Proximity-Induced Spin Polarization of Graphene ... - ACS Publications

Proximity-Induced Spin Polarization of Graphene in Contact with Half-Metallic Manganite Seiji Sakai,*,†,‡,⊥ Sayani Majumdar,§ Zakhar I. Popov,# Pavel V. Avramov,∥ Shiro Entani,† Yuri Hasegawa,⊥ Yoichi Yamada,⊥ Hannu Huhtinen,Δ Hiroshi Naramoto,† Pavel B. Sorokin,†,□,# and Yasushi Yamauchi†,‡ †

Quantum Beam Science Research Directorate, National Institutes for Quantum and Radiological Science and Technology QST, 2-4 Shirakata, Tokai, Naka, Ibaraki 319-1106, Japan ‡ National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan ⊥ Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8577, Japan § Department of Applied Physics, Aalto University School of Science, FI-00076 Aalto, Finland ∥ Department of Chemistry, College of Natural Sciences, Kyungpook National University, Daegu 702-701, Republic of Korea # National University of Science and Technology MISiS, 4 Leninskiy Prospekt, Moscow 119049, Russian Federation Δ Wihuri Physical Laboratory, Department of Physics and Astronomy, University of Turku, 20014, Turku, Finland □ Technological Institute of Superhard and Novel Carbon Materials, 7a Centralnaya Street, Troitsk, Moscow 142190, Russian Federation S Supporting Information *

ABSTRACT: The role of proximity contact with magnetic oxides is of particular interest from the expectations of the induced spin polarization and weak interactions at the graphene/magnetic oxide interfaces, which would allow us to achieve efficient spin-polarized injection in graphenebased spintronic devices. A combined approach of topmost-surfacesensitive spectroscopy utilizing spin-polarized metastable He atoms and ab initio calculations provides us direct evidence for the magnetic proximity effect in the junctions of single-layer graphene and half-metallic manganite La0.7Sr0.3MnO3 (LSMO). It is successfully demonstrated that in the graphene/LSMO junctions a sizable spin polarization is induced at the Fermi level of graphene in parallel to the spin polarization direction of LSMO without giving rise to a significant modification in the π band structure. KEYWORDS: graphene, magnetic oxides, interface, proximity effect, spintronics, spin polarization, half-metals

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case of direct contacts, while a large spin polarization of conduction electrons and a spin-filtering effect at the graphene/ magnetic metal interface have been theoretically predicted,16−19 the magnitudes of the spin polarization (P) estimated from the magnetoresistance ratio in current-perpendicular-to-plane spin valves, which allow us to intensively analyze the spin transport properties associated with graphene/electrode interfaces, composed of single-layer graphene (SLG) and multilayer graphene (MLG) are ∼10% at 1.4 K11,12,15 and 50 K14 and less than several % at higher temperatures.10,11,13,14 Exceptionally, a large magnitude of the spin polarization (−42%) has

raphene has attracted great attention in spintronics due to its high potentiality for spin-polarized transport based on the unusual properties such as the small spin−orbit and hyperfine coupling, quantum transport, and excellent charge carrier mobility.1 The effective injection of spin-polarized carriers in graphene is of critical importance in developing spintronic devices by taking advantage of the potential spin transport properties of graphene. Numerous studies have been carried out for the study of the magnetoresistance effect associated with the spin-polarized injection of carriers from ferromagnetic metal (FM) electrodes into graphene by employing both direct (transparent) contacts with graphene/FM junctions and tunneling contacts through insulating barrier layers.2−15,20 However, the effective spin injection in graphene-based devices remains challenging. In the © 2016 American Chemical Society

Received: April 10, 2016 Accepted: July 15, 2016 Published: July 20, 2016 7532

DOI: 10.1021/acsnano.6b02424 ACS Nano 2016, 10, 7532−7541

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Figure 1. (a) Schematic illustration of the SPMDS measurements for the SLG/LSMO junctions. The direct information on the local electronic structure and the spin polarization states of SLG in the junction can be obtained by measuring the intensity and energy distribution of ejected electrons and their changes by altering the direction of the He(23S) spin with respect to the majority spin direction of LSMO, which are represented by black arrows. The spin detection in SLG is based on the spin-selective deexcitation process of He(23S) on the topmost surface to satisfy the Pauli principle. AFM images of SLG/LSMO(001) observed after annealing (b) at 800 K, (c) at 1000 K, and (d) after removing SLG by UV-ozone subsequent to the 1000 K annealing, respectively. The color scale shown on the right side is common for (b)−(d). (e) STM image after 1000 K annealing. Scanning parameters: sample bias = −5 mV; tunneling current = 80 pA. The blue scale bar corresponds to 1 nm.

