RESEARCH ARTICLE www.small-journal.com Large-Area Synthesis of Ferromagnetic Fe5−xGeTe2/ Graphene van der Waals Heterostructures with Curie Temperature above Room Temperature Hua Lv,* Alessandra da Silva, Adriana I. Figueroa, Charles Guillemard, Iván Fernández Aguirre, Lorenzo Camosi, Lucia Aballe, Manuel Valvidares, Sergio O. Valenzuela, Jürgen Schubert, Martin Schmidbauer, Jens Herfort, Michael Hanke, Achim Trampert, Roman Engel-Herbert, Manfred Ramsteiner,* and Joao Marcelo J. Lopes* 1. Introduction Van der Waals (vdW) heterostructures combining layered ferromagnets and other 2D crystals are promising building blocks for the realization of 2D ferromagnetic materials are anticipated to play a crucial role in the development ultracompact devices with integrated magnetic, electronic, and optical of future spin-based devices (e.g., spin- functionalities. Their implementation in various technologies depends valves and random-access magnetic mem- strongly on the development of a bottom-up scalable synthesis approach ories) with high energy efficiency and ultra- allowing for realizing highly uniform heterostructures with well-defined compact dimensions.[1–3] Moreover, com- interfaces between different 2D-layered materials. It is also required that each bining them with other layered materials such as graphene and 2D semiconductors material component of the heterostructure remains functional, which ideally (e.g., WS2 and MoSe2 ) into van der Waals includes ferromagnetic order above room temperature for 2D ferromagnets. (vdW) heterostructures is a promising path- Here, it is demonstrated that the large-area growth of Fe5−x GeTe2 /graphene way for the realization of novel devices with heterostructures is achieved by vdW epitaxy of Fe5−x GeTe2 on epitaxial integrated magnetic, optical , and electronic graphene. Structural characterization confirms the realization of a continuous functionalities.[4] In this case, besides the vdW heterostructure film with a sharp interface between Fe5−x GeTe2 and individual properties of each 2D crystal, proximity-induced coupling effects across graphene. Magnetic and transport studies reveal that the ferromagnetic order the vdW gap offer unique opportunities to persists well above 300 K with a perpendicular magnetic anisotropy. In tailor properties via a layering design.[4,5] addition, epitaxial graphene on SiC(0001) continues to exhibit a high In order to enable a broad range of electronic quality. These results represent an important advance beyond applications, it is critical to identify 2D nonscalable flake exfoliation and stacking methods, thus marking a crucial ferromagnets exhibiting a Curie tempera- ture (TC ) at least at room temperature.[2,6] step toward the implementation of ferromagnetic 2D materials in practical Among a vast selection of 2D materials with applications. magnetic order investigated recently,[6] the Fe5−x GeTe2 (FGT) metallic system (with x The ORCID identification number(s) for the author(s) of this article H. Lv, A. da Silva, J. Herfort, M. Hanke, A. Trampert, R. Engel-Herbert, can be found under https://doi.org/10.1002/smll.202302387 M. Ramsteiner, J. M. J. Lopes [+] Present address:Departament de Física de la Matèria Condensada, Uni- Paul-Drude-Institut für Festkörperelektronik versitat de Barcelona, and Institut de Nanociència i Nanotecnologia, Uni- Leibniz-Institut im Forschungsverbund Berlin e.V versitat de Barcelona, Barcelona 08028, Spain 10117 Berlin, Germany E-mail: lv@pdi-berlin.de; ramsteiner@pdi-berlin.de; lopes@pdi-berlin.de © 2023 The Authors. Small published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution A. I. Figueroa[+] , I. F. Aguirre, L. Camosi, S. O. Valenzuela License, which permits use, distribution and reproduction in any Catalan Institute of Nanoscience and Nanotechnology (ICN2) medium, provided the original work is properly cited. CSIC and BIST Campus UAB DOI: 10.1002/smll.202302387 Bellaterra, Barcelona 08193, Spain Small 2023, 19, 2302387 2302387 (1 of 10) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 39, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202302387 by Readcube (Labtiva Inc.), Wiley Online Library on [30/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com usually between 0 and 0.4)[7–10] has been found to offer a great 2. Results and Discussion prospect, as it has been reported to exhibit a TC of around 300 K.[7,10,11] FGT is an itinerant ferromagnetic metal[11] with mag- 2.1. Structural Properties netic properties that can be tuned via electrostatic gating,[12] strain, or doping when interfacing a topological insulator,[13] as MBE was employed to grow ≈15 nm thick FGT films on epi- well as elemental doping.[14,15] Similar to other 2D Fe–Ge–Te taxial graphene on SiC (0001) templates synthesized by surface compounds like Fe3 GeTe2 [16] and Fe4 GeTe2 [17] with below room graphitization.[22] Rutherford backscattering spectrometry (RBS) temperature TC , it possesses a layered structure with each single analyses (see Figure S1 in the Supporting Information) revealed layer being formed of Fe and Ge 2D slabs encapsulated by lay- the films to exhibit an average composition close to x = 0, namely ers of Te.[7,8] The weak vdW interaction between interlayers per- Fe4.8 GeTe2 . This is similar to what has been reported for bulk sin- mits to exfoliate micrometer-sized flakes from FGT bulk single gle crystals synthesized by chemical vapor transport.[7,23] In situ crystals and to fabricate vdW heterostructures combining FGT growth monitoring by reflection high-energy electron diffraction and other 2D crystals via sequential flake stacking.[18–20] Such an (RHEED) showed a pattern composed of isolated streaks which approach has recently been used to realize FGT/graphene spin- indicates growth of a film with a smooth surface (Figure 1a, bot- valve devices demonstrating room-temperature operation.[18] tom panel). The position of the streaks with respect to the pattern FGT/graphene vdW heterostructures are in general very promis- measured for epitaxial graphene on SiC(0001) before growth (for ing for the design of all-2D logic/multiplexer devices[4] in SiC(0001), the in-plane (IP) lattice parameter is 3.08 Å)[24] attested which FGT is the spin injector/detector and graphene the spin- the evolution to a larger in-plane lattice constant around 4.06 Å, transport channel. as expected for FGT formation.