-tion at the injector e2Dsν(EF)∇µs = Pinjj. In theabove equations, µs indicates spin accumulation gen-erated by the injection current density j, which atthe detector is measured as a non-local voltage Vnl =−Pdetµs(d), Ds is the spin diffusion constant, τs spinrelaxation time, ωL =g∗µBBh̄ is the Larmor frequencyat the external magnetic field B, with g∗ = 2 being theLandé factor, ν(EF) is theDOS at the Fermi level,µB isthe Bohr’s magneton and h̄ the reduced Planck’s con-stant. From the fitting curves, we obtain the values ofP, Ds and τs, with the latter two giving the spin dif-fusion length λs =√Dsτs. The extracted value of Pis P=√PinjPdet. To minimize errors in fitting thesethree variables, we first fitted the normalized Hanledata (Rnl, B −Rnl, offset)/(Rnl, 0T −Rnl, offset) to extractτs and Ds from the shape of the curves and then wefitted the raw data with the extracted values fromthe normalized fits. Therefore, P was the only vari-able in the second fit. As can be seen in figure 4(a)the fits (shown as a black line) match the experi-mental data quitewell. Fitting the parallelHanle curvegives τs = 0.447 ns, Ds = 0.0210 m2s , and P= 18.4% ,whereaswe obtain τs = 0.415 ns,Ds = 0.0199 m2s , andP= 18.3% in the anti-parallel configuration.We performed Hanle measurements in the par-allel configuration for different injection currents,back gate voltages and at different temperatures. Thefull set of results for sample A and B can be foundin the supplementary figures S6 and S2, respectively.The fitting results for P of sample A are summar-ized in figure 5. In the following section, we discussin more detail the obtained results.3.4. DiscussionAs can be seen in figure 5, we have obtaineda fairly high injection efficiency, reaching up to40%, which is significantly higher than that repor-ted for structures without tunnel barriers [29].However, this is a low estimate of Pinj. When linearlyextrapolating Pdet(1.5K) = P(I= 0,T= 1.5K) to be≈ 17%, the spin injection efficiency is estimated tobe Pinj(−200µA,45V,1.5K) = 93%. Consistent withthe spin valve signal ∆Rnl, P is larger for the neg-ative back gate voltages, i.e. in the electron regime,as shown in figure 5(c), and for negative injectioncurrents, i.e. for the case of electron injection. Pdecreases, while sweeping the injection current fromnegative to positive values as illustrated in figure 5(a).As bias affects only the injector, the decrease in Pwith current is attributed to a decrease of Pinj. P alsodecreases with increasing temperature for T⩾ 50K,although at T= 1.5 K, P is lower than at T= 50K,both in the electron and hole regime, as shown infigure 5(b).In order to properly interpret the current depend-ence of the spin injection efficiency (as shown infigure 5(a)), it is helpful to have some knowledgeabout the spin polarization of Fe3GeTe2. To thisend, we performed DFT calculations of the elec-tronic band structure of the bulk FGT (see thesupplementary information IV for details), includ-ing the spin-resolved DOS. A measure for the degreeof spin polarization of the injected current is the62D Mater. 12 (2025) 045008 J Bärenfänger et alFigure 6. (a) DFT-calculated band structure of bulk Fe3GeTe2. Red (blue) lines correspond to spin up (down). (b) Thecorresponding spin-resolved density of states. Negative (positive) values for the DOS reflect a spin down (up) polarization, asindicated by the blue (red) arrow. (c) Calculated results for the spin polarization PN, PNv and PNv2 .tunneling density of states (TDOS), which is definedvia the product of DOS and the velocity of the Blochbands [48]. It should be noted that this calculationdoes not take into account any properties of the inter-face, the barrier or the second contact. Based on spin-resolved DOS, N↑/↓, and Bloch band velocities in thez-direction (perpendicular to the Fe3GeTe2 layers), vz,we calculate the DOS spin polarization PN of the bulkFGT and the TDOS spin polarization PNv and PNv2 asfollows [48]:PN =N↑ −N↓N↑ +N↓, (2)PNv =⟨Nvz⟩↑ −⟨Nvz⟩↓⟨Nvz⟩↑ + ⟨Nvz⟩↓(3)PNv2 =⟨Nv2z⟩↑ −⟨Nv2z⟩↓⟨Nv2z⟩↑ + ⟨Nv2z⟩↓. (4)In figures 6(a) and (b) we show the calcu-lated FGT band structure and its spin-resolved DOS,respectively. Additionally, the spin polarization PN,together with the TDOS PNv and PNv2 are shown infigure 6(c). We note that at the Fermi level EF and athigher