been reported for the Ni/MLG/Al2O3/Co spin valve20 despite the similar device configuration to that reported in ref 12. The small spin polarization of injected carriers in most reports can be attributed not only to the conductivity mismatch problem20 but also to the local electronic states introduced at the graphene/FM interfaces. Neither the obvious spin polarization nor spin splitting has been confirmed in the spin-resolved photoemission spectroscopy of the SLG/Co(0001) junction,22 being contradictory to the earlier study,23 and the large probing depth (more than several atoms layers) in this spectroscopy makes the spin analysis of graphene rather problematic because of the strong background signals form the underlying magnetic substrate. In addition, the modification of the electronic states of graphene at the graphene/FM interface24−26 could lead to the declination of the spin transport properties. In the case of tunneling contacts with insulating barrier layers, while higher spin polarization of injected carriers has been reported,4,7,8 the high contact resistance limits the potential benefits of graphene as a spin transport material.27−29 It has been also pointed out that the fabrication of an oxide tunneling barrier on graphene by utilizing conventional deposition processes such as radio frequency magnetron sputtering and pulsed laser deposition (PLD) has a large impact on the quality of graphene at the interface by the introduction of disorder and defects.30,31 A direct contact with magnetic oxide (MO) is of current interest as a promising way to control spin-polarized carriers in graphene because of the possible spin polarization induced by the proximity effect in graphene/insulating MO junctions32−36 and weak chemical interactions at the interfaces of graphene and oxides.34,37,38 A recent study has demonstrated that SLG contacted with yttrium iron garnet shows the anomalous Hall effect, which is generally observed in magnetic materials with non-negligible spin polarization and spin−orbit interactions.35 In view of realizing effective spin injection in graphene and related materials, perovskite manganite La 0.7 Sr 0.3 MnO 3 (LSMO) is attractive as a spin injector because of the halfmetallic character (i.e., ∼100%, spin polarization of conduction electrons) at low temperature and the persistence of

ferromagnetism even at room temperature.39,40 It is also expected that the lower conductivity of LSMO compared to FM is useful for minimizing the conductivity mismatch21 with graphene. However, the effects of the proximity contact between graphene and LSMO on the electronic and spin polarization states of graphene in the junction have not been clarified so far. In the present study, we successfully provide direct evidence for the magnetic proximity effect in the junctions of SLG and LSMO thin films by employing spin-polarized metastable atom deexcitation spectroscopy (SPMDS).41 The extremely high sensitivity of SPMDS to the local electronic state of the topmost surface allows us to figure out the spin polarization state of SLG separately from that of the underneath LSMO layer. It is demonstrated that the graphene π band in the SLG/ LSMO junctions is spin-polarized along the spin polarization direction of LSMO without giving rise to visible modification of the band structure. It is also shown that the magnitude of the spin polarization in the vicinity of the Fermi level is significantly large in SLG, different from that on the LSMO surface without SLG. Furthermore, comprehensive DFT calculations reveal an important role of interfacial oxygen atoms in the local spin polarization of SLG by the indirect exchange interaction at the SLG/LSMO interface.

RESULTS AND DISCUSSION The (001) orientation with the c-axis out-of-plane and the (110) orientation with the c-axis in-plane of the LSMO thin films are selected for providing different crystallographic environments around SLG at the interface (see Figure S1 in the Supporting Information). The (001)- and (110)-oriented LSMO thin films with a 150 nm thickness were synthesized on SrTiO3(001) and (110) substrates (10 × 10 × 0.5 mm3), respectively, by PLD (see ref 42 for more details). The film deposition was conducted at 973 K in the O2 gas (0.2 Pa), and the samples were postannealed for 10 min at the same temperature under atmospheric pressure of O2. Carbon contaminant on the LSMO surface was removed by the UV/ 7533


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ACS Nano ozone treatment in air.43 In the preparation of the SLG/LSMO samples, single-layer graphene was grown on a polycrystalline Cu foil by the low-pressure chemical vapor deposition (CVD) method. The SLG was transferred from the Cu foil to the LSMO surface by using a poly(methyl methacrylate) (PMMA)assisted transfer process. Subsequently, the samples were dried at 380 K in air. Prior to the SPMDS measurements, the samples of SLG/LSMO and LSMO without SLG were annealed at 800 K in the ultrahigh vacuum (UHV) chamber with a base pressure of 3 × 10−8 Pa for surface cleaning and for removing residual water existing at the interface.44,45 The spin-dependent electronic structure of SLG in the SLG/ LSMO junctions was investigated by SPMDS, which is illustrated in Figure 1a. The details of the SPMDS measurements have been described elsewhere.41,46 In brief, a thermal beam of the spin-polarized metastable He(23S) atoms generated by optical pumping with circularly polarized radiation was perpendicularly irradiated on the sample set in a UHV chamber. The spot size of the He(23S) beam was less than 3 mm in diameter. The deexcitation of the He(23S) atom occurs with surface electrons spilling out toward the vacuum side at a distance of a few angstroms above the sample surface. The intensity (SPMDS intensity, hereafter) of the ejected electrons by the He(23S) deexcitation was measured as a function of their kinetic energy with a retarding-potential electron analyzer. The SPMDS spectrum thus provides the direct information on the local electronic states of the topmost atomic layer on the sample surface. Furthermore, in the He(23S) deexcitation process, only surface electrons with a spin opposite the electrons of He(23S) are allowed to transit to the 1s hole of He(23S) following the Pauli principle. This spin-selective deexcitation process allows us to elucidate the spin polarization of the local electronic states by measuring the spin-dependent asymmetry (the spin asymmetry, hereafter) of the SPMDS intensities (I↑↑ and I↑↓) under the parallel and antiparallel configurations of the He(23S) spin with respect to the magnetization direction (the majority spin direction) of the sample. The experimentally measured spin asymmetry A is defined as A (E ) =

