Magnetically Hard Fe3Se4 Embedded in Bi2Se3 Topological Insulator

Dec 10, 2015 - ABSTRACT: We investigated the structural, magnetic, and electronic properties of Bi2Se3 epilayers containing Fe grown on GaAs(111) by...
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Magnetically Hard Fe3Se4 Embedded in Bi2Se3 Topological Insulator Thin Films Grown by Molecular Beam Epitaxy Hugo Menezes do Nascimento Vasconcelos,†,‡ Mahmoud Eddrief,† Yunlin Zheng,† Dominique Demaille,† Sarah Hidki,† Emiliano Fonda,§ Anastasiia Novikova,§ Jun Fujii,⊥ Piero Torelli,⊥ Benjamin Rache Salles,⊥,∥ Ivana Vobornik,⊥ Giancarlo Panaccione,⊥ Adilson Jesus Aparecido de Oliveira,‡ Massimiliano Marangolo,† and Franck Vidal*,† †

Sorbonne Universités, UPMC Univ Paris 06, CNRS-UMR 7588, Institut des NanoSciences de Paris, F-75005 Paris, France Departamento de Física, Universidade Federal de São Carlos, CP 676, 13565-905 São Carlos, SP, Brazil § Synchrotron Soleil, L’Orme des Merisiers Saint-Aubin BP 48, 91192 Gif-sur-Yvette Cedex, France ⊥ Istituto Officina dei Materiali (IOM) - CNR, Laboratorio TASC, in Area Science Park, S.S.14, Km 163.5, I-34149 Trieste, Italy ∥ Instituto de Física, Universidade Federal do Rio de Janeiro, 21941-972 Rio de Janeiro, RJ, Brazil ‡

ABSTRACT: We investigated the structural, magnetic, and electronic properties of Bi2Se3 epilayers containing Fe grown on GaAs(111) by molecular beam epitaxy. It is shown that, in the window of growth parameters leading to Bi2Se3 epilayers with optimized quality, Fe atom clustering leads to the formation of FexSey inclusions. These objects have platelet shape and are embedded within Bi2Se3. Monoclinic Fe3Se4 is identified as the main secondary phase through detailed structural measurements. Due to the presence of the hard ferrimagnetic Fe3Se4 inclusions, the system exhibits a very large coercive field at low temperature and room temperature magnetic ordering. Despite this composite structure and the proximity of a magnetic phase, the surface electronic structure of Bi2Se3 is preserved, as shown by the persistence of a gapless Dirac cone at Γ. KEYWORDS: topological insulator, molecular beam epitaxy, ferrimagnetism, phase segregation, hybrid compounds materials with distinct physical and structural properties.18−20 In such a context, the control of the structural quality of the interfaces and of the composition of the phases is of primary importance, and this calls for detailed studies on the growth of thin-film heterostructures including TIs. This has been a key issue in the demonstration of magnetic proximity effects.21,22 Another interesting and active area currently investigated is the issue of diluted magnetic TIs, where a fraction of cations is replaced by magnetic ions in order to generate ferromagnetic order. Such materials have a strong spin−orbit coupling that can be exploited in spintronics devices.23 Indeed, ferromagnetism has been demonstrated in Mn-doped Bi2Te3, Mn-doped Bi2Se3, and Cr-doped Bi2Se3.21,24−28 The possibility of doping TIs with Fe was also explored.29,30 In this last case, the results are somewhat controversial, and further work is needed in order to determine whether true diluted TIs based on Fe doping can be grown in thin films. In particular, the issue of Fe substitution versus Fe segregation and the impact of multiphase

hree-dimensional topological insulators (TIs),1−12 such as the prototypical compounds Bi2Te3 and Bi2Se3, form a class of materials that has attracted considerable and increasing attention in recent years because of their peculiar electronic structure: while they are insulating within the bulk, gapless surface states ensure surface conductivity. Moreover, the spin and momentum of the surface states are locked at the Fermi surface, and backscattering is strongly suppressed.9 These properties make TIs appealing for fundamental studies on new physical effects and future applications such as spintronics devices or quantum computing. Interactions of the topologically protected surface state with magnetically ordered or superconducting materials are drawing increasing attention. Theoretically, it has been proposed that superconductor/TI heterostructures could be used in order to detect Majorana Fermions.6,13 Also, ferromagnet/TI heterostructures have been predicted as a potential path toward the design of artificial Weyl semimetals.14,15 This last type of heterostructures may also be of great interest in the quest for giant magneto-optical effects.16 Experimental tests of these predictions and successful use of TIs in future devices17 rely on our ability to design and grow heterostructures based on

