High-Temperature Magnetism in Graphene Induced by Proximity to EuO

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High-Temperature Magnetism in Graphene Induced by Proximity to EuO Dmitry V. Averyanov, Ivan Sokolov, Andrey M. Tokmachev, Oleg E. Parfenov, Igor A. Karateev, Alexander N. Taldenkov, and Vyacheslav G. Storchak ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04289 • Publication Date (Web): 28 May 2018 Downloaded from http://pubs.acs.org on May 28, 2018

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ACS Applied Materials & Interfaces

High-Temperature Magnetism in Graphene Induced by Proximity to EuO Dmitry V. Averyanov,† Ivan S. Sokolov,† Andrey M. Tokmachev,† Oleg E. Parfenov,† Igor A. Karateev,† Alexander N. Taldenkov,† Vyacheslav G. Storchak†* †

National Research Center “Kurchatov Institute”, Kurchatov Sq. 1, Moscow 123182, Russia

KEYWORDS: graphene, magnetism, anomalous Hall effect, EuO, spintronics

ABSTRACT – Addition of magnetism to spectacular properties of graphene may lead to novel topological states and design of spin logic devices enjoying low power consumption. A significant progress is made in defect-induced magnetism in graphene – selective elimination of ‫݌‬௭ orbitals (by vacancies or adatoms) at triangular sublattices tailors graphene magnetism. Proximity to a magnetic insulator is a less invasive way, which is being actively explored now. Integration of graphene with the ferromagnetic semiconductor EuO has much to offer, especially in terms of proximity-induced spin-orbit interactions. Here, we synthesize films of EuO on graphene using reactive molecular beam epitaxy. Their quality is attested by electron and X-ray diffraction, cross-sectional electron microscopy, Raman and magnetization measurements. Studies of electron transport reveal a magnetic transition at ܶ஼∗ ≈ 220 K, well above the Curie temperature 69 K of EuO. Up to ܶ஼∗ , the dependence ܴ௫௬ ሺ‫ܤ‬ሻ is strongly non-linear suggesting the

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presence of the anomalous Hall effect. The role of synthesis conditions is highlighted by studies of an overdoped structure. The results justify the use of the EuO/graphene system in spintronics.

INTRODUCTION Graphene holds high promise for spintronics, a promise of both fundamental spin transport phenomena and spin devices with low power consumption.1-3 In particular, graphene can be an outstanding spin channel material – it exhibits room-temperature spin transport with very long spin diffusion lengths, high electron mobility and gate-tunable carrier concentration.1 However, experimental observation of spin-related phenomena and modulation of spin currents in graphene is challenging due to intrinsically weak spin-orbit coupling (SOC) and the absence of ferromagnetism (FM),1,4 which limits the prospects of graphene in spintronics.5 2D magnetism offers hope of ultra-compact spintronics: a significant progress has recently been made in engineering materials with intrinsic 2D FM;6-9 graphene, however, must rely upon extrinsic ways to induce a magnetic state. The most direct approach is to generate some asymmetry between the two triangular sublattices of graphene. Uneven distribution of vacancies and/or adatoms (removing ‫݌‬௭ orbitals) between the C sublattices results in defect-induced graphene magnetism.1,3,10 The same principles apply to graphene shaped into nanoribbons with welldefined 'zigzag' edges comprising atoms from only one sublattice of the bipartite honeycomb lattice.11 However, in practice such chemical functionalization strongly impacts the electronic states of graphene and introduces disorder which may have adverse effects on both charge and spin transport. Putting graphene in direct contact with an FM material seems to be a milder approach, both preserving chemical bonding in graphene and avoiding scattering by random