energies, theDOS is highly spin-polarizedwiththe majority of spin-down states. However, slightlybelow EF, the DOS decreases, particularly for spin-down states. BothPN andPNv change sign belowEF. Incontrast to this, PNv2 tends to stay positive in the closevicinity ofEF, indicating that the current is dominatedby the spin-up charge carriers. However, the degreeof spin polarization of PNv2 decreases towards largerenergies. In the experiment, we tune the alignmentof the Fermi-level of graphene and FGT by chan-ging the bias across the junction and we note that thecalculated decrease of PNv2 towards larger energies isvery consistent with the measured decrease of spininjection efficiency towards larger positive currents,shown in figure 5(a).In order to obtain a comprehensive under-standing of the tunneling, it is necessary tocalculate the coherent tunneling for the entireFGT/hBN/hBN/graphene structure. This calcula-tion requires precise knowledge of the band structureand the exact twist angles of each layer. However,as we lack access to this structural information,and given the focus of this paper on the experi-mental realization of efficient spin injection anddetection in all vdW heterostructures, these calcu-lations cannot be performed and are beyond thescope of the presented work. Nevertheless, a changeof sign at or near the EF is evident for all calcu-lated spin polarizations PN, and PNv, and PNv2 ofFGT, which might provide an explanation for thecurrent dependence of the non-local signal height(see figure 3(b)) and the spin injection efficiency(see figure 5(a)).Let us now discuss the obtained spin transportparameters. The extracted values for τs are in therange from ∼300 to ∼600 ps and Ds spans from∼0.004 to ∼0.04m2 s−1. There is no clear depend-ence of both variables on current and on gate voltage.However, fit results of τs and Ds both suggest lar-ger values for negative than for positive back gatevoltages, i.e. in the hole conduction regime (seefigure S6). Furthermore, a small dependence of τs andDs on temperature is observed. Whereas the valuesextracted for T= 1.5 K are larger than at T= 50K,for T⩾ 50K the spin relaxation time increases fromτs = 0.3 ns at T= 50K to 0.5 ns at T= 150K in theelectron regime and a similar effect can also be seenin the hole regime (see figure S6). Also Ds increaseswith temperature in a similar way as τs, so the calcu-lated spin diffusion length λs =√Dsτs doubles from72D Mater. 12 (2025) 045008 J Bärenfänger et al1.5µm to 3.1µm in the electron regime and increasesfrom 2.24µm to 2.78µm in the hole regime.Surprisingly, the extracted values of Ds are sig-nificantly lower than the values of the charge diffu-sion constant Dc at the same temperatures and gatevoltages, as obtained from transport measurements,which are in the range 0.08− 0.12m2 s−1 (see sup-plementary figure S8). This discrepancy between thecharge and spin diffusion constants and the tem-perature dependence of τs could be explained bythe presence of magnetic moments, which wouldbe consistent with the spin valve measurements.Resonant scattering at magnetic impurities intro-duces a temperature-dependent scattering rate [49]and results in narrower Hanle curves due to the addi-tional exchange field [43]. This exchange field can betaken into account in the Hanle curve fitting, takinga larger effective g-factor g∗ > 2. During the above-described fitting of the Hanle curves, a constant g-factor of g∗ = 2 was assumed, which in the presenceofmagneticmoments results in incorrect values ofDs.To correct for this, we performed an alternative fit-ting, where we fixed Ds = Dc and extracted from thefitting the effective g-factor. However, this resulted invery large values of the effective g-factor, reaching ashigh as g∗eff = 23. This would indicate the presence of asubstantial exchange field or a significant amount ofmagnetic moments in the graphene channel, whoseorigin is unknown to us.Another explanation for the peculiar temperaturedependence of τs and Ds, and the low values of Ds,could be provided by the assumption, that under con-tacts both τs and Ds are strongly suppressed becauseof the influence of the ferromagnetic FGT. As thefitting was performed assuming uniform τs and Dsthroughout the channel, the extracted values of bothparameters could be underestimated. As with