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© XXXX American Chemical Society

Received: October 13, 2015 Accepted: December 7, 2015

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DOI: 10.1021/acsnano.5b06430 ACS Nano XXXX, XXX, XXX−XXX

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In order to probe the chemical environment of Fe in the epilayers, we performed X-ray absorption spectroscopy at the K-edge. Figure 2a shows the absorption spectrum of a Bi2Se3

growth on the topological properties clearly deserve further investigation. Here, we address the growth of Bi2Se3 epilayers by molecular beam epitaxy (MBE) with Fe content in the 8− 20% range. It is shown that coevaporation of Fe during growth under optimized conditions for high-quality Bi2Se3/GaAs(111) epilayers31,32 leads to the formation of FexSey platelets embedded within the TI matrix. Despite the presence of such buried phases, the system conserves its topological surface electronic properties. Moreover, the secondary phases exhibit interesting magnetic properties, making the system a promising platform for experiments on proximity effects in multifunctional materials combining topological electronic properties and collective order.

RESULTS AND DISCUSSION Fe segregates during the growth of Bi2Se3 epilayers (see Methods for a complete description of the growth conditions), forming Fe-rich regions as illustrated in Figure 1, where energy-

Figure 2. (a) Normalized X-ray absorption spectra of a Bi2Se3/ GaAs(111) epilayer with 20% Fe atomic content (line) and of a reference Fe foil (dashed line) taken at the Fe K-edge. (b) Spectrum of a Bi2Se3/GaAs(111) epilayer with 8% Fe atomic content. Inset: Extended X-ray absorption fine structure Fourier transform of the Fe K-edge signal.

epilayer with 20% Fe atomic content at the Fe K-edge together with the spectrum of a metallic Fe foil taken as a reference. It should be noted that these data were acquired in fluorescence mode in order to probe the whole Fe content of the system without being limited to the surface/subsurface region. The spectral fingerprint of the Fe-rich phases in Bi2Se3 differs from that of metallic Fe, ruling out the possibility of metallic Fe clustering within the epilayers. In the case of the 8% Fe sample, the spectrum recorded at the Fe K-edge is also quite distinct from the one of metallic Fe (Figure 2b). There are some differences in the spectra when the Fe content varies; most notably, the pre-edge feature is more pronounced in the spectrum of the 8% Fe sample. This may indicate that the composition of the Fe-rich phases changes with the total Fe content. A direct determination of the formula and stoichiometry of these phases based on X-ray absorption data only is not possible due to the lack of reference spectra of, for example, FexSey compounds. Qualitatively, we can only note that the spectrum of the 8% sample shares some features with spectra of tetragonal FeSe reported in the literature.33 Before going further, we summarize here the principal and salient features of the rich Fe−Se phase diagram. Close to x = y, FexSey compounds are known to exist in two different crystalline structures: a tetragonal PbO-type phase with FeSe stoichiometry that can sustain Se vacancies (we will refer to this phase as FeSe1−δ, with δ the amount of Se vacancies, in what follows) and a hexagonal NiAs-type phase for compounds with formula FeSe1+η with η ≥ 0. In the 0−0.33 η range, deviations to the NiAs unit cell can occur; for example, for η = 0.143 (corresponding to Fe7Se8), ordering of Fe vacancies leads to