[9] Despite their great utility for fundamental studies and testing To study the in-plane structure in more detail, synchrotron- device concepts, exfoliation-based methods are not scalable and based grazing incidence diffraction (GID) was employed. difficult to integrate into established device fabrication schemes. Figure 1b shows the in-plane intensity distribution plotted Hence, achieving bottom-up synthesis of 2D heterostructures in hexagonal coordinates according to the symmetry of the comprising FGT and graphene is crucial for implementing SiC(0001) substrate. This map was obtained for the 15 nm FGT these materials into future devices. In this work, we demon- film covered by a 5 nm thick Te capping layer, which was used strate the synthesis of large-area FGT/graphene vdW heterostruc- to reduce FGT surface oxidation upon prolonged air exposure. tures exhibiting ferromagnetic order persisting well above 300 For convenience, we use the four-component vector notation for K and perpendicular magnetic anisotropy (PMA). For this, vdW hexagonal symmetry: (H K. L) = (H K −[H + K] L). The only epitaxy of FGT films on single-crystalline graphene/SiC(0001) substrate contribution within the investigated area in reciprocal templates was realized by molecular beam epitaxy (MBE), a ̄ space appears as a single (21.0)-type reflection. The other most in- method which has recently been employed for the growth of FGT tense and prominent diffraction features originate from the FGT on substrates such as Al2 O3 (0001), WS2 (001)/Al2 O3 (0001), and film. There are two different sets of reflections, namely FGT(11.0) SrTiO3 (111).[9,13,21] Importantly, the underlying graphene layers and (22.0), and FGT(H0.0) with [H = 1,…,3]. They both refer to exhibit a sharp interface to the FGT and conserve their high elec- orientations with their c-axes along the surface normal. The three tronic quality, which are key ingredients for the future develop- most intense diffraction features at (11.0), (22.0), and (30.0) re- ment of devices in which interfacial effects can be used to control veal an FGT in-plane lattice parameter of 4.049(13) Å. This is, in properties and functionalities. general, agreement with the value obtained by RHEED, as well as literature values for Fe5−x GeTe2 (0 < x ≤ 0.4) bulk crystals[7,8,10] and thin films.[9,13] The intensity modulation along the FGT(11.0) ring exhibits a maximum in the azimuth which is aligned along the SiC[21.0] ̄ in- C. Guillemard, L. Aballe, M. Valvidares plane substrate direction. This indicates a preferential in-plane ALBA Synchrotron Light Source orientation, i.e., the (11.0) net planes of FGT and SiC are par- Cerdanyola del Valles allel, as we previously observed for Fe3 GeTe2 films grown on Barcelona 08290, Spain graphene.[25] Nevertheless, the FGT arc maxima are broader than I. F. Aguirre Universitat Autònoma de Barcelona those of Fe3 GeTe2 . For the arc crossing the (11.0) reflection, a Bellaterra, Barcelona 08193, Spain full width at half maximum of 8.9° (Figure 1b, inset) is found, S. O. Valenzuela whereas for Fe3 GeTe2 this value is about 4.7°.[25] Even though Institució Catalana de Recerca i Estudis Avançats (ICREA) this relatively large value is not uncommon for epitaxially grown Barcelona 08010, Spain vdW materials,[26–28] it reveals that the in-plane mosaicity in the J. Schubert FGT film is clearly larger than that measured for the Fe3 GeTe2 Peter Grünberg Institut (PGI-9) Forschungszentrum Jülich one (grown with virtually identical conditions, except the Fe flux). 52425 Jülich, Germany This difference might be associated with the higher energies re- J. Schubert quired for the stabilization of layered Fe–Ge–Te phases exhibiting JARA-Fundamentals of Future Information Technology a Fe composition higher than 3.[17] The existence of in-plane mo- Jülich-Aachen Research Alliance saicity can also be inferred from the rather diffuse streaks of the 52425 Jülich, Germany RHEED pattern (Figure 1a, bottom panel). Similar RHEED pat- M. Schmidbauer terns were reported by Ribeiro et al.[9] for Fe5 GeTe2 films grown Leibniz-Institut für Kristallzüchtung (IKZ) 12489 Berlin, Germany by MBE on Al2 O3 (0001) substrates. Interestingly, they could Small 2023, 19, 2302387 2302387 (2 of 10) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 39, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202302387 by Readcube (Labtiva Inc.), Wiley Online Library on [30/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com Figure 1. Structural characterization of 15 nm thick FGT films on graphene. a) In situ RHEED patterns taken perpendicular to the SiC〈100〉 direction, ̄ reflection, as well as contribu- before (upper panel) and after FGT growth (lower panel). b) GID in-plane reciprocal space map. It contains the SiC (21.0) tions along particular rings (corresponding to discrete in-plane lattice parameters) which can be well attributed to FGT, FeTe, and the Te capping layer. The inset image (left side, bottom) is an azimuthal intensity profile intersecting the maximum of the (11.0) FGT arc. c) Out-of-plane omega-2theta scan for an uncapped film containing a series of FGT peaks, whose intensities and positions match well with calculated ones. It also shows contributions from FeTe and the SiC substrates. d) AFM height image taken for an uncapped film. The inset depicts a higher magnification image where it is possible to better visualize the nanostructures present at the surface. observe an evolution from diffuse to sharp RHEED streaks by ingly, the c-axis of tetragonal FeTe points along the direction per- performing postgrowth annealing of the films at a temperature pendicular to the surface as well. Finally, a sequence of low-index of 550 °C under Te flux. This suggests that postgrowth thermal polyrings with different lattice spacings is also detected, which is treatment as well as higher growth temperatures might improve originating from the topmost Te capping layer. in-plane order for FGT films grown on graphene. This will be the Complementary to GID we have performed a laboratory-based topic of further investigations. There are two more ring-like con- omega/2theta measurement at an X-ray wavelength of 1.54056 Å, tributions detectable in the map and in the attached line profile which probes exclusively out-of-plane features of the layer (see taken at K = 0, which cannot be related to FGT. These are most Figure 1c). A set of FeTe reflections confirms the presence of probably due to a smaller fraction of tetragonal FeTe agreeing very tetragonal FeTe with its out-of-plane orientation along SiC [00.1] well with FeTe(200) and (220) in-plane lattice spacing. Interest- as it could already be anticipated from the GID in-plane data. A