800 K and (c, e) 1000 K annealing and (d) the UV-ozone treatment subsequent to the 1000 K annealing, respectively. After the annealing at 800 K, many wrinkles 10−30 nm in width and a few nanometers in height are observed among the smooth regions (Figure 1b). It is known that the transfer of CVD-grown graphene on substrate often results in the formation of wrinkles in graphene.47 The number of wrinkles decreases with increasing the annealing temperature, and an atomically smooth surface with only a small number of wrinkles is obtained after the annealing at 1000 K (Figure 1c). The presence of SLG on the sample surface is confirmed from the atomic resolution STM image (Figure 1e). We can see the honeycomb lattice of graphene with a small corrugation of less than 0.1 nm. The surface morphology of the LSMO substrate underneath SLG is confirmed by removing SLG from SLG/ LSMO(001) through the UV-ozone treatment (Figure 1d). After this treatment, the graphene wrinkles as observed in Figure 1c have disappeared and instead flat LSMO grains of ∼100 nm in diameter were observed on the sample surface. The root-mean-square of the surface roughness estimated from the AFM images in Figure 1c and d are 1.2 and 1.4 nm for the surfaces of SLG and LSMO(001) in SLG/LSMO(001), respectively. The comparable surface roughness except for the graphene wrinkles and grain boundaries in the former and latter cases implies a sufficiently high degree of SLG adhesion to the LSMO surface, which enables us to investigate the proximity effect in the junction. Raman spectroscopy provides us abundant information on the quality and electronic states of graphene from the vibrational features.48−52 Figure 2a shows a set of Raman spectra of SLG/LSMO(001) after drying at 380 K and annealing at 800 and 1000 K, respectively. In the inset, the two spectra obtained from the same SLG/LSMO(001) sample after 1000 K annealing and after the subsequent UV-ozone treatment for removing SLG on the surface are presented over a broader energy range for comparison. The Raman signals were collected from a 100 μm × 100 μm area of the sample in a backscattering geometry by using a 488 nm laser. One can see the prominent peaks of the G band (∼1590 cm−1) and 2D band (∼2700 cm−1), which are characteristic of the graphitic structure, and a weak peak of the defect-induced D band (∼1350 cm−1). The symmetric shape of the 2D band and the large intensity ratio of the 2D/G bands (I(2D)/I(G) > 1) guarantee the single atomic layer of graphene. In the inset, one can also recognize the absence of graphene-related peaks after the UV-ozone treatment. As a measure of the crystal quality of SLG, the average distance (LD) between defects is estimated from the intensity ratio of the D and G bands.52 Although the presence of the relatively strong background signals from the underneath LSMO/SrTiO3 substrate in the low-energy region (see the spectrum with red markers in the inset of Figure 2a) makes the quantitative estimation difficult, the values of LD are estimated to be ∼45, ∼40, and ∼20 nm after 380, 800, and 1000 K annealing from the D/G ratios of ∼5% (380 K), ∼7% (800 K), and ∼25% (1000 K), respectively. Large blue shifts of the G and 2D bands and a decrease of I(2D)/I(G) are observed when the annealing temperature is raised to 1000 K. These peak shifts can be attributed to the p-type doping and compressive strain in SLG.50,51 Figure 2b shows the X-ray photoemission spectra of SLG/ LSMO(001), which covers the energy range of the C 1s, La 4s, and Sr 3p core levels. The photoemission measurements were performed with Al Kα radiation (1486.6 eV). In the spectrum

1 ⎛ I(E)↑↑ − I(E)↑↓ ⎞ ⎜⎜ ⎟⎟ PHe ⎝ I(E)↑↑ + I(E)↑↓ ⎠

where PHe is the polarization degree of the He(23S) spin (PHe = 0.94). According to the measurement principle, the positive (negative) spin asymmetry sign in SPMDS indicates the negative (positive) spin polarization of the electronic states. In the present study, the SPMDS and spin asymmetry spectra were measured for the SLG/LSMO and LSMO samples with increasing the annealing temperature from 800 to 1000 K. All SPMDS measurements were performed at 100 K because of the significant decrease of the remanent magnetization (Mr) of LSMO at higher temperature. The sample was magnetized inplane transverse to the He(23S) beam by applying a pulsed magnetic field of 1000 G, and the He(23S) polarization direction with respect to the magnetization direction was switched in between parallel and antiparallel directions by altering the helicity of the pumping radiations. Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) reveal the evolution of the surface morphology of the SLG/LSMO samples with the increase of the annealing temperature. Figure 1b−e show the (b−d) AFM and (e) STM images of the SLG/LSMO(001) sample after (b) 7534