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impurities. FM metals can be used to mediate magnetism but shunt current away from graphene. Therefore, FM insulators proximity coupled to graphene are actively explored to control spin generation and modulation without compromising graphene properties.5,12-24 The basic idea of such studies is that the magnetic insulator induces a magnetic exchange field (MEF) in the π-system of the adjacent graphene layer. The magnetic material of choice is the ferrimagnetic insulator yttrium iron garnet (YIG). Integration of YIG with graphene leads to observation of effects such as magnetoresistance and spin-to-charge conversion,12,13 modulation of spin transport by exchange fields,14 chiral charge pumping,15 anomalous Hall effect (AHE).16,17 It is important that the effects persist even at room temperature. The system is extensively studied theoretically15,18,19 and the results can be extended to bilayer graphene on YIG.20 The studies suggest a strong influence of the quality of YIG on the properties: the symmetry breaking is associated with screw dislocations;15 the sharp drop in the AHE signal in the temperature range of 2-80 K is attributed to distance change between YIG and graphene16 (not detected for graphene supported by ℎ-BN17); large discrepancy in the reported exchange coupling14-16,20 is attributed to different interaction strengths between YIG and graphene.19 The role of substrate seems to be particularly important for 2D systems.25 Moreover, very recent theoretical studies demonstrate that changes induced in graphene by proximity to a ferromagnet are very complex.21,22 Simple exchange field is accompanied by other terms of the same order of magnitude, such as magnetic orbital couplings and proximity-induced SOC.21 They lead to a variety of phases with different topological features, including AHE regimes, which depend strongly on the balance between those couplings.21 The limit of large SOC comparable with exchange coupling is especially important.22 A significant SOC in graphene can be induced by heavy atoms.21,26,27 In this and other respects, Eu


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chalcogenides (EuX) – a canonical class of magnetic semiconductors – may provide a distinctly new playground for studies of emergent spin-dependent phenomena. These compounds are routinely used to introduce spin imbalance into non-magnetic materials such as transition metal dichalcogenides,28 topological insulators,29 silicon.30 Most remarkably, in the EuS/Bi2Se3 bilayer the induced FM state persists up to room temperature,29 well above the Curie temperature of EuS (~17 K), signifying the crucial role of SOC and the emergence of a topological magnetic state. An important technological advantage of Eu chalcogenides with respect to YIG is that EuX can be readily grown on graphene to enrich the spectrum of possible applications. EuS films are probably the easiest to produce – EuS/graphene heterostructures demonstrate a sizable MEF;23 low-temperature deposition of EuS leads to a substantially enhanced carrier mobility in graphene.24 However, in theoretical studies EuO seems to be more popular than EuS, probably due to its significantly higher Curie temperature; also, the changes in the graphene electronic structure induced by EuO and EuS are quite different.19 Exchange splitting in EuO/graphene is predicted to be 5 meV,31 this value is later revised to 36 meV with 24% spin-polarization of the π-states of graphene.32 Predictions for the EuO/graphene structure include spin filter and spin valve effects,33 spin-valley beam splitting effect,34 topologically non-trivial states due to intervalley interactions;35 the structure can be used to construct an efficient single-electron transistor,36 a highly tunable non-local transistor,37 a junction with a large anisotropic magnetoresistance.38 In Ref. [39], EuO is synthesized on graphene as well as highly-oriented pyrolytic graphite; however, the structural quality of the EuO/graphene films is unclear and spindependent properties of the functionalized graphene are not demonstrated. Optimization of EuO growth leading to textured EuO films on graphene is reported in Ref. [40, 41] but again spin-


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dependent transport properties of the structure are not addressed, probably because graphene is placed on a metallic substrate. Here, we draw on our experience in the epitaxial integration of EuO with various substrates42-45 to couple EuO with graphene. The structure of the films is determined with a number of techniques – electron and X-ray diffraction, electron microscopy, Raman spectroscopy – and confirmed by magnetization studies. Transport measurements demonstrate high-temperature magnetism as revealed by magnetoresistance and anomalous Hall conductivity. The important role of the interface between EuO and graphene is illustrated recruiting the example of overdoped samples.

RESULTS AND DISCUSSION Interfacing EuO with graphene is a major challenge: in addition to difference in lattice symmetries and a significant lattice mismatch, it adds the complexity of joining an ionic system (EuO) to covalent structure (graphene); synthesis of EuO can easily end up as the higher oxides Eu3O4 and Eu2O3. The best films of EuO are produced by molecular beam epitaxy (MBE).43,45,46 A typical synthesis is carried out in the adsorption-controlled (distillation) regime – a larger than stoichiometric flux of Eu is employed with excessive Eu atoms evaporated from the hot substrate. However, in the case of graphene, intense distillation causes formation of an admixture of polar EuOሺ111ሻ and intercalation of graphene by Eu40 – both processes may strongly affect transport and magnetic properties of functionalized graphene. Therefore, we prefer synthesis of EuO employing close to stoichiometric fluxes of the reactants. That makes the range of growth parameters rather narrow.