Figure 1. Energy-filtered cross-sectional imaging of Fe segregation within Bi2Se3/GaAs(111) epilayers (Fe atomic content 8%), leading to the formation of Fe-rich secondary phases. (a) Zero-loss image of a region containing a secondary phase. (b) Same region as in (a) imaged at the Fe L-edge, revealing an Fe-rich secondary phase. (c) Zero-loss image of another region where secondary phase formation occurs. (d) Same as in (c) imaged at 60 eV loss, revealing the Fe-rich inclusion.

filtered transmission electron microscopy cross sections are shown. Chemical mapping of the samples reveals the formation of iron-rich phases with platelet shapes: as can be seen in Figure 1b,d, such regions have a lateral extension on the order of ∼50−100 nm while their thickness is on the order of 10 nm. These inclusions are located at varying depth within the Bi2Se3 epilayer, near the subsurface region in Figure 1d and near the interface with the GaAs substrate in Figure 1b. Considering the fact that Fe, Bi, and Se are coevaporated at constant fluxes during the MBE growth of the system, the formation of these Fe-rich segregated regions indicates strong Fe diffusion at the growth temperature, including diffusion along the growth axis, leading eventually to formation of FexSey platelets, as will be shown in what follows. B

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Figure 3. (a) θ−2θ scan of an FexSey:Bi2Se3/GaAs(111) epilayer (20% Fe atomic content). Sheared plain rectangles denote monoclinic Fe3Se4 peaks, and rectangles denote PbO-type FeSe peaks; the asterisk located near 31° denotes a possible NiAs-type FeSe peak (see text for details on the indexation of the diffraction peaks). (b) Same as (a) for 18% Fe. (c) Same as (a) for 8% Fe.

rectangles in Figure 3 are detected. Such additional features are fingerprints of the presence of FexSey compounds in the system. The peaks labeled by a sheared plain rectangle in Figure 3 can all be indexed as peaks related to the presence of a monoclinic phase with a = 6.20 Å, b = 3.50 Å, c = 11.28 Å, and β = 91.8°. Such values are fully consistent with the values reported previously for the monoclinic Fe3Se4 compound. It should be noted that the simultaneous detection of 1̅01, 002, 101, and 110 peaks allows the determination of a, b, c, and β unequivocally. This result attests the presence of Fe3Se4 within FexSey:Bi2Se3 epilayers. Aside from the clear presence of Fe3Se4, other FexSey phases cannot be completely excluded from our X-ray diffraction measurements. Indeed, l ̅0l Bragg peaks of monoclinic Fe3Se4 with the aforementioned parameters are located quite close to the 00l peaks of PbO-type FeSe (see rectangle labels in Figure 3). The deduced c-axis lattice parameter for PbO-type FeSe would be 5.51 ± 0.01 Å, to be compared with the values found in the literature for the bulk tetragonal phase: 5.52 Å. It is thus possible that Fe3Se4 and PbO-type FeSe coexist within the samples. In addition, the weak peak at 2θ = 30.5° labeled by an asterisk in Figure 3a may be indexed as 002 of NiAs-type FeSe with c = 5.86 Å, in agreement with values of the c-axis parameter reported in the literature. Thus, from such analysis of the diffraction pattern, the presence of several iron−selenium phases in the system, namely, PbO-type FeSe, NiAs-type FeSe, and monoclinic Fe3Se4, is possible. Interestingly, this indicates a wide range of stoichiometry among the FeSe species: PbO-type FeSe is known to be stable in slightly Se-deficient systems,