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3p peaks at 268.5 eV (3p3/2) and 278.6 eV (3p1/2) and the La 4s peak at ∼274.5 eV]. After 1000 K annealing, a small peak at 289 eV, which is assigned to the CO bond,53 appears in the spectrum. The energy position of the main C 1s peak after 800 K annealing is well in agreement with that of highly oriented pyrolytic graphite (HOPG). The annealing at 1000 K induced a shift of the main C 1s peak by −0.4 eV. Since the C 1s peak shift can be related to the Fermi-level shift of graphene,54,55 it can be said that after 1000 K annealing the Fermi level (EF) of SLG is somewhat (∼0.4 eV) below the Dirac point with reference to C 1s of HOPG. The p-type doping of SLG is consistent with the Raman spectroscopic analysis and can be interpreted by the charge transfer55 from SLG to LSMO considering the difference in work function between graphene (4.5 eV) and LSMO (4.8−4.9 eV).56,57 It is reasonably considered that the increase in adhesion between graphene and LSMO after high-temperature annealing leads to a decrease in the wrinkles (Figure 1b and c) as well as the p-type doping of SLG. The appearance of the 289 eV peak in the C 1s spectrum and the decrease of LD in Raman analysis indicate that the hightemperature annealing causes partial oxidization and defect formation in SLG possibly through reaction with oxygen from LSMO, as has been reported for SLG/SrTiO3(001).45 It is also notable that the intensities of the Sr 3p peaks increase relatively to the other peaks after 1000 K annealing. This can be attributed to the Sr segregation in LSMO,58−61 as confirmed by angle-resolved X-ray photoemission spectroscopy (see Figure S3 in the Supporting Information). Both SLG/LSMO and LSMO samples display ferromagnetism with the Curie temperature well above room temperature (Figure 2c), in agreement with the previous study for LSMO thin films.42 For each sample annealed at 800−1000 K, the coercive force (HC) and the ratio of the remanence to saturation magnetization (Mr/Ms) are HC = ∼10 Oe and Mr/Ms = 75−80% at 100 K, respectively (Figure 2d). A similar magnetic property is also observed for the LSMO samples without SLG. Figure 3 shows the SPMDS and spin asymmetry spectra of SLG/LSMO(001) annealed at 800 and 1000 K, respectively, together with the spectra of LSMO(001) annealed at 1000 K. In Figure 3a, SLG/LSMO(001) annealed at 800 K shows the spectral features that agree with those reported for graphite.62 The broad intense structure involving a linear increase just below the Fermi level (EF = 15.3 eV) and two maxima located

Figure 2. (a) From the bottom to the top: Raman spectra of SLG/ LSMO(001) dried at 380 K (green) and UHV-annealed at 800 K (black) and 1000 K (blue), respectively. The inset illustrates a comparison of the spectra collected from the same SLG/ LSMO(001) sample after 1000 K annealing (blue) and after the UV-ozone treatment subsequent to the 1000 K annealing (red) over a broader energy range. The sharp peaks with weak and strong intensity at 1557 and 2331 cm−1 are attributed to ambient O2 and N2 gases, respectively. The gradual intensity increase with decreasing the wavenumber in the region below ∼1500 cm−1 is caused by the LSMO/SrTiO3 substrate. The numbers near the D and G bands indicate the peak positions of the respective bands. (b) X-ray photoemission spectra of SLG/LSMO(001) at the C 1s (∼284 eV), La 4s (∼274.5 eV), and Sr 3p3/2, 1/2 (∼268.5 and ∼278.6 eV) core levels, which were measured after 800 K (black) and 1000 K (blue) annealing, respectively. (c, d) Magnetization data of SLG/LSMO(001) after 1000 K annealing. (c) Temperature (T) dependence of magnetization (M(T)) measured under the field-cooled-warming condition at the in-plane magnetic field of H = 500 Oe, which is enough to saturate the magnetization of the LSMO(001) thin film, in the temperature range of T = 5−300 K. The degree of magnetization is normalized by the quantity at 5 K (M(5 K)). (d) Magnetization curve normalized by the saturation magnetization (MS) measured at 100 K.

after 800 K annealing, one can see the peaks originating from SLG [the main C 1s peak at 284.5 eV and the weak peak at 291 eV, which is assigned to the π−π* shakeup satellite of the aromatic sp2 carbon], as well as the peaks from LSMO [the Sr

Figure 3. (a, c) SPMDS and (b, d) spin asymmetry spectra of SLG/LSMO(001) and LSMO(001), respectively. (a and b) SLG/LSMO(001) after 800 K (black) and 1000 K (blue) annealing. (c and d) LSMO(001) after 1000 K annealing. The measurements were performed at 100 K. EF denotes the Fermi level. π and σ* denote the spectral components associated with the graphene π band and σ* band, respectively. O and L1−5 denote the oxygen- and LSMO-related peaks. 7535