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We synthesized two types of films of EuO on graphene. The first type employs the weak distillation mode (5% excess of Eu) on a graphene substrate heated to 370 °C. The synthesis of the second type consists of two steps: the first 5 monolayers of EuO are grown at room temperature, then the temperature of the substrate is increased and the rest of the film is grown in the weak distillation mode. The films are remarkably different; the most pronounced difference between the films is in the EuO/graphene interface region. The main feature of the roomtemperature growth is that excessive Eu atoms are not re-evaporated and can concentrate near the interface. We mark such films as EuO/graphene(+Eu) while those produced in the distillation as EuO/graphene. EuO layers in all the films are grown thick (≥ 20 nm) to avoid any changes in the Curie temperature and magnetic moment associated with ultrathin EuO films.47 The structure of the films is controlled in situ with reflection high-energy electron diffraction (RHEED). Figure 1 shows the evolution of RHEED images from pristine graphene to thin EuO films to fully grown EuO films. The streaks ሼ01ത1ሽ and ሼ011തሽ in the RHEED image of graphene are somewhat dim due to the ultimate thinness of the graphene monolayer. The resulting image of EuO on graphene is formed by 2 sets of streaks – outer ሼ01തሽ and ሼ01ሽ and inner ሼ1ത1തሽ and ሼ11ሽ; their positions allow for determination of the lateral lattice parameter of EuO: 5.14 ± 0.02 Å from the outer streaks and 5.15 ± 0.02 Å from the inner streaks, both consistent with the lattice parameter of bulk EuO. The simultaneous presence of these 2 sets of streaks points at textured EuO films (as in Ref. [40]); the intensity of the streaks depends on the azimuth. The streaks are a little spotty which may indicate some roughness of the films. Ex situ measurements require protection of EuO from air; otherwise, the monoxide is readily oxidized by atmospheric oxygen. Therefore, all the films are covered with amorphous SiOx – a standard capping material.


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The atomic structure of the films is further studied ex situ. X-ray diffraction (XRD) methods provide information on the phase purity of the structure and orientation of EuO. Figure 2a presents θ-2θ scan for the EuO/graphene film which confirms the conclusion that the MBE synthesis results in EuO. Both RHEED and XRD determine the orientation of EuO as ሺ001ሻ, i.e. the EuO/graphene interface couples squares of EuO with hexagons of graphene. The same orientation of EuO is detected in other works attempting to grow EuO on graphene.39,40 It is natural to assume that the hexagonal lattice of graphene supports fcc EuO in the ሺ111ሻ orientation; this lattice coupling explains why EuOሺ111ሻ/graphene rather than EuOሺ001ሻ/graphene is invariably employed in the electronic structure calculations.19,32,35 The coupling of EuO with graphene is a very important issue since it strongly influences the proximity effects in the heterostructure. In particular, incommensurate coupling of EuO with graphene may by itself locally break the equivalence between the two graphene sublattices and produce interactions fluctuating over finite-size regions.21 Preferential formation of EuOሺ001ሻ is explained by minimization of surface energies39 and lattice mismatch.39,40 According to lowenergy electron diffraction, sides of EuO squares are aligned along ‘zigzag’ directions of graphene (see Scheme 1 for a ball-and-stick model) which results in a commensurate 2:1 superstructure though only along this particular direction.40 Rotation of EuO crystallites by 30° leads to symmetry equivalent structures. This point is illustrated by ϕ-scan of the EuOሺ022ሻ reflection in the EuO/graphene structure (Figure 2c). As expected, it exhibits 12 periodically distributed maxima for ϕ ranging from 0° to 360°, corresponding to equivalent crystallites. However, the ϕ-scan also has a high average intensity which is probably a consequence of the significant orientational disorder of graphene flakes on the substrate in our macroscopic samples. XRD study of the EuO/graphene(+Eu) structure (Figure 2b) shows that growth at room