Small 2023, 19, 2302387 2302387 (3 of 10) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 39, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202302387 by Readcube (Labtiva Inc.), Wiley Online Library on [30/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com further set of eight subsequent FGT(00.L) reflections with L = in which the thickness of a single layer is larger than that of 3, 6, …, 24 proves (in conjunction with the presented in-plane Fe5 GeTe2 has not been reported. In a simple picture, such a data) the formation of FGT with a Fe5 GeTe2 structure,[8] since thickness increment is anticipated if one considers that “filling” they fit well to the position and intensity ratios of the numeri- the 2D backbone structure of Fe5 GeTe2 with more Fe atoms cally derived powder pattern (black bars). Based on the positions will result in the formation of additional slabs of Fe, following of the contributions at L = 3, 6, 18, and 21, we have deduced an the same trend as experimentally observed when transitioning out-of-plane lattice parameter of 29.16(9) Å, in agreement with from Fe3 GeTe2 to Fe5 GeTe2 (for which the single-layer thickness previous reports for bulk crystals and thin films.[7–10,13] The for- evolves from ≈0.8 to ≈1.0 nm).[7,8,16,34] In fact, Liu et al.[35] used mation of [00.1]-oriented Fe3 GeTe2 can be excluded as it would first-principles calculations to study different Fe–Ge–Te-layered result into a clearly different diffraction pattern.[16,29] Note that phases including Fe6 GeTe2 and Fe7 GeTe2 . They predicted them these measurements were performed for uncapped FGT films, to exhibit a thickness for single layers of around 1.1 and 1.2 nm, which explain the absence of contributions related to pure Te. respectively. Our XRD data (Figure 1c) do not directly confirm The surface morphology was probed by atomic force mi- the formation of such phases, which shows that they are not croscopy (AFM). Figure 1d shows a typical AFM height image dominant in the film. However, the shift of the (00.9) reflection for an uncapped FGT film. Its coverage appears mainly con- toward a lower angle (in comparison to the value calculated for tinuous over the micrometer-sized graphene/SiC terraces, with Fe5 GeTe2 ), as well as the existence of an additional, unidentified the exception of some holes appearing close to surface steps. reflection closed to (00.18) (superimposed to SiC (00.6)), could These substrate regions are known to exhibit few-layer thick be related to the existence of layered phases with larger c lattice graphene ribbons and patches, whereas the terraces are mostly constants. covered in monolayer—and, to a lesser extent, bilayer—thick Even though at the present stage we cannot unambiguously graphene.[28,30] A similar effect was observed in Fe3 GeTe2 grown identify their composition and structure, the existence of single on graphene/SiC(0001),[25] which we associated with the lower layers with thickness larger than that of Fe5 GeTe2 is strong ev- surface reactivity of the thicker graphene areas relative to that idence for the formation of new Fe–Ge–Te-layered phases with of the terraces,[22] leading to a lower FGT growth rate near the Fe composition exceeding 5, which are possibly metastable.[17] steps. Moreover, it is possible to observe nanoislands with tetrag- This illustrates the great potential of vdW epitaxy via MBE for onal shapes scattered over the surface (Figure 1d, inset). Based on the large-scale realization of metastable phases of 2D magnets, the GID and X-ray diffraction (XRD) findings, we suggest that similar to what have been observed for 2D semiconducting they are composed of tetragonal FeTe,[31] forming due to phase dichalcogenides.[36,37] Further investigations are needed in order segregation during growth or during subsequent sample cool- to identify the reasons for the formation of such phases and to ing. In spite of these unwanted surface effects—which are ex- controllably realize them. Finally, note that the average RBS com- pected to be mitigated by further refinement of both the FGT and position obtained for our films (Fe4.8 GeTe2 ) does not pose a con- graphene growth protocols—it is reasonable to conclude that the tradiction to the existence of layers with higher Fe concentration. FGT film offers a good surface homogeneity over large areas. The It is known that Fe5−x GeTe2 single layers are usually Fe-deficient root-mean-square (RMS) roughness for different surface regions with 0 < x ≤ 0.4[8,10,38,39] (x can be even close to 1 if the Fe4 GeTe2 (free of FeTe nanoislands) varies between 0.8 and 1.1 nm, which structure stabilized by Seo et al.[17] is considered, which interest- is close to the thickness of one monolayer of Fe5 GeTe2 (a single ingly also exhibits a single-layer thickness of around 1 nm). Fe layer corresponds to 1/3 of the unit cell with c = 29.16(9) Å). It deficiency taking place in the “normal” FGT layers could, in prin- is important to note that the AFM investigations discussed here ciple, compensate for the higher amount of Fe present at the less were conducted on noncapped FGT films. Although oxidation in abundant, thicker layers. FGT[32] is not as strong as in 2D magnetic halides,[33] surface ox- Another very important feature observed in the STEM images idation taking place within a few hours after air exposure might is the sharp nature of the FGT/graphene interface. Figure 2d de- increase the surface roughness. picts the same area as in Figure 2a, rendered using a custom The structure of the FGT/graphene heterostructure was color map to aid the visualization of the graphene layers, since also investigated by scanning transmission electron mi- the HAADF-STEM images provide an atomic number (Z) con- croscopy (STEM). Figure 2a depicts a high-angle annular trast. The continuous graphene layers are observed (dark green dark-field (HAADF)-STEM image of the FGT film grown on color) directly interfacing the crystalline FGT film (in orange). A graphene/SiC(0001). It is possible to observe the layered struc- more detailed view of the FGT/graphene interface, obtained for ture of the film; in that single layers are clearly separated by vdW another sample region, is provided in Figure 2e–h. In agreement gaps, in agreement with previous TEM characterization of FGT with the previous reports,[22] they confirm the existence of a bi- bulk crystals and thin films.