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ACS Nano at ∼13 and ∼8 eV originate from the π band of graphene. The two maxima are associated with the electronic states around the K point and the M and Γ points of the graphene Brillouin zone, respectively. The small peak at 3.7 eV can be assigned to the final state structure reflecting the unoccupied σ* band.62 The above consistency also ensures that the He* deexcitation on the surface of SLG/LSMO(001) occurs via the Auger deexcitation (AD) process63 in a similar manner to that on graphite.62 The 1000 K annealing results in an overall shift of the SPMDS spectrum to the lower kinetic energy side and also the appearances of additional peaks derived from oxygen63 and LSMO (see below for the details), which are labeled O and L3, respectively. The overall shift after the annealing indicates an increase in work function in agreement with the p-type doping of SLG. The appearance of the O and L3 peaks seems to coincide with the partial oxidation (and the associated decomposition) of SLG as described above. However, it can be said that most of the surface area of SLG/LSMO(001) is covered with SLG even after the high-temperature annealing, judging not only from the AFM and STM images (Figure 1) but also from the similar extent of the Raman (Figure 2a), C 1s (Figure 2b), and SPMDS (Figure 3a) signal intensities from SLG between 800 and 1000 K. In Figure 3b, SLG/LSMO(001) shows a negative spin asymmetry, indicating a positive spin polarization in the π band region. Only a small asymmetry of about −1% is observed around EF after 800 K annealing, and its magnitude increases significantly with the increase of the annealing temperature. After 1000 K annealing, the spin asymmetry in the vicinity of EF becomes A(EF) = −4.5%. In spite of the incomplete magnetization (∼80%) of SLG/ LSMO(001) in remanence (see Figure 2d), the magnitude of A(EF) is remarkably large compared to that of the SLG/ Ni(111) junction (A(EF) = −2%)25 and is rather close to those of ferromagnetic metals: Fe, Co, and Ni single crystals with various crystal orientations (A(EF) = +3% to +10%).25,41,64−69 While a quantitative method of the determination of the spin polarization from the SPMDS spin asymmetry has not been established, the present result clearly demonstrates that SLG has a sizable spin polarization of conduction electrons in the SLG/LSMO(001) junction. In Figure 3c, the L1−L4 peaks in the SPMDS spectrum of LSMO(001) are assigned to the Mn 3d−O 2p hybridized states, which have large contributions from the Mn 3d orbitals (L1 and L2) and O 2p orbitals (L3 and L4), respectively.70−73 The L5 peak can be assigned to the Mn 3d-derived satellite caused by the final state effect.73 In the spin asymmetry spectrum (Figure 3d), one can see two peaks with negative spin asymmetry at 15.2 and 13.7 eV. These peaks can be interpreted in terms of the positive spin polarization of the Mn 3d eg and t2g states.71 This clearly shows that the MnO6 octahedral coordination of LSMO still survives on the LSMO(001) surface even after the high-temperature annealing. It is also noteworthy that on the LSMO surface the He(23S) atoms deexcite via the AD process, as suggested from the good correspondence between the SPMDS and photoemission data.63 In order to discuss the details of the electronic and spin polarization states near the Fermi level, the magnified SPMDS and spin asymmetry spectra of SLG/LSMO(001) and LSMO(001) after 1000 K annealing are replotted in Figure 4a and b as a function of the binding energy E − EF. In SLG/ LSMO(001), the SPMDS intensity shows a linear decrease with a decrease in the binding energy in the range of E − EF < −1 eV

Figure 4. Magnified (a) SPMDS and (b) spin asymmetry spectra of SLG/LSMO(001) (blue) and LSMO(001) (green) after 1000 K annealing as a function of the binding energy E − EF in the energy region near the Fermi level. Unmagnified (c) SPMDS and (d) spin asymmetry spectra of SLG/LSMO(110) after annealing at 1000 K. The gray dashed-dotted lines in (a) and the blue and green solid curves in (b) are guides to the eye.

(Figure 4a). Note that the nonlinear intensity change seen just at ∼EF is attributed to the He(23S) atom deexcitation with the short electron transition time, which causes the energy level blurring with an exponential tail according to Heisenberg’s uncertainty principle.74 Since, in the case of the AD process of He(23S), the SPMDS spectra directly reflect the local density of states (DOS) of the topmost atomic layer,63 the linear spectral behavior demonstrates that the π band structure of SLG around the Fermi level is preserved in the junction. This behavior is clearly distinct from the plateau-like feature of the L1 peak in LSMO(001) (Figure 4a) and also an appearance of additional peaks in SLG/Ni(111) around EF,25 which is attributed to the strong π−d hybridization at the graphene/FM interfaces.24,25,75 The magnitude of the negative spin asymmetry in SLG/ LSMO(001) linearly increases up to EF with decreasing the binding energy in the energy range just below the Fermi level (Figure 4b). In addition, one can see a small negative spin asymmetry peak at −1.7 eV. The same negative spin asymmetry sign in between SLG/LSMO(001) and LSMO(001) in the region close to EF indicates that the graphene π band is spinpolarized parallel to LSMO with the positive spin polarization. The above SPMDS features evidence that a sizable spin polarization is induced in the π band of SLG by the ferromagnetic exchange coupling with LSMO without giving rise to significant changes in the electronic structures of graphene. Interestingly, the magnitude of the spin asymmetry in the vicinity of EF is significantly larger in SLG/LSMO(001) compared with that in LSMO(001) (Figure 4b), which suggests that the spin polarization of conduction electrons in SLG is significantly higher than that on the LSMO(001) surface without SLG. In Figure 4c and d, SLG/LSMO(110) exhibits SPMDS and spin asymmetry spectra with similar features to those in SLG/ LSMO(001), except for minor differences in the energies of the Fermi level and σ* peak position, which are indicative of the work function difference, and the absence of the LSMO-derived peaks. In SLG/LSMO(110), the SPMDS intensity and the spin asymmetry decrease and increase up to EF, respectively, in a similar manner to those in SLG/LSMO(001), and the magnitude of the spin asymmetry at the Fermi level is also comparable to that in SLG/LSMO(001). The above spectral similarities between SLG/LSMO(001) and SLG/LSMO(110) 7536