increas-ing temperature the magnetization of FGT decreases(see figure S9), so does its possible detrimental effecton the spin dynamics in graphene. As a result, theextracted τs andDs would increase. In order to invest-igate a potential magnetic proximity effect [13, 15]at the FGT/hBN/graphene interface, we performedDFT calculations with a two-layer hBN tunnel barrier(see supplementary information IV for details). In thecalculated band structure of the heterostructure, theDirac states of graphene remain spin-degenerate, andno magnetic moments are induced. Consequently, aproximity effect in graphene due to the FGT can beruled out and cannot explain the discrepancy of Dcand Ds.4. ConclusionIn conclusion, we report on efficient electrical spintransport and spin precession in an all-vdW 2Ddevice. Non-local signals are as large as∆Rnl ≈ 1.9Ω,showing a strong current dependence, and even lead-ing to the inversion of the signal. The clear Hanlesignal allowed for a full gate-, temperature-, andcurrent-dependent characterization of the spin trans-port properties. A low estimate of the spin injec-tion efficiencies results in P(−200µA,45V,1.5K) =40%. The observed bias dependence of the spin injec-tion efficiency, and the inversion of the spin valvesignal are consistent with the calculated TDOS. Thepresence of a small dip in the non-local spin valvemeasurements as well as the discrepancy between Dsand Dc suggest the presence of magnetic moments,whose origin, however, remains unknown.Overall, this work demonstrates the potentialof all-vdW heterostructures for realizing high-performance spintronic devices. Future advance-ments in this field could exploit the precise angu-lar alignment of the constituent vdW materials.This concept, often termed as twistronics [50, 51],is a powerful tool to tune the interface propertiesof a given heterostructure. The twist angle betweenstacked layers can be a decisive parameter that influ-ences proximity effects and the resulting band struc-ture of the heterostructure [52–55]. Consequently,angle-dependent studies on similar all-vdW spintransport devices would be highly valuable to unraveland control the spin injection and detection mech-anisms. Ultimately, utilizing the twist-angle betweenvdW materials could pave the way for realizing fullycoherent tunneling spintronic devices with unpre-cedented spin injection efficiencies.Data availability statementThe data that support the findings of this study areopenly available at the following URL/DOI: https://doi.org/10.5283/epub.76433.AcknowledgmentsJ B, J E, K Z, L C and J F gratefully acknowledgesupport from the Deutsche Forschungsgemeinschaft(DFG, German Research Foundation) SFB 1277(Project No. 314695032, sub-Project B07, A09), SPP2244 (Project No. 443416183), the EU GrapheneFlagship project 2DSPIN-TECH (Project No.101135853), and FLAGERA project 2DSOTECH.K.W. and T.T. acknowledge support from theJSPS KAKENHI (Grant Numbers 21H05233 and23H02052) , the CREST (JPMJCR24A5), JSTand World Premier International Research CenterInitiative (WPI), MEXT, Japan. M. C. acknow-ledges support by the National Science Centre,Poland, Project No. 2022/45/B/ST5/04292 of OPUS-23 call. We would also like to thank C Strunkfor facilitating access to the reactive ion etchingsystem.8https://doi.org/10.5283/epub.76433https://doi.org/10.5283/epub.764332D Mater. 12 (2025) 045008 J Bärenfänger et alORCID iDsJan Bärenfänger 0009-0006-1900-8047Klaus Zollner 0000-0002-6239-3271Lukas Cvitkovich 0000-0003-2453-507XKenji Watanabe 0000-0003-3701-8119Takashi Taniguchi 0000-0002-1467-3105Jaroslav Fabian 0000-0002-3009-4525Jonathan Eroms 0000-0003-2212-9537Dieter Weiss 0000-0002-9630-9787Mariusz Ciorga 0000-0003-2441-7874References[1] Novoselov K S, Mishchenko A, Carvalho A and CastroNeto A H 2016 2D materials and van der Waalsheterostructures Science 353 aac9439[2] Deiseroth H-J, Aleksandrov K, Reiner C, Kienle L andKremer R K 2006 Fe3GeTe2 and Ni3GeTe2– two new layeredtransition–Metal compounds: crystal structures, HRTEMinvestigations and magnetic and electrical properties Eur. J.Inorg. 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Introduction 2. Experimental details 3. Results and discussion 3.1. Electrical characterization 3.2. Non-local spin valve 3.3. Hanle signal 3.4. Discussion 4. Conclusion References