[7,9] Most of the film is formed of layer thick graphene (with an interlayer distance of 0.34 nm), and well-oriented layers with the expected FGT single-layer thickness the carbon-rich buffer layer very close to SiC. (≈1 nm, see Figure 2b). This value agrees with the c lattice con- stant obtained from the XRD data. Stacking and rotational faults are also evident. Strikingly, STEM images reveal that, although 2.2. Magnetic and Electric Properties the overall vdW-layered structure is preserved, single layers with a thickness larger than 1 nm are also present. Figure 2c, which The magnetic and electric properties were investigated via mag- shows an enlarged area from an interface region in Figure 2a, netotransport measurements, superconducting quantum inter- reveals some layers as thick as 2 nm. To the best of our knowl- ference device (SQUID) magnetometry, X-ray absorption spec- edge, the experimental realization of Fe–Ge–Te-layered phases troscopy (XAS), and X-ray magnetic circular dichroism (XMCD). Small 2023, 19, 2302387 2302387 (4 of 10) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 39, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202302387 by Readcube (Labtiva Inc.), Wiley Online Library on [30/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com Figure 2. STEM of the 15 nm thick FGT film on graphene/SiC(0001). a) HAADF-STEM image of a region showing the FGT film on graphene. b) Magnified region from the green dashed rectangle in panel (a) showing the homogeneous layer formation and its line profile (in red). The interlayer distance is 0.97 (±0.10) nm. c) Another enlarged area (blue dashed rectangle) from panel (a), showing the formation of single layers with different thicknesses. d) The same image as in panel (a) rendered using a custom colormap to aid visualization. e) Bright field-STEM image showing another region of the sample where the graphene layers are better resolved. f) HAADF-STEM image of the same area in panel (e). g) Colormap version of the same area in panel (e) to aid visualization of the graphene layers. h) Enlarged region of the yellow rectangular box in panel (g). These measurements were performed on pieces cut from the temperature originates from the anomalous Hall effect (AHE), same 1 cm2 sample consisting of an ≈15 nm FGT film (capped its signal strength being proportional to the out-of-plane mag- with 5 nm Te) grown on graphene/SiC(0001). netization. The nearly square-shaped loops and the finite coer- Figure 3a displays the Hall resistivity 𝜌xy measured via mag- civity observed for temperatures up to 360 K provide clear evi- netotransport during upward and downward sweeps of an exter- dence for ferromagnetism well above room temperature with a nal magnetic field H, where the arrows indicate the field sweep- robust PMA. Similar results obtained for an ≈10 nm FGT film ing directions. The linear background is caused by the ordinary are shown in Figure S2 (Supporting Information). For the de- Hall effect (OHE) with the negative slope observed for the whole termination of the Curie temperature, we adopt the commonly temperature range suggesting electron-like transport behavior. used method based on the Arrott plot;[17,40] 𝜌A 2 is plotted against However, the measured OHE results from parallel conduction μ0 H/𝜌A , where 𝜌A is the AHE part of the Hall resistivity after sub- through the FGT and graphene transport channels exhibiting tracting the linear background of the OHE contribution (see also hole- and electron-like transport characteristics, respectively.[25] Figure S3 in the Supporting Information). From the linear fit in Consequently, the sign as well as the absolute value of the OHE the high-field ranges of the Arrott plots in Figure 3b, the inter- slope depend on the relative conductivities of the FGT and cepts at the 𝜌A 2 axis are extracted for each temperature. As shown graphene films. The extracted effective carrier density of 1.0 × in Figure 3c, these intercepts exhibit a linear temperature depen- 1014 cm−2 and the sheet resistance of 176 Ω sq−1 (at room tem- dence in accordance with Arrott’s theory.[40] Finally, a Curie tem- perature) in our FGT/graphene heterostructure are close to the perature of TC ≈ 390 K is extracted from the extrapolation to zero values obtained for the previously studied Fe3 GeTe2 /graphene intercept by a linear fit (red line in Figure 3c; see also Figure S2 heterostructure (7 × 1013 cm−2 and 150 Ω sq−1 , respectively).[25] in the Supporting Information). The same TC is obtained from The observed hysteresis loop superimposed onto the OHE at each the temperature decays of the remanent AHE resistivity and the Small 2023, 19, 2302387 2302387 (5 of 10) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 39, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202302387 by Readcube (Labtiva Inc.), Wiley Online Library on [30/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com Figure 3. Curie temperature determination using magnetotransport measurements for a Te capped, 15 nm thick FGT film on graphene/SiC(0001). a) Hall resistivity 𝜌xy measured during upward and downward sweeps of an external magnetic field at various temperatures with the sweeping directions indicated by arrows. The square-shaped hysteresis loops result from the AHE. b) Arrott plot (𝜌A 2 against μ0 H/𝜌A ) extracted from AHE data, where 𝜌A is the AHE part of the Hall resistivity. The linear dependencies in the large field range are indicated as red lines. c) Intercepts at the 𝜌A 2 axis of Arrott plots as a function of temperature. The Curie temperature TC is determined from the extrapolation to zero intercept from a linear fit (red line) of the data. coercive field (Figure S3, Supporting Information). This value is significantly higher than those previously reported for single crystals (or flakes) as well as large-area films of Fe3 GeTe2 (TC ≈ 220 K),[25,29,41,42] Fe4 GeTe2 (TC ≈ 270 K),[17] and Fe5−x GeTe2 (0 < x ≤ 0.4, TC ≈ 280–320 K).[7,9,21,39,43] Only the transition temperature recently reported for MBE-grown Fe5−x GeTe2 /Bi2 Te3 heterostruc- tures (TC as high as 570 K) surpasses this value.[13] The high TC value approaching 400 K is in agreement with a recent theoretical prediction for the Fe5 GeTe2 phase;[17] how- ever, it differs from other experimental reports for Fe5−x GeTe2 . This could be related to the existence of thicker single vdW FGT layers, as observed by STEM. It is anticipated that a gradual in- crease of the Fe content within individual layers will result in a larger number of nearest Fe neighbors per Fe atom, enhancing the spin–pair exchange interaction and consequently the Curie temperature. In fact, recent theoretical investigations predicted PMA and TC values of 450 and 570 K for layered Fe6 GeTe2 and Fe7 GeTe2 , respectively.