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where N↑(E) and N↓(E) are spin up and spin down DOS contributions, respectively. A reasonable agreement with experiments is obtained for the SLG/LSMO(001) structure with the interface partially terminated by oxygen atoms on the topmost layer of LSMO(001) as illustrated in Figure 5b−d (the green, gray, red, and pink spheres represent Sr, La, O, and Mn atoms, respectively). The SLG/LSMO(001) structure, which has three terminating oxygen atoms per supercell (the maximum number of terminating oxygen atoms is 24), displays a positive spin polarization of SLG in the vicinity of the Fermi level (Figure 5a), which linearly increases to EF with decreasing the binding energy, consistently with the negative increase of the spin asymmetry in SPMDS (Figure 4b). The density of states displays a weak but definite p-type doping of SLG (Figure 5a, inset). The overall similarity of the SLG/ LSMO(001) DOS shape to that of pristine SLG leads to the conclusion that, despite some distortion around the Dirac cone, the the graphene π band is not significantly disturbed in the SLG/LSMO(001) structure. The experimentally observed positive spin asymmetry peak located at E − EF = −1.7 eV (Figure 4b) might be related to the oscillatory behavior of the theoretically calculated spin polarization (Figure 5a), which strongly depends on the concentration of the interfacial oxygen atoms at the surface layer of LSMO (see Figure S4 in the Supporting Information). The spatial distribution of the spin density close to the Fermi level (Figure 5b) allows us to elucidate the nature of the induced spin polarization in SLG. The spin polarization of the LSMO fragment, which is mostly localized on the MnO layers, causes the spin polarization of SLG at the interface through the indirect exchange coupling between manganese atoms and SLG mediated by the interfacial oxygen atoms. These oxygen atoms draw the graphene’s electrons toward LSMO (Figure 5d), which affects the doping level of SLG and causes wide variations of the graphene Dirac point energy from −1 to 2 eV in binding energy depending on the oxygen concentration at the interface (Figure S4). This result explains the experimentally observed p-type doping of SLG, which therefore can be originated by the presence of oxygen atoms at the junction interface. The spatial distribution of the charge density in SLG/ LSMO(001) allows us to understand the nature of the interfacial interactions in the junction (see Figure 5c and d). The absence of the charge density between SLG and the LSMO fragments (Figure 5c) evidences that the interaction between SLG and LSMO(001) is physical in nature. The analysis of the differential charge density between SLG/LSMO(001) and the fragments of freestanding SLG and LSMO(001) (Figure 5d) clearly shows that the electrostatic interactions between the π orbitals of graphene and the O 2p orbitals at the interface are responsible for the adhesion between SLG and LSMO since no overlapping of the charge density is recognized in the interface region. The adhesion energy of SLG to the LSMO surface and the distance between SLG and LSMO are estimated to be 0.2 eV/carbon atom and 2.01 Å, respectively, which are comparable to those of SLG on metals, such as Co (0.16−0.18 eV/carbon atom and 2.05−2.08 Å),56,76 Ni (0.12−0.13 eV/carbon atom and 2.05−2.10 Å),56,75−77 Ru (0.14 eV/carbon atom and 2.22 Å),75 and sapphire (0.11−0.13 eV/carbon atom and 2.7−2.9 Å).38 The strong electrostatic interactions at the interface and the small interlayer distance facilitate the exchange splitting of the π band of graphene with keeping intact the key features of the band structure such as the Dirac cone and π-electronic

are in contrast to the considerable differences between LSMO(001) (Figure 3c and d) and LSMO(110) (see Figure S2a and b in the Supporting Information), which can be attributed to the difference in the atomic configuration of the topmost layer of LSMO at the interface depending on the crystallographic orientation (Figure S1). The spectral similarities between SLG/LSMO(001) and SLG/LSMO(110) indicate the similar exchange mechanism in these junctions irrelevant to the crystallographic environment at the interface. Comprehensive DFT calculations were carried out in order to investigate the mechanism of the spin polarization of SLG in the SLG/LSMO junctions (Figure 5). Taking into account the

Figure 5. (a−d) Simulated atomic structures and electronic properties of SLG/LSMO(001). (a) Spin polarization (the density of states is presented in the inset), spatial distributions of (b) differential spin density from −0.5 to 0 eV (the Fermi level), (c) total charge density, and (d) differential charge density of the SLG/ LSMO(001) structure, respectively. The differential spin and charge densities are those in the SLG/LSMO(001) structure relative to the freestanding SLG and LSMO fragments. In (b), the upper and the lower panels show the side and top views of the SLG/LSMO(001) structure, respectively. In (b)−(d), Sr, La, O, and Mn atoms are represented by the green, gray, red, and pink spheres, respectively. In (d), the loss and gain of the charge are denoted by yellowish and bluish colors, respectively. (e) Spin polarization at the surface layers of the LSMO(001) fragment (enclosed by the dashed light blue line in (b)) in the SLG/ LSMO(001) structure (blue line) and in the freestanding LSMO(001) fragment (green line) with the same lattice parameters and the atomic structure.