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temperature produces EuO in the same ሺ001ሻ orientation. However, the θ-2θ scan also suggests a noticeable amount of higher Eu oxides – within the corresponding broad feature in Figure 2b we discern peaks which can be ascribed to reflexes (040) and (320) of Eu3O4. Their formation may be caused by slow migration of Eu atoms on the surface at room temperature and shortage of Eu during the nucleation stage of the growth due to competing process of graphene intercalation. Alternatively, Eu may segregate at room temperature with subsequent intercalation at the elevated temperature. The structure of the films is refined with analytical electron microscopy. A cross-section of the EuO/graphene film in the high angle annular dark field (HAADF) mode (Figure 3a) confirms the ሺ001ሻ orientation of EuO as well as direct coupling of EuO and graphene. Fast Fourier transform pattern (Figure 3b) identifies the grown film as a rock-salt structure with EuO lattice parameters. The result agrees with the data of electron energy loss spectroscopy (EELS) which determines the valent state of Europium as Eu(II) (Figure 3c). A similar study of the EuO/graphene(+Eu) film also demonstrates formation of EuO on graphene at room temperature (Figure 3d). However, it also detects crystallites of the higher Eu oxide Eu3O4 which can spread from top to bottom of the EuO layer (Figure 3e). The Eu3O4 crystallites are identified with fast Fourier transforms of the corresponding areas of the cross-sections – an example is given in Figure 3g. Most importantly, the electron microscopy study indicates intercalation of Eu under the graphene layer: we detect small regions (puddles) under graphene enriched with Eu (Figure 3f). As the ratio of Eu and O2 fluxes is maintained close to EuO stoichiometry, such segregation explains simultaneous formation of Eu puddles and Eu3O4 in the EuO/graphene(+Eu) structure – Figure 3e presents a fragment of the film where both Eu3O4 crystallite and Eu puddle are formed.


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Before the actual properties of the EuO/graphene structure are studied, it is important to establish that graphene remains graphene after EuO deposition. Raman spectroscopy is just the technique to prove the case. The spectra of graphene and EuO/graphene are demonstrated in Figure 4. The main result is that single-layer graphene is not destroyed by EuO synthesis; also, the MBE growth does not result in any significant amount of defects. The salient property of EuO is magnetism. Therefore, magnetization study helps to establish the quality of EuO integrated with graphene. Figure 5 shows temperature dependence of the normalized magnetization for EuO/graphene and EuO/graphene(+Eu) in a magnetic field of 100 Oe. As expected, the synthesis procedure has little effect on the magnetic properties: both magnetization curves are quite similar and the Curie temperatures ܶ஼ are close to the bulk value of 69 K. Indeed, the magnetization measurements probe the bulk of the films and carry almost no information on the interface region. It should be noticed that a small increase of ܶ஼ of EuO due to graphene proximity is suggested in Ref. [40]. Other characteristics also correspond to FM of EuO: the remnant moment in the EuO/graphene structure vanishes around ܶ஼ ; magnetic field dependence of magnetization exhibits a standard hysteretic behavior (inset in Figure 5). The magnetization studies do not detect a magnetic state of graphene, probably due to insufficient sensitivity. In contrast, studies of electrical transport are more appropriate to detect changes in the electronic structure of graphene monolayer caused by proximity to EuO and get insight into a magnetic state of graphene itself. In fact, while huge EuO signal dwarfs tiny response from graphene in magnetization measurements, electrical transport studies present information on graphene layer rather than EuO. The resistivity of EuO depends strongly on the amount of oxygen vacancies and can be as low as 0.1 Ω cm at low temperature below the metal-


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insulator transition.45 However, our growth procedure is aimed at stoichiometric and thus highly insulating films of EuO: such films on insulating substrates demonstrate resistivity exceeding 1 MΩ cm even at room temperature – thus, the EuO contribution to the overall conductivity of the EuO/graphene structure is exceedingly small. One may suspect that we inadvertently synthesize EuO films with a noticeable amount of O vacancies. However, such films would exhibit a very strong metal-insulator transition at ܶ஼ of EuO, up to 11 orders of magnitude45 – a feature which is absent in our measurements of the EuO/graphene structure. Figure 6a shows temperature dependence of sheet resistivity of the films as compared to graphene whose conductivity is temperature-insensitive. The resistivity of both EuO/graphene and EuO/graphene(+Eu) is lower than that of pristine graphene. The likely reason is electron doping of graphene by EuO as predicted by theoretical studies of the EuO/graphene structure.19 However, there is a striking difference between the two films: the EuO/graphene(+Eu) structure exhibits a characteristic featureless highly conducting metallic behavior – it probably comes from a strong doping due to Eu intercalation of graphene. The dependence is drastically different in the case of the less conductive EuO/graphene structure. The latter exhibits a transition around ܶ஼∗ ≈ 220 K – a feature absent in pristine graphene. The steeper increase of the resistivity above the transition temperature is consistent with scattering at paramagnetic centers above ܶ஼∗ coming from the magnetic disorder. To reveal the nature of the transition, we study the ܴ௫௫ ሺܶሻ dependence in a transverse magnetic field of 9 T (Figure 6b). The magnetic field strongly affects the feature associated with the transition – it appears shifted and partially suppressed. This result suggests a magnetic nature of the transition. The shift of ܶ஼∗ in a magnetic field towards a larger temperature is an indication