[35] Therefore, in the case of our samples, the existence of similar phases with Fe composition higher than Figure 4. Remanent magnetization M obtained by SQUID magnetome- 5 could, in principle, lead to an effective TC value that is superior try as a function of temperature for IP and OP configurations on the same sample piece of a Te-capped, 15 nm thick FGT film on graphene/SiC(0001) to that of pure Fe5−x GeTe2 . Nevertheless, in order to verify this with an FGT volume of about 1.07×10−4 mm3 . The background signal orig- hypothesis, future growth experiments will be conducted aim- inating from the SiC substrate was subtracted (Figure S4, Supporting In- ing at the stabilization, and isolation, of such phases. Finally, formation). note that the tetragonal FeTe phase detected by GID and XRD (Figure 1b,c) and correlated to the surface nanoislands observed by AFM (Figure 1d) is not expected to contribute to the observed Supporting Information). The clearly larger remanent magneti- ferromagnetism due to its anti-ferromagnetic nature (Néel tem- zation in the OP configuration confirms the PMA in our FGT perature of around 70 K).[31] films. Furthermore, it is clear from the decay of the OP rema- The magnetic properties of the FGT film were further investi- nent magnetization that ferromagnetic order persists well above gated by SQUID magnetometry. Figure 4 displays the remanent room temperature, in agreement with the result extracted from magnetization obtained for both out-of-plane (OP) and in-plane the AHE measurements. The Fe atoms on the five inequivalent (IP) configurations as a function of temperature (a background lattice sites in Fe5 GeTe2 are expected to exhibit rather different signal caused by the SiC substrate has been subtracted; Figure S4, magnetic moments.[7,38] The average magnetic moment per Fe Small 2023, 19, 2302387 2302387 (6 of 10) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 39, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202302387 by Readcube (Labtiva Inc.), Wiley Online Library on [30/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com Figure 5. Temperature dependence of the Fe L3 XMCD maximum in re- manence (0 T), normalized by its value in saturation (6 T). The external magnetic field was applied along the direction perpendicular to the sam- ple surface (out-of-plane, red triangles) and at 20° from the sample plane (in-plane, black circles). Inset: Hysteresis loops recorded at the Fe L3 edge at 3 K for both configurations, where data have been normalized by the value in saturation (6 T). Figure 6. Longitudinal magnetoresistance and weak localization in an FGT/graphene heterostructure. a) Rxx versus H measured at 4.3 K. The atom obtained from the SQUID data measured at 2 K is 1.37 dashed arrows indicate the sweeping directions. The red line indicates a H2 -dependent positive magnetoresistance characteristic for carrier trans- μB , considering the average Fe concentration of 4.8 as obtained port in graphene whereas the pink area highlights the resistance up-turn by RBS measurements. This result falls into the range between due to weak localization. b) The magnetic-field induced variation in the 0.8 and 2.6 μB per Fe atom covered by reported experimental longitudinal conductivity Δ𝜎 xx (H) = 𝜎 xx (H) – 𝜎 xx (0) can be well fitted by values.[7] the HLN equation (pink line) in the low field range during a down sweep of XAS and XMCD measurements were employed to investigate the external field. The obtained fitting parameters correspond to a phase the atomic-scale magnetic properties.[44] Figure 5 shows the re- coherence length of lϕ = 211 nm. The longitudinal conductivity has been calculated using the relation 𝜎 xx = 1/(Rxx × W/L) with W and L being the manence of the dichroic signal at B = 0 T (normalized by its value sample width and contact length, respectively. at B = 6 T), for both grazing (20° from the sample plane) and nor- mal incidence geometries, as a function of temperature. At low temperature, the sample has a PMA, as evidenced by the larger re- A TC above room temperature, as observed in XMCD, is in manence in the perpendicular direction (red triangles). However, good agreement with literature values for Fe5−x GeTe2 with x above T = 220 K, the measured magnetization undergoes an in- ranging from 0 to 0.3.[9,10,21,39] Certain discrepancy observed be- plane reorientation transition. The spontaneous magnetization tween XMCD, SQUID, and magnetotransport results, in terms of persists above room temperature (Figures S6 and S7, Supporting remanence, anisotropy and TC may be attributed to the fact that Information). the different experiments probe different parts of the FGT film. We performed a sum-rule analysis on the XAS–XMCD spec- While SQUID probes the whole film, the detection in XMCD is tra at T = 3 K to extract the atomic magnetic moments of Fe restricted to the topmost FGT layers (≈5 nm). As observed by atoms (Figure S5, Supporting Information). We obtained an ef- STEM (Figure 2a), this area is formed exclusively of single layers fective total magnetic moment in normal incidence (i.e., not cor- with the interlayer spacing expected for Fe5−x GeTe2 with x being rected from the intra-atomic dipole operator Tz ) Meff = 1.13 ± close to 0. Also, the grazing geometry measured in XMCD does 0.15 μB per Fe atom, in reasonable agreement with the value not correspond to a purely in-plane component of the magnetiza- obtained by SQUID. The lower value found from this analysis tion, but instead it has a contribution from the out-of-plane one. might be due to the fact that not all Fe atoms probed in XAS For the magnetotransport (AHE) measurements, the actual cur- contribute to the magnetic signal obtained by XMCD (e.g., those rent distribution between the different FGT components and the part of anti-ferromagnetic FeTe nanoislands do not contribute); graphene film is essential but difficult to determine. thus, this value is an underestimation of the actual Fe mag- Finally, it is important to verify the functionality of the netic moment in FGT. This analysis also allowed us to extract graphene film after FGT synthesis. Figure 6 displays the longi- the orbital moments measured in normal incidence mL⊥ = 0.03 tudinal magnetoresistance (Rxx ) of the FGT/graphene stack mea- μB /Fe and the difference between perpendicular and in-plane or- sured at 4.3 K under a perpendicular magnetic field. The appar- bital moments ΔmL = mL⊥ − mL∥ that is proportional to the macro- ent hysteresis is caused by the magnetization switching in the scopic anisotropy in itinerant magnetism materials according to FGT film, while the H2 -dependent positive magnetoresistance Bruno’s model.