multiple cells with either 3c or 4c lattice parameter along the hexagonal axis (c being the lattice parameter of the simple hexagonal NiAs-type structure). 34,35 For η near 0.33, corresponding to the formula Fe3Se4, a monoclinic structure is found. Such structure can also be viewed as deriving from a parent pseudohexagonal NiAs structure with Fe vacancies ordering over multiple NiAs-like cells.34,35 The secondary phase formation is quite interesting: it offers the possibility to grow multifunctional systems combining the peculiar electronic properties of Bi2Se3 and physical properties implying collective order in FexSey. Tetragonal FeSe1−δ has been shown to exhibit superconductivity below TC = 8.5 K.36−40 Moreover, a dramatic increase of the critical temperature has been demonstrated recently in the case of ultrathin FeSe epilayers grown on SrTiO3(001).41,42 Hexagonal FeSe1+η exhibits ferrimagnetic ordering in a wide range of stoichiometries, with critical temperature exceeding room temperature: the critical temperature of Fe3Se4 is 315 K, and the one of Fe7Se8 exceeds 400 K.43 Moreover, Fe3Se4 is a hard ferrimagnetic compound with a huge magnetocrystalline anisotropy at low temperature.45−48 In order to determine the crystallographic structure of the Fe-rich regions identified by transmission electron microscopy, we performed X-ray diffraction measurements. Figure 3 shows θ−2θ scans of Bi2Se3/GaAs(111) epilayers with varying Fe content. Peaks related to the substrate and Bi2Se3 epilayers are observed as expected considering the epitaxy of Bi2Se3 on GaAs(111) with the hexagonal basal plane of Bi2Se3 parallel to (111)GaAs. Additional peaks labeled by rectangles and sheared C

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ACS Nano while the other compounds exist in the Fe-deficient side of the phase diagram. More insight on the nature of the FexSey phases can be gained through analysis of extended X-ray absorption fine structure (EXAFS) data, such as the one depicted in the inset of Figure 2b. For all of the samples, the analysis reveals a single broad peak in the EXAFS Fourier transform, corresponding to the first coordination shell. An Fe−Se mean distance of 2.44 ± 0.01 Å is extracted in the case of the 20% Fe sample. In the case of the 8% sample, we get 2.36 ± 0.01 Å. Such values should be compared to the Fe−Se distances expected in bulk unstrained FexSey compounds: 2.379 Å in the case of PbO-type FeSe49 and 2.48 Å in the case of Fe3Se4. This latter bond length is a mean value of the Fe−Se bond lengths in the monoclinic structure. Therefore, the measured Fe−Se bond length seems to be consistent with Fe3Se4 at higher Fe content and with a local tetragonal FeSe-like structure at lower Fe content. These results combined together with the X-ray diffraction results point toward the following picture: at higher Fe content, the Fe-rich phases are composed primarily of Fe3Se4 with possible trace of NiAs-like and PbO-type FeSe, and at lower Fe content, PbOtype FeSe coexists with monoclinic Fe3Se4. With regard to the formation of the PbO-type structure, it should be noted that a recent study reports the formation of a tetragonal FeSe-like structure upon annealing of Fe deposited at low temperature, in the submonolayer range, on the surface of a Bi2Se3 crystal.44 Having identified the main FexSey phases present in the system and knowing the percentage of Fe atoms in the system, we can determine the volume fraction occupied by the platelets in the system, hereafter labeled ξ. For 8% Fe atomic content, we get ξ = 4.6%. Taking into account the size of these objects, we estimate that the density of platelets is on the order of 1015 cm−3. For 20% Fe atomic content, we get ξ = 11.4%, assuming that most of the iron selenide platelets are composed of Fe3Se4. As expected given the presence of Fe3Se4 in the system, magnetic order is present in the epilayers. This is illustrated by the results displayed in Figure 4a, showing the magnetic response of samples with 20% Fe at T = 5 K. The coercive field is quite large, around 2.5 T. Such a huge value indicates the presence of a strong magnetic anisotropy in the system. The shape anisotropy of the platelets cannot explain such an anisotropy because Ms values of iron selenide ferrimagnetic compounds are not large enough: the shape anisotropy, given by (1/2)μ0Ms2 is too weak to account for the measured coercivity. We are thus left with the conclusion that the origin of the magnetic anisotropy is magnetocrystalline. Indeed, Fe3Se4 is a compound that exhibits a huge magnetocrystalline anisotropy (1.2 × 106 J m−3 predicted, between 1.0 × 106 and 0.5 × 106 J m−3 measured in nanoparticles45−48). Given the fact that monoclinic Fe3Se4 is detected by X-ray diffraction, we naturally assign the hard magnetic response to the Fe3Se4 inclusions. The fact that the magnetic response comes from Fe3Se4 is further substantiated by data in Figure 4b. At T = 250 K, Ms has dropped by a factor of 2; furthermore, the coercivity dropped from ∼2.5 to ∼0.5 T. This is fully consistent with the characteristics of the ferrimagnetic Fe3Se4 compound with TC = 314 K and a uniaxial anisotropy varying as Ms3 and dropping to much smaller values when approaching room temperature47 when compared to its huge value at low temperature. Although the value of the coercivity reported here is smaller than the one found in nanoparticles (4 T),47 it is fairly large. Possible reasons for the lowering of these values may reside in (i) the composite nature of the system inducing