survival of the MnO6 octahedron and possible Sr segregation at the LSMO surface, the calculations were carried out for the SLG/LSMO(001) structures with a SrO termination of LSMO at the interfaces. The spin polarization (Figure 5a) was derived from the spin-resolved partial density of states (the inset of Figure 5a) using the following equation: P(E) =

N ↑(E) − N↓(E) N↑(E) + N ↓(E) 7537


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junctions by analyzing the spin asymmetry of the He(23S) deexcitaion signals in SPMDS. We demonstrate that a significant spin polarization is proximity-induced in SLG without giving rise to the visible modification of the graphene electronic structure. The comprehensive DFT calculations reveal that the indirect exchange interaction between SLG and LSMO under the strong electrostatic interactions at the interface is responsible for the large magnetic proximity effect. The calculations also point out that the contact with graphene increases the local spin polarization of the surface LSMO layer, which eventually leads to the sizable spin polarization of SLG in the junction. It is expected that the use of graphene/LSMO contacts in graphene-based devices allows us to improve the spin injection efficiency into graphene with minimizing the conductivity mismatch21 and also the declination of the spin transport properties of graphene at the interface. We hope that opportunities for designing high-performance spintronic devices would be opened up by employing spin-polarized graphene/MO junctions in graphene spintronics.

system. This is remarkably different from the strong chemical interactions and the spin polarization of the π−d hybridized states at the graphene/FM interfaces.24,25,75 Here, we theoretically argue the contrasting spin polarization behaviors in the vicinity of the Fermi level in between SLG of SLG/LSMO(001) and the LSMO(001) surface without SLG (Figure 4b). In Figure 5e, we compare the calculated results of the spin polarization at the surface LSMO layers (marked by the dashed rectangle in Figure 5b) of the LSMO fragment in the SLG/LSMO(001) structure and also at the corresponding surface layer of the freestanding LSMO fragment without SLG, but with the same lattice parameters and atomic structure in order to avoid the influence of the strain on the spin polarization. In both cases, we can see two peaks, 1(1*) and 2(2*), of the spin polarization near the Fermi level. These two peaks are associated with the interfacial oxygen atoms at the surface LSMO layer and caused by the ferromagnetic exchange coupling between the interfacial oxygen atoms and the MnO layers of LSMO inside. It is found that both peaks in freestanding LSMO(001) (1 and 2) and in SLG/LSMO(001) (1* and 2*) are different in both the position and the shape. In the case of freestanding LSMO(001), the spin polarization of peak 2 decreases in the region E − EF > −0.5 eV, in good agreement with the spin asymmetry spectrum of the LSMO(001) sample (Figure 4b). On the other hand, the same LSMO fragment incorporated in SLG/LSMO(001) displays shifts and shape changes of the peaks due to the electrostatic and exchange interactions between LSMO and SLG. This leads to the increase of the spin polarization near EF up to almost 100% at the surface LSMO layer of SLG/ LSMO(001), as shown in Figure 5e, and then induces the sizable polarization of SLG as elucidated in SPMDS. Half-metallicity is known to be lost at the surface and interface of half-metals, while it is kept in the deeper region.78,79 Our calculations demonstrate that in the SLG/LSMO junction half-metallicity is recovered at the surface layer of LSMO by contact with SLG. It is therefore reasonably considered that the spin polarization of SLG can be further improved by maximizing the transfer efficiency of the spin polarization between the SLG and the LSMO surface through optimizing the surface structure of LSMO with intercalated species and its concentration, e.g. by controlling the concentration of the interfacial oxygen atoms (see Figure S4). It should be noted that the concentration of the interfacial oxygen atoms can also affect the exchange interaction and magnetism of LSMO in the region near the interface possibly due to the change in the density of itinerant electrons.80 The similar exchange interactions induced under the electrostatic interfacial interactions can take place not only in other graphene/MO junctions35,36 but also at the first molecular layer of organic molecules in molecule/MO junctions depending on the molecular orbital structure. We believe that the present finding of the proximity-induced interfacial spin polarization in physical contact would open up intriguing perspectives for “spinterface” science in the field of molecular spintronics, in which the ability to modify the local spin polarization at the organic molecule/ magnetic metal interface has been studied focusing on the spindependent orbital hybridization between molecules and magnetic metals involved in chemical contact.81−83