[45] We found ΔmL < 0.01 μB , consistent with (red line) and the peak induced by weak localization at zero field the small anisotropy observed in normal (red curve) and grazing (pink square area) are typical characteristics of the carrier trans- (black curve) hysteresis loops shown in the inset of Figure 5. port in graphene.[46–50] This observation reveals the occurrence Small 2023, 19, 2302387 2302387 (7 of 10) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 39, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202302387 by Readcube (Labtiva Inc.), Wiley Online Library on [30/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com of strong quantum interference effects for the electron motion, sphere. Both H-etching and graphene growth were performed at a pres- demonstrating that the high quality of the graphene film is main- sure of 900 mbar and a gas flow rate of 500 sccm. Further information on tained during the overgrowth by FGT. In the presence of a mag- the formation of the epitaxial graphene on SiC and the influence of the SiC morphology can be found elsewhere.[30,53] netic field, the strength of weak localization in the 2D case can MBE Growth of Fe5−x GeTe2 : The ≈10 and ≈15 nm thick FGT films be described by the Hikami–Larkin–Nagaoka (HLN) equation[51] were grown in ultrahigh vacuum (UHV, a base pressure of around 5 × [ ( ) ( )] 10−11 mbar) using high purity Fe, Ge, and Te evaporated from effusion e2 1 H𝜙 H𝜙 cells at 1330, 1050, and 308 °C (hot lip at 440 °C), respectively. The flux for Δ𝜎xx = 𝛼 𝜓 + − ln (1) 𝜋h 2 H H each element was obtained by measuring the beam equivalent pressure employing a pressure gauge. Based on the film composition obtained by where Δ𝜎 xx (H) = 𝜎 xx (H) – 𝜎 xx (0) is the corresponding change RBS analyses, the flux ratios were calibrated aiming at the growth of films in the longitudinal conductivity. Here, 𝜓 is the Digamma func- with the nominal composition, i.e., Fe5 GeTe2 . The graphene/SiC(0001) templates were outgassed at 450 °C for at least 1 h and then cooled to tion, Hϕ is the phase coherence characteristic field, h is the Planck 300 °C for FGT growth. Prior to that, they were coated with 1 μm of Ti on constant, and e is the elementary electron charge. In the low field the backside via electron beam evaporation to allow noncontact heating range (from 0.2 to 0 T during downsweep of the external field by radiation. In situ growth monitoring was performed by RHEED. Finally, to avoid the influence of magnetization switch), our data can be the FGT films were capped in situ with an ≈ 5 nm thick Te layer deposited well fitted by the HLN equation using 𝛼 and Hϕ as free parame- after sample cooling to room temperature. This procedure was adopted ters as shown by the pink line in Figure 6b. The obtained 𝛼 = 0.45 in order to avoid significant surface oxidation upon air exposure. Only the AFM, XRD, and RBS analyses were performed using uncapped samples. is in accordance with previously reported values and, therefore, For AFM and XRD, the measurements could be started within a few min- confirms the quantum origin of the resistance upturn.[51] Based utes after the sample was unloaded from the MBE system. For the RBS on the extracted 𝜇 0 Hϕ = 3.67 mT, a phase coherence length of lϕ analysis, the Te capping is unwanted in order to facilitate the composi- = 211 nm is deduced from 𝜇 0 Hϕ = h/8𝜋elϕ 2 . The obtained lϕ is tional analyses. In this case, the samples were measured about 3 days comparable to a previously reported value for epitaxial graphene after unloading from the MBE system. on SiC.[49,52] Rutherford Backscattering Spectrometry: The composition of the sam- ples was investigated by means of RBS using 1.4 MeV He+ ions with a scattering angle of 170°. Their stoichiometry was determined by integra- tion of the areal density from the well-separated elemental signals of the 3. Conclusion different elements (Fe, Ge, and Te). Atomic Force Microscopy: The surface topography of the FGT films was Our results demonstrate the successful growth of a large-area investigated via standard tapping mode measurements performed in am- 2D heterostructure with high quality of the individual FGT and bient conditions. The measurements were performed in uncapped sam- graphene components, as well as of the interface between them, ples within hours after MBE growth. which was previously achieved only by manually assembling X-Ray Diffraction: Lab-based omega/2theta scans were recorded with flakes of the different materials. This constitutes a critical step a conventional X’Pert Pro MRD from Malvern Panalytical using Cu-K𝛼1 toward the implementation of these materials in future technolo- radiation (𝜆 = 1.54056 Å) as selected by a hybrid monochromator. The measurements were performed on uncapped samples within hours after gies. Morphological and structural characterization revealed the MBE growth. fabrication of continuous heterostructure films with a sharp in- Synchrotron-Based GID: This experiment was performed at the BM25- terface between FGT and graphene. Interestingly, stacking faults SpLine beamline at The European Synchrotron (ESRF) in Grenoble. An related to the presence of single vdW layers with thicknesses ex- incidence angle of the illuminating X-rays of 0.2° sufficiently suppresses ceeding those expected for the Fe5 GeTe2 phase are identified by strong scattering by the substrate, and thus makes this method highly sur- STEM. We expect these to be FGT phases with Fe composition face sensitive. An X-ray wavelength of 0.729 Å (corresponding to a photon higher than 5 and potentially enhanced magnetic properties. Fu- energy of 17 keV), selected by a Si(111) monochromator, enables the in- spection of a comparatively large area in reciprocal space, which is impor- ture investigations will be carried out to clarify this aspect. Dif- tant for accessing multiple reflections of the same lattice plane family. ferent methods used to study the magnetic and transport prop- Transmission Electron Microscopy: The thin lamellas imaged in the TEM erties revealed that the ferromagnetic order persists well above experiments were prepared utilizing a JEOL JIB-4501 focused ion-beam room temperature with perpendicular magnetic anisotropy. In (FIB) microscope. A JEOL ARM 200F aberration-corrected microscope was addition, the underlying graphene film