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of an FM order below ܶ஼∗ .48 The temperature dependence of magnetoresistance (inset in Figure 6b) – weak changes at low temperature with a maximum at ܶ஼∗ followed by a strong decline above ܶ஼∗ – is also in agreement with the suggested transition from a magnetically ordered state into a disordered (paramagnetic) state. The presented data are not related to low-field magnetoresistance effects reported for YIG/graphene.12 The most striking result is that ܶ஼∗ is much higher than the Curie temperature of EuO. This is in sharp contrast to YIG/graphene where extrapolation of the AHE signal suggests the Curie temperature to be significantly lower than that of YIG.17 The high ܶ஼∗ of EuO/graphene reminds the structure EuS/Bi2Se3, which exhibits an FM state up to room temperature despite the low Curie temperature of EuS (~17 K).29 The remarkable increase in the interfacial Curie temperature is probably due to an exceptional role of SOC associated with heavy Eu. We note that the magnetic transition cannot be ascribed to Eubased compounds since they all are paramagnetic (PM) at ܶ஼∗ . We also conclude that the magnetic transition is not related to defect-induced magnetism.49 First, the synthesis of EuO is carried out in mild conditions, not conducive to defect formation. Second, Raman spectra do not show formation of defects, in contrast to defect-induced magnetism where Raman spectra exhibit a strong D peak and strongly suppressed 2D peak.49 Third, our samples do not demonstrate a semiconducting behavior characteristic for graphene with a high density of defects.49 Another argument against defect-induced nature of the magnetism in EuO/graphene is its absence in EuO/graphene(+Eu) where the concentration of defects is expected to be higher. The emergence of a magnetic state in graphene is further supported by measurements of transverse resistance. Figure 7a demonstrates dependence of ܴ௫௬ on the magnetic field ‫ ܤ‬at ܶ = 2 K. ܴ௫௬ ሺ‫ܤ‬ሻ is linear for both pristine graphene and the EuO/graphene(+Eu) structure. The slope of ܴ௫௬ ሺ‫ܤ‬ሻ indicates that pristine graphene layer is ‫݌‬-doped. In contrast, graphene is strongly ݊-


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doped in EuO/graphene(+Eu) due to Eu intercalation. Both longitudinal and transverse resistances show that this overdoped structure does not exhibit any noticeable magnetism although the band structure of graphene is left intact by Eu intercalation, according to angleresolved photoelectron spectroscopy.50 The EuO/graphene structure proves to be remarkably different – ܴ௫௬ ሺ‫ܤ‬ሻ is essentially non-linear. The Hall resistance in high magnetic fields reveals an expected change of the effective carriers from holes to electrons caused by EuO – as stated above, calculations predict a strong electron doping of graphene by EuO (as well as EuS and YIG).19 The electrical transport measurements reveal a 20% loss of carrier mobility in EuO/graphene with respect to graphene, analogous to the result of Ref. [39] for a similar structure. The high value of carrier mobility in the EuO/graphene structure is another evidence of transport via graphene states – both Eu and Eu oxides exhibit rather low electron mobility. Nonlinearity of ܴ௫௬ ሺ‫ܤ‬ሻ suggests AHE; though, other mechanisms may be responsible. The simplest way to establish that the non-linearity arises from the AHE is to find a correlation between the emergence of an FM order and the ܴ௫௬ ሺ‫ܤ‬ሻ. Figure 7b presents the AHE resistance ܴ஺ுா ሺ‫ܤ‬ሻ (produced by subtraction of the linear part from ܴ௫௬ ሺ‫ܤ‬ሻ). The data suggest that the non-linearity is strongly reduced as soon as temperature exceeds ܶ஼∗ . The same conclusion can be drawn from temperature dependence of the saturated value of the AHE resistance (taken as ܴ஺ுா (-9 T)). The AHE signal increases weakly with temperature up to ܶ஼∗ (as in several other compounds51-53); in contrast, the saturated value of AHE decreases steeply above ܶ஼∗ . This behavior is indicative of an FM transition at ܶ஼∗ – it is difficult to imagine a mechanism based on PM centers leading to such temperature dependence. Another argument against the PM origin of the observed AHE comes from comparison of the samples: the overdoped system EuO/graphene(+Eu) has a significant amount of Eu concentrated at graphene, but does not show