METHODS CVD Growth of SLG. A Cu foil placed in a quartz furnace was annealed at 1000 °C for 90 min under a reducing atmosphere of Ar (50 Pa) and H2 (20 Pa). Subsequently, CVD of graphene was performed for 5 min at the same temperature and atmosphere by additionally introducing CH4 (2 Pa) into the furnace. After the CVD growth, the sample was rapidly cooled to room temperature in the same atmosphere. PMMA-Assisted Transfer Process of SLG onto LSMO. The SLG grown on a Cu foil was spin-coated with a thin PMMA layer. A sheet of SLG supported with PMMA was prepared by removing the Cu foil with a FeCl3 solution, and the sheet of the PMMA-supported SLG (PMMA/SLG) was washed with distilled water several times and floated on a water surface for 2 days in order to remove ionic residues. The PMMA/SLG sheet was transferred onto the LSMO thin film, and the PMMA layer was removed by dissolving in hot acetone. The obtained SLG/LSMO sample was immersed in acetone for 2 days to remove organic residues and was dried at 380 K in air. No iron contamination and residual PMMA were detected by XPS and Raman spectroscopy, respectively. DFT Calculations. All calculations were performed in the framework of spin-polarized DFT theory by the VASP code84 using the plane wave basis set and projector augmented wave method.85,86 Exchange−correlation effects were taken into account in the framework of the generalized gradient approximation (GGA) by the Perdew−Burke−Ernzerhof (PBE) functional.87 To address the strong correlation effects in LSMO, the GGA+U approach was used.88 The U = 2 and J = 0.7 eV parameters were set in compliance with refs 89 and 90. The Brillouin zone was sampled with a (2 × 2 × 1) Monkhorst− Pack mesh91 of k-points for geometry optimization and (7 × 6 × 1) for electronic structure calculations. The cutoff energy for the plane-wave basis set was set to be 400 eV. To allow van der Waals interactions between the SLG and LSMO fragments, Grimme correction92 was applied. To avoid artificial interactions between periodic replicas of low-dimensional nanoclusters, a vacuum interval of 10 Å was introduced in all 2D supercells. Structures were visualized in VESTA.93

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02424. Schematic representations of the (001)- and (110)oriented LSMO lattices and their planes on the surface (Figure S1); SPMDS and spin asymmetry spectra of LSMO(110) (Figure S2); analyzed result of the angleresolved X-ray photoemission spectroscopy data col-

CONCLUSIONS In summary, the presence of the proximity-induced spin polarization in graphene is evidenced for the SLG/LSMO 7538


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ACS Nano

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lected for SLG/LSMO(001) after annealing at 1000 K (Figure S3); DFT calculation results of the SLG/ LSMO(001) structure under two extreme conditions of concentration of the interfacial oxygen atoms terminating the LSMO surface (Figure S4) (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: (S. Sakai) [email protected]. Author Contributions

S.S., Y.Y., and S.E. fabricated the SLG/LSMO samples and performed the measurements of SPMDS, XPS, Raman spectroscopy, and also the magnetic property. S.M. and H.H. prepared the LSMO thin films. Y.Y. and E.H. performed the AFM and STM observations. Z.I.P., P.V.A., and P.B.S. performed the theoretical DFT calculations. H.N. contributed to the analysis of the experimental data. S.S. wrote the manuscript. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge Dr. Yuki Fukaya (ASRC-JAEA), Dr. Shin-ichi Shamoto (ASRC-JAEA), Dr. Atsuo Kawasuso (QST), Mr. Takahiro Watanabe (Univ. of Tsukuba), Dr. Liubov Antipina (Technological Institute for Superhard and Novel Carbon Materials), Dr. Artem V. Kuklin (Siberian Federal University), Dr. Alex A. Kuzubov (Siberian Federal University), and the members of QST and ASRC-JAEA for fruitful discussion, assistance with AFM observations, and theoretical support. This work was supported by JSPS KAKENHI Grant Number 16H03875 and JST Paris. The authors gratefully acknowledge the financial support of the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISiS” (No. K22015-033) and Russian Science Foundation (Grant No. 14-1300139). P.B.S. acknowledges the financial support of the RFBR, according to the research project No. 16-32-60138 mol_a_dk. S.M. and H.H. acknowledge Academy of Finland (Grant No. 13293916), Kone Foundation and Wihuri Foundation for the financial support. REFERENCES (1) Han, W.; Kawakami, R. K.; Gmitra, M.; Fabian, J. Graphene Spintronics. Nat. Nanotechnol. 2014, 9, 794−807. (2) Hill, E. W.; Geim, A. K.; Novoselov, K.; Schedin, F.; Blake, P. Graphene Spin Valve Devices. IEEE Trans. Magn. 2006, 42, 2694− 2696. (3) Nishioka, M.; Goldman, A. M. Spin Transport through Multilayer Graphene. Appl. Phys. Lett. 2007, 90, 252505. (4) Tombros, N.; Jozsa, C.; Popinciuc, M.; Jonkman, H. T.; van Wees, B. J. Electronic Spin Transport and Spin Precession in Single Graphene Layers at Room Temperature. Nature 2007, 448, 571−574. (5) Goto, H.; Kanda, A.; Sato, T.; Tanaka, S.; Ootuka, Y.; Okada, S.; Miyazaki, H. Gate Control of Spin Transport in Multilayer Graphene. Appl. Phys. Lett. 2008, 92, 212110. (6) Han, W.; Pi, K.; Bao, W.; McCreary, K. M.; Li, Y.; Wang, W. H.; Lau, C. N.; Kawakami, R. K. Electrical Detection of Spin Precession in Single Layer Graphene Spin Valves with Transparent Contacts. Appl. Phys. Lett. 2009, 94, 222109. (7) Han, W.; McCreary, K. M.; Li, Y.; Wong, J. J. I.; Swartz, A. G.; Kawakami, R. K. Tunneling Spin Injection into Single Layer Graphene. Phys. Rev. Lett. 2010, 105, 167202. 7539


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