continues to show quan- operated in STEM mode at 200 kV with a beam current of 68 pA. A HAADF tum features in its electronic properties, attesting to the conser- detector was used to acquire the images with inner and outer collection angles of 54 and 220 mrad, respectively, whereas the bright field detector vation of its high quality after vdW epitaxy of FGT. had a collection angle of 36 mrad. Magnetotransport Characterization: The magnetotransport measure- ments were carried out on a rectangular strip of the FGT/graphene het- 4. Experimental Section erostructure bonded on a chip carrier with Al contact wires. A constant current of Ixx = 200 mA was applied along the long side of the strip (x- Epitaxial Graphene on SiC: Semi-insulating 4H-SiC(0001) substrates direction) through two contacts close to the edges of the strip. For the de- with sizes of either 1.0 cm × 1.0 cm or 0.5 cm × 1.0 cm were chemically termination of the longitudinal resistance, the voltage (Vxx ) between two cleaned in n-butyl acetate, acetone, and propanol. After that, they were inner contacts was measured. The Hall voltage (Vxy ) was measured in the loaded in an induction-heated furnace pumped down to a base pressure of orthogonal direction (y-direction) between two contacts close to the center 10−4 mbar. In order to remove surface irregularities and create well-defined of the strip. The measurements were performed at temperatures between stepped surfaces, an H-etching treatment was performed at 1400 °C 4.3 and 400 K under vacuum conditions (10−6 –10−5 mbar) with an exter- for 15 min in forming gas atmosphere (95% Ar and 5% H2 ) before nal perpendicular (z-direction) magnetic field of up to 0.8 T. graphene growth. Synthesis of epitaxial graphene was finally achieved via SQUID Magnetometry: The magnetometry measurements were per- surface graphitization at a temperature of 1600 °C for 15 min in Ar atmo- formed by a Quantum Design MPMS 5XL SQUID at various temperatures Small 2023, 19, 2302387 2302387 (8 of 10) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 39, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202302387 by Readcube (Labtiva Inc.), Wiley Online Library on [30/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com between 2 and 375 K with an applied magnetic field of up to 4.5 T along [2] Q. H. Wang, A. Bedoya-Pinto, M. Blei, A. H. Dismukes, A. Hamo, S. both OP and IP directions, using the same sample. Jenkins, M. Koperski, Y. Liu, Q.-C. Sun, E. J. Telford, H. H. Kim, M. XAS and XMCD: XAS spectra at the Fe L3 and L2 edges were recorded Augustin, U. Vool, J.-X. Yin, L. H. Li, A. Falin, C. R. Dean, F. Casanova, in BL-29 (BOREAS) beamline[54] at the ALBA synchrotron (Spain), which R. F. L. Evans, M. Chshiev, A. Mishchenko, C. Petrovic, R. He, L. Zhao, provides an ultrahigh-vacuum sample environment with a base tempera- A. W. Tsen, B. D. Gerardot, M. Brotons-Gisbert, Z. Guguchia, X. Roy, ture of ≈3 K and a magnetic field B of up to 6 T. Measurements used total- S. Tongay, et al., ACS Nano 2022, 16, 6960. electron-yield detection, where the drain current was measured from the [3] A. Bedoya-Pinto, J.-R. Ji, A. K. Pandeya, P. Gargiani, M. Valvidares, P. sample to the ground. The magnetic field was applied along the X-ray beam Sessi, J. M. Taylor, F. Radu, K. Chang, S. S. P. Parkin, Science 2021, both at normal and grazing incidence (20°) relative to the sample plane in 374, 616. order to obtain information on the out-of-plane as well as in-plane mag- [4] J. F. Sierra, J. Fabian, R. K. Kawakami, S. Roche, S. O. Valenzuela, Nat. netization, respectively. XMCD was obtained by calculating the difference Nanotechnol. 2021, 16, 856. between XAS spectra obtained with the photon helicity vector antiparallel [5] C. Gong, X. Zhang, Science 2019, 363, eaav4450. and parallel to the magnetic field (see the Supporting Information). [6] H. Kurebayashi, J. H. Garcia, S. Khan, J. Sinova, S. Roche, Nat. Rev. Phys. 2022, 4, 150. Supporting Information [7] A. F. May, D. Ovchinnikov, Q. Zheng, R. Hermann, S. Calder, B. Huang, Z. Fei, Y. Liu, X. Xu, M. A. McGuire, ACS Nano 2019, 13, 4436. Supporting Information is available from the Wiley Online Library or from [8] J. Stahl, E. Shlaen, D. Johrendt, Z. Anorg. Allg. Chem. 2018, 644, 1923. the author. [9] M. Ribeiro, G. Gentile, A. Marty, D. Dosenovic, H. Okuno, C. Vergnaud, J.-F. Jacquot, D. Jalabert, D. Longo, P. Ohresser, A. Hallal, M. Chshiev, O. Boulle, F. Bonell, M. Jamet, npj 2D Mater. Appl. 2022, Acknowledgements 6, 10. [10] A. F. May, C. A. Bridges, M. A. McGuire, Phys. Rev. Mater. 2019, 3, The authors would like to thank H.-P. Schönherr, C. Hermann, C. Stemm- 104401. ler, and K. Morgenroth for their dedicated maintenance of the MBE sys- [11] H. Zhang, R. Chen, K. Zhai, X. Chen, L. Caretta, X. Huang, R. V. tem. The authors also appreciate the critical reading of the manuscript by Chopdekar, J. Cao, J. Sun, J. Yao, R. Birgeneau, R. Ramesh, Phys. Rev. A. Hernández-Mínguez and S. Sadofiev. They also acknowledge the pro- B 2020, 102, 064417. vision of beamline under the projects HC-4068 and HC-4972 at the Euro- pean Synchrotron Radiation Facility (ESRF), located in Grenoble (France), [12] C. Tan, W.-Q. Xie, G. Zheng, N. Aloufi, S. Albarakati, M. Algarni, J. Li, and at ALBA synchrotron in Spain under the project 2021035105. Dr. J. Partridge, D. Culcer, X. Wang, J. B. Yi, M. Tian, Y. Xiong, Y.-J. Zhao, Juan Rubio-Zuazo and Dr. Jesus López-Sánchez are acknowledged for L. Wang, Nano Lett. 2021, 21, 5599. their assistance with the GID measurements. ICN2 acknowledges sup- [13] E. Georgopoulou-Kotsaki, P. Pappas, A. Lintzeris, P. Tsipas, S. port from the European Union Horizon 2020 research and innovation Fragkos, A. Markou, C. Felser, E. Longo, M. Fanciulli, R. Mantovan, programme under Grant Agreement No. 881603 (Graphene Flagship) F. Mahfouzi, N. Kioussis, A. Dimoulas, Nanoscale 2023, 15, 2223. and of the Spanish Research Agency (AEI), Ministry of Science and [14] X. Chen, Y.-T. Shao, R. Chen, S. Susarla, T. Hogan, Y. He, H. Zhang, Innovation, MCIN/AEI/10.13039/501100011033, under grants PID2019- S. Wang, J. Yao, P. Ercius, D. A. Muller, R. Ramesh, R. J. Birgeneau, 111773RB-I00 and CEX2021 001214-S (Severo Ochoa Centres of Excel- Phys. Rev. Lett. 2022, 128, 217203. lence programme). A.I.F. acknowledges support of a fellowship from “la [15] C. Tian, F. Pan, S. Xu, K. Ai, T. Xia, P. Cheng, Appl. Phys. Lett. 2020, Caixa” Foundation (ID 100010434) with code LCF/BQ/DI18/11660030 116, 202402. and of H2020 Marie Skłodowska-Curie grant No. 713673. A.I.F. is a Serra [16] A. F. May, S. Calder, C. Cantoni, H. Cao, M. A. McGuire, Phys. Rev. B Húnter Fellow. 