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the AHE, in contrast to EuO/graphene where these PM centers are absent. Most tellingly, thorough measurements of the AHE in EuO/graphene demonstrate a pronounced hysteresis (Figure 8) with the coercive field ~40 Oe. In fact, observation of the AHE indicates a significant SOC induced in graphene. The sign of the AHE in EuO/graphene differs from that in YIG/graphene which highlights the difference between the two magnetic insulators – the sign of proximity-induced AHE in graphene is predicted to be controlled by Hamiltonian parameters.22 Thus, the transport measurements indicate that proximity to EuO induces high-temperature magnetism in the graphene layer.

CONCLUSION Graphene is known for many remarkable properties. However, magnetism is not one of them. The quest for magnetism in graphene has produced two major solutions: defect-induced magnetism and proximity to a magnetic insulator. A strong advantage of the latter approach is that the electronic structure of graphene is less affected. Experimental works concentrate on coupling of graphene with YIG but theoretical studies suggest EuO as a viable alternative to YIG capable to produce new properties of the combined system. In our work, we employ different growth regimes to produce films coupling EuO with graphene. The films are thoroughly characterized by a combination of techniques. Most importantly, the influence of EuO on transport properties of graphene is studied in details. Remarkably, a high-temperature FM transition is found in films grown in the weak distillation regime. The magnetism disappears when graphene is overdoped which shows the utter importance of the synthesis conditions. The emergence of an FM order is firmly established by temperature dependence of the resistance,


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magnetoresistance and AHE. This work is particularly enticing for spintronic applications. We envisage that our results would provide a new platform for studies of magnetism in graphene. The fascinating result that EuO induces magnetism in graphene at a temperature which is 3 times as high as the Curie temperature of EuO calls for new theoretical studies. METHODS Synthesis. The samples of EuO on graphene are synthesized in a Riber Compact 12 system for molecular beam epitaxy furnished with an ultra-high vacuum (UHV) system comprising a TiTan ion pump and Titanium sublimation pump (Gamma Vacuum), a cryopump and cryopanels cooled by liquid nitrogen. The base pressure maintained in the growth chamber does not exceed 10-10 Torr. The system's capacity is sufficient for stable ratios of Eu and oxygen fluxes without any accumulation in the growth chamber. Large-area graphene layers wet transferred to SiO2/Siሺ001ሻ (coverage exceeds 98%, grain size is up to 20 µm, Graphenea) are employed as substrates for EuO films growth. Graphene samples exhibit ‫݌‬-type conductivity with a mobility around 103 cm2 V-1 s-1. Before synthesis of Eu oxides, the substrates are annealed for 10 min at 600 °C under UHV conditions. 4N Eu is supplied from a Knudsen cell effusion source. Molecular oxygen (6N) is injected with stable intensity into the chamber; its flux is tuned with a gas flow system comprising a mass flow controller and Baratron manometer. The cell and substrate temperatures are controlled with thermocouples; however, the absolute temperature of the substrate is determined with a PhotriX ML-AAPX/090 infrared pyrometer (LumaSense Technologies) operating at a wavelength 0.9 µm. The intensity of molecular beams at the substrate site is measured with a Bayard-Alpert ionization gauge. To prevent degradation by air, all the films are covered at room temperature by a capping layer of SiOx.


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Characterization. The surface of the films is controlled in situ with a reflection high-energy electron diffractometer furnished with the kSA 400 Analytical RHEED system (k-Space Associates, Inc.). The atomic structure is further characterized ex situ by X-ray diffraction; the experiments are carried out with a Rigaku SmartLab 9kW spectrometer (CuKα X-ray source). The cross-sectional samples for electron microscopy are prepared in a Helios (FEI) scanning electron microscope/focused ion beam (FIB) dual beam system: a 2 µm layer of Pt is deposited on top of the capping layer; FIB milling with 30 keV Ga+ ions produces 2 µm-thick crosssections; further thinning with 5 keV Ga+ ions and final cleaning with 2 keV Ga+ ion beams results in electron transparency. The specimens are studied with a 300 kV TEM/STEM Titan 80300 (FEI) microscope equipped with HAADF and BF detectors, a spherical aberration (Cs) corrector, a Si(Li) energy dispersive X-ray spectrometer, and a postcolumn Gatan energy filter. The images are analyzed with Digital Micrograph (Gatan) and Tecnai Imaging and Analysis (FEI) software. The magnetic properties of the films are measured with an MPMS XL-7 SQUID magnetometer (Quantum Design) using the reciprocating sample option; the samples are mounted in plastic straws holding orientation of the film surface with respect to an external magnetic field with an accuracy better than 5 °. Transport studies are carried out using the van der Pauw configuration with a LakeShore 9709A Hall effect measurement system for temperatures ranging from 2 to 300 K and magnetic fields up to 9 T. Ohmic contacts are fabricated by depositing an Ag–Sn–Ga alloy onto each terminal.