Figure 4. (a) Magnetic cycle of an FexSey:Bi2Se3/GaAs(111) epilayer (20% Fe atomic content) at T = 5 K. The magnetic field is applied in the plane of the sample. (b) Same as (a) for T = 250 K. (c) Magnetization of a sample with 20 atom % Fe as a function of the temperature; data collected at remanence after cooling the sample from 400 to 1.8 K under a magnetic field μ0Happl = 7 T. (d) Magnetization of a sample with 18 atom % Fe as a function of the temperature; data collected after a field-cooling procedure, at an applied field μ0Happl = 0.01 T.

some disorder, leading to a smaller magnetocrystalline anisotropy, and (ii) strain within the embedded Fe 3Se4 platelets, inducing a magnetoelastic contribution to the magnetic anisotropy. The magnetic cycles in Figure 4a,b exhibit two inflections in the vicinity of μ0H = 0. Such unusual shape can be understood as resulting from the contribution of two distinct cycles, a hard one (preponderant) and a soft one. This indicates the possible presence of a minor magnetic phase distinct from Fe3Se4. Figure 4c,d displays M(T) measurements obtained for samples containing 20 and 18% Fe (same samples as the one measured by diffraction; see Figure 3). In the case of the 20% Fe sample, a critical temperature TC = 330 K is measured. This temperature is slightly larger than the value in bulk Fe3Se4. This may be due to the presence of trace of hexagonal NiAstype FeSe with larger TC, as suggested by the extra peak in Figure 3a and the softer, minor, magnetic response detected in magnetic hysteresis loops. In another 20% Fe sample, where no magnetically ordered species other than Fe3Se4 can be detected by X-ray diffraction, the measured critical temperature is TC = 320 K, very close to the bulk value. At 18% Fe content, again with no magnetic species structurally detected other than Fe3Se4, we measure TC = 315 K, in excellent agreement with the critical temperature of the bulk phase. The magnetic behavior of the 8% Fe sample (not shown here) confirms the general picture, displaying a large magnetic anisotropy that decreases when the temperature increases. From magnetization measurements and the calibration of the Fe content, we can evaluate the mean magnetic moment carried by the Fe ions in the system. We obtain between 0.26 and 0.32 μB/Fe for the set of samples explored here. Such values are D