2016, 93, 014411. Open access funding enabled and organized by Projekt DEAL. [17] J. Seo, D. Y. Kim, E. S. An, K. Kim, G.-Y. Kim, S.-Y. Hwang, D. W. Kim, B. G. Jang, H. Kim, G. Eom, S. Y. Seo, R. Stania, M. Muntwiler, J. Lee, K. Watanabe, T. Taniguchi, Y. J. Jo, J. Lee, B. I. Min, M. H. Jo, H. W. Conflict of Interest Yeom, S.-Y. Choi, J. H. Shim, J. S. Kim, Sci. Adv. 2020, 6, eaay8912. The authors declare no conflict of interest. [18] B. Zhao, R. Ngaloy, S. Ghosh, S. Ershadrad, R. Gupta, K. Ali, A. M. Hoque, B. Karpiak, D. Khokhriakov, C. Polley, B. Thiagarajan, A. Kalaboukhov, P. Svedlindh, B. Sanyal, S. P. Dash, Adv. Mater. 2023, Data Availability Statement 35, 2209113. [19] S. Albarakati, W.-Q. Xie, C. Tan, G. Zheng, M. Algarni, J. Li, J. Partridge, The data that support the findings of this study are available from the cor- M. J. S. Spencer, L. Farrar, Y. Xiong, M. Tian, X. Wang, Y.-J. Zhao, L. responding author upon reasonable request. Wang, Nano Lett. 2022, 22, 6166. [20] N. Liu, C. M. Gallaro, K. Shayan, A. Mukherjee, B. Kim, J. Hone, N. Vamivakas, S. Strauf, Nanoscale 2021, 13, 832. Keywords [21] S. Liu, Z. Li, K. Yang, E. Zhang, A. Narayan, X. Zhang, J. Zhu, W. Liu, Z. Liao, M. Kudo, T. Toriyama, Y. Yang, Q. Li, L. Ai, C. Huang, J. Sun, X. 2D magnets, 2D spintronics, Fe5−x GeTe2 , graphene, van der Waals epitaxy, Guo, W. Bao, Q. Deng, Y. Chen, L. Yin, J. Shen, X. Han, S. Matsumura, van der Waals heterostructures J. Zou, Y. Xu, X. Xu, H. Wu, F. Xiu, Natl. Sci. Rev. 2022, 9, nwab117. [22] M. Heilmann, A. S. Prikhodko, M. Hanke, A. Sabelfeld, N. I. Borgardt, Received: March 21, 2023 J. M. J. Lopes, ACS Appl. Mater. Interfaces 2020, 12, 8897. Revised: May 10, 2023 [23] Y. Gao, Q. Yin, Q. Wang, Z. Li, J. Cai, T. Zhao, H. Lei, S. Wang, Y. Published online: May 25, 2023 Zhang, B. Shen, Adv. Mater. 2020, 32, 2005228. [24] J. M. J. Lopes, Prog. Cryst. Growth Charact. Mater. 2021, 67, 100522. [25] J. M. J. Lopes, D. Czubak, E. Zallo, A. I. Figueroa, C. Guillemard, M. [1] M. Gibertini, M. Koperski, A. F. Morpurgo, K. S. Novoselov, Nat. Nan- Valvidares, J. Rubio-Zuazo, J. López-Sanchéz, S. O. Valenzuela, M. otechnol. 2019, 14, 408. Hanke, M. Ramsteiner, 2D Mater. 2021, 8, 041001. Small 2023, 19, 2302387 2302387 (9 of 10) © 2023 The Authors. Small published by Wiley-VCH GmbH 16136829, 2023, 39, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202302387 by Readcube (Labtiva Inc.), Wiley Online Library on [30/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-journal.com [26] M. Nakano, Y. Wang, Y. Kashiwabara, H. Matsuoka, Y. Iwasa, Nano [39] T. T. Ly, J. Park, K. Kim, H. Ahn, N. J. Lee, K. Kim, T. Park, G. Duvjir, N. Lett. 2017, 17, 5595. H. Lam, K. Jang, C. You, Y. Jo, S. K. Kim, C. Lee, S. Kim, J. Kim, Adv. [27] M. T. Dau, M. Gay, D. Di Felice, C. Vergnaud, A. Marty, C. Beigné, Funct. Mater. 2021, 31, 2009758. G. Renaud, O. Renault, P. Mallet, T. L. Quang, J.-Y. Veuillen, L. [40] A. Arrott, Phys. Rev. 1957, 108, 1394. Huder, V. T. Renard, C. Chapelier, G. Zamborlini, M. Jugovac, [41] K. Kim, J. Seo, E. Lee, K.-T. Ko, B. S. Kim, B. G. Jang, J. M. Ok, J. Lee, V. Feyer, Y. J. Dappe, P. Pochet, M. Jamet, ACS Nano 2018, 12, Y. J. Jo, W. Kang, J. H. Shim, C. Kim, H. W. Yeom, B. Il Min, B.-J. Yang, 2319. J. S. Kim, Nat. Mater. 2018, 17, 794. [28] J. E. Boschker, L. A. Galves, T. Flissikowski, J. M. J. Lopes, H. Riechert, [42] Z. Fei, B. Huang, P. Malinowski, W. Wang, T. Song, J. Sanchez, W. Yao, R. Calarco, Sci. Rep. 2016, 5, 18079. D. Xiao, X. Zhu, A. F. May, W. Wu, D. H. Cobden, J.-H. Chu, X. Xu, Nat. [29] S. Liu, X. Yuan, Y. Zou, Y. Sheng, C. Huang, E. Zhang, J. Ling, Y. Liu, Mater. 2018, 17, 778. W. Wang, C. Zhang, J. Zou, K. Wang, F. Xiu, npj 2D Mater. Appl. 2017, [43] G. K. R. Nair, Z. Zhang, F. Hou, A. Abdelaziem, X. Xu, S. W. Q. Yang, 1, 30. N. Zhang, W. Li, C. Zhu, Y. Wu, H. Weiling, L. Kang, T. Salim, J. Zhou, [30] L. A. Galves, J. M. Wofford, G. V. Soares, U. Jahn, C. Pfüller, H. L. Ke, J. Lin, X. Li, W. Gao, Z. Liu, Nano Res. 2022, 15, 457. Riechert, J. M. J. Lopes, Carbon 2017, 115, 162. [44] G. van der Laan, A. I. Figueroa, Coord. Chem. Rev. 2014, 277–278, 95. [31] L. Kang, C. Ye, X. Zhao, X. Zhou, J. Hu, Q. Li, D. Liu, C. M. Das, J. [45] P. Bruno, Phys. Rev. B 1989, 39, 865. Yang, D. Hu, J. Chen, X. Cao, Y. Zhang, M. Xu, J. Di, D. Tian, P. Song, [46] S. Cho, M. S. Fuhrer, Phys. Rev. B 2008, 77, 081402. G. Kutty, Q. Zeng, Q. Fu, Y. Deng, J. Zhou, A. Ariando, F. Miao, G. [47] E. Pallecchi, F. Lafont, V. Cavaliere, F. Schopfer, D. Mailly, W. Poirier, Hong, Y. Huang, S. J. Pennycook, K.-T. Yong, W. Ji, X. Renshaw Wang, A. Ouerghi, Sci. Rep. 2014, 4, 4558. et al., Nat. Commun. 2020, 11, 3729. [48] R. S. Singh, X. Wang, W. Chen, A. T. S. Wee, Appl. Phys. Lett. 2012, [32] D. Kim, S. Park, J. Lee, J. Yoon, S. Joo, T. Kim, K. Min, S.-Y. Park, C. 101, 183105. Kim, K.-W. Moon, C. Lee, J. Hong, C. Hwang, Nanotechnology 2019, [49] W. A. de Heer, C. Berger, X. Wu, P. N. First, E. H. Conrad, X. Li, T. Li, 30, 245701. M. Sprinkle, J. Hass, M. L. Sadowski, M. Potemski, G. Martinez, Solid [33] D. Shcherbakov, P. Stepanov, D. Weber, Y. Wang, J. Hu, Y. Zhu, State Commun. 2007, 143, 92. K. Watanabe, T. Taniguchi, Z. Mao, W. Windl, J. Goldberger, M. [50] T. Schumann, K.-J. Friedland, M. H. Oliveira, A. Tahraoui, J. M. J. Bockrath, C. N. Lau, Nano Lett. 2018, 18, 4214. Lopes, H. Riechert, Phys. Rev. B 2012, 85, 235402. [34] H.-J. Deiseroth, K. Aleksandrov, C. Reiner, L. Kienle, R. K. Kremer, Eur. [51] S. Hikami, A. I. Larkin, Y. Nagaoka, Prog. Theor. Phys. 1980, 63, 707. J. Inorg. Chem. 2006, 2006, 1561. [52] P. Willke, J. A. Amani, A. Sinterhauf, S. Thakur, T. Kotzott, T. Druga, [35] Q. Liu, J. Xing, Z. Jiang, Y. Guo, X. Jiang, Y. Qi, J. Zhao, Commun. Phys. S. Weikert, K. Maiti, H. Hofsäss, M. Wenderoth, Nano Lett. 2015, 15, 2022, 5, 140. 5110. [36] Y. Wei, C. Hu, Y. Li, X. Hu, K. Yu, L. Sun, M. Hohage, L. Sun, Nan- [53] M. H. Oliveira, T. Schumann, M. Ramsteiner, J. M. J. Lopes, H. otechnology 2020, 31, 315710. Riechert, Appl. Phys. Lett. 2011, 99, 111901. [37] D. K. Singh, G. Gupta, Mater. Adv. 2022, 3, 6142. [54] A. Barla, J. Nicolas, D. Cocco, S. M. Valvidares, J. Herrero-Martın, P. [38] S. Ershadrad, S. Ghosh, D. Wang, Y. Kvashnin, B. Sanyal, J. Phys. Gargiani, J. Moldes, C. Ruget, E. Pellegrin, S. Ferrer, J. Synchrotron Chem. Lett. 2022, 13, 4877. Radiat. 2016, 23, 1507. Small 2023, 19, 2302387 2302387 (10 of 10) © 2023 The Authors. Small published by Wiley-VCH GmbH