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Scheme 1. A ball-and-stick model of the structure EuO/graphene showing the relative orientation of the lattices.


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Figure 1. RHEED images of (a) the graphene substrate; (b) the EuO/graphene structure after growth of 10 nm of EuO; (c) the EuO/graphene structure after growth of 20 nm of EuO. The images correspond to the prevailing ሾ100ሿ azimuth of EuO. The position of the streaks is marked red.


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Figure 2. X-ray diffraction scans of the films: (a) θ-2θ scan of EuO/graphene; (b) θ-2θ scan of EuO/graphene(+Eu); (c) ϕ-scan of reflections for EuOሺ022ሻ of the EuO/graphene structure indicating periodicity of 30°. Asterisks denote peaks from the Siሺ001ሻ substrate.


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Figure 3. Atomic structure of the films according to electron microscopy: (a) cross-sectional HAADF-STEM image of EuO/graphene on the SiOx/Si substrate; inset shows a magnified image of the EuO/graphene interface; (b) fast Fourier transform pattern of EuO in EuO/graphene; (c) EELS from EuO region in the EuO/graphene structure; (d) cross-sectional HAADF-STEM image of EuO/graphene(+Eu) on the SiOx/Si substrate; (e) cross-sectional HAADF-STEM image of a defect region of EuO/graphene(+Eu) demonstrating simultaneous formation of a Eu3O4 crystallite and Eu puddle at the interface; (f) a magnified image of a Eu puddle; (g) fast Fourier transform pattern of a Eu3O4 crystallite.


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Figure 4. Raman spectra of graphene before (green) and after (blue) EuO deposition. Fast Fourier transform filtered spectra are shown by red curves.


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Figure 5. Temperature dependence of the normalized magnetization of the films in a magnetic field 100 Oe: the EuO/graphene structure (blue) and the EuO/graphene(+Eu) structure (green). The remnant curve for the EuO/graphene film is also shown (red). Inset: Magnetic field dependence of normalized magnetization of the EuO/graphene(+Eu) film at 2 K.


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Figure 6. Temperature dependence of sheet resistivity of the films: (a) pristine graphene (green), the EuO/graphene structure (blue) and the EuO/graphene(+Eu) structure (red); (b) the EuO/graphene structure in zero magnetic field (blue) and in a magnetic field of 9 T (red, notice different scales); inset: magnetoresistance (MR) of the EuO/graphene structure.


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Figure 7. (a) Magnetic field dependence of transverse resistance in pristine graphene (green), the EuO/graphene structure (blue) and the EuO/graphene(+Eu) structure (red); (b) magnetic field dependence of the anomalous Hall effect resistance in the EuO/graphene structure at 2 K (blue), 50 K (green), 200 K (red), 220 K (black), 250 K (yellow) and 300 K (magenta). Inset: temperature dependence of the saturated (taken at -9 T) anomalous Hall effect resistance in the EuO/graphene structure.


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Figure 8. Hysteretic magnetic field dependence of transverse resistance in the EuO/graphene structure at 2 K. Inset shows the magnified region of low magnetic fields.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (VGS) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is partially supported by NRC “Kurchatov Institute”, the Russian Foundation for Basic Research (grants 16-07-00204, 16-29-03027 and 17-07-00170), and the Russian Science Foundation (grant 14-19-00662). The measurements have been carried out using the equipment of the resource centres of electrophysical, laboratory X-ray and electron microscopy techniques of NRC “Kurchatov Institute”. We are grateful to Dr. S.M. Novikov (Laboratory of Nanooptics and Plasmonics, Moscow Institute of Physics and Technology) for Raman spectra measurements. REFERENCES 1.

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High-Temperature Magnetism in Graphene Induced by Proximity to EuO

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