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Fe (8% atomic content). The surface is smooth with large terraces separated by steps of 0.9 nm height. Such height corresponds to what is expected from the crystal structure of Bi2Se3, composed of quintuple layers stacked along the hexagonal c-axis. Atomic resolution imaging, see inset in Figure 5a, reveals the hexagonal structure of the Bi2Se3 surface. These results show that the surface is not perturbed by the presence of Fe in buried platelets. Figure 5b shows a high-resolution transmission electron microscopy cross section of the same system, acquired in the vicinity of the film surface. As can be seen, the only phase detected is Bi2Se3. Indeed, no secondary phase could be detected within a few nanometers depth below the surface. The persistence of the topological surface state in FexSey:Bi2Se3/GaAs(111) epilayers was checked by angleresolved photoemission, and the results are displayed in Figure 5c for a sample containing 8% Fe. The characteristic features of the electronic band structure around the Fermi level of Bi2Se3 are present: the Dirac cone related to the topological state with a Dirac point located at ∼0.42 eV below the Fermi level and the bulk band characterized by an inverted dispersion in the 0.6− 0.8 eV binding energies region. Such features are found to be identical to that previously observed in the case of undoped Bi2Se3 epilayers grown using the same parameters (temperature and flux).31,32 This shows that the band structure, in particular, the Dirac cone of the topological state, is unaffected by the presence of Fe in the system. Interestingly, the persistence of the Dirac cone is also demonstrated at 20% Fe, as shown by the data in Figure 5d. Although the opening of a small gap around the Dirac point cannot be completely excluded from our measurements, the 6 meV energy resolution of the spectrometer sets an upper limit for such a gap. It should be stressed that our measurements are global: the beam size is 50 × 100 μm2 and does not allow us to probe selected areas with a spatial resolution comparable to the size of the patches. Moreover, given the density and dimensions of the platelets and the fact that the remanent magnetization lies in the plane, the fraction of the surface that is susceptible of showing a perturbed electronic structure is at most a few percent. Thus, we have access to the average surface electronic properties, and any perturbation related to a proximity effect is hidden in our measurements. Despite these facts, we stress that the observation of the Dirac surface state still constitutes an interesting result because it is a prerequisite for further studies on possible proximity effects on a local scale. Such studies may be envisaged using scanning tunneling spectroscopy in the vicinity of the platelets/TI interfaces in order to explore possible magnetic proximity effects or to track the possible appearance of topological superconductivity.

quite consistent with a ferrimagnetic order of the Fe ions and with the values reported in the literature for Fe3Se4.46 The possible presence of the tetragonal FeSe phase in the system, most notably at lower Fe content, raises naturally the question of superconductivity. Indeed, our magnetic measurements did not show clear evidence of a superconducting phase transition at low temperature. This may be due to the fact that the stoichiometry range where superconductivity is present in FeSe is very narrow, corresponding to a slight deficiency in Se.37 Another possibility to explain the absence of superconductivity is the proximity of magnetically ordered monoclinic Fe3Se4 compounds within the embedded patches. The composite structure of the samples may have an influence on the topological surface state of Bi2Se3. We have thus studied the surface electronic properties of the Fe-doped samples. Figure 5a shows a typical scanning tunneling microscopy image of the surface of a Bi2Se3 epilayer containing

Figure 5. Surface properties of FexSey:Bi2Se3/GaAs(111) epilayers. (a) STM image of the surface of an FexSey:Bi2Se3 epilayer (8% Fe atomic content) (300 × 300 nm2). Inset: atomic resolution image showing the hexagonal symmetry of Bi2Se3. Bottom curve: height profile taken along the line drawn on the STM topographic image, showing steps with quintuple layer height as expected from the Bi2Se3 structure. (b) High-resolution transmission electron microscopy image acquired in cross-section geometry (GaAs [110] zone axis), same sample as in (a). The ∼5 nm thick gray zone above the surface is the Se capping layer; the lighter zone above this capping layer is a carbon protective layer. (c) Angleresolved photoemission spectrum of the FexSey:Bi2Se3 thin film (8% Fe atomic content) acquired with a 60 eV photon energy and linear polarization along the K̅ −Γ̅ −K̅ direction at T = 77 K. (d) Angleresolved photoemission spectrum of an FexSey:Bi2Se3 thin film with 20% Fe atomic content acquired with a 50 eV photon energy and linear polarization along the K̅ −Γ̅ −K̅ direction at T = 300 K.

CONCLUSIONS In this paper, it is shown that composite nanostructures, FexSey patches embedded in Bi2Se3 thin films epitaxially grown on GaAs(111), combining the peculiar electronic properties of TIs and magnetic order can be grown by molecular beam epitaxy. Among the FexSey phases forming during the growth, monoclinic Fe3Se4 with a huge magnetic anisotropy was clearly identified by structural and magnetic measurements and is the dominant species. Another phase with a local tetragonal PbOtype structure is also observed when the Fe content is decreased. These results suggest paths for the growth of ferrimagnetic/topological insulator nanocomposites. Fe3Se4/ Bi2Se3 nanocomposites may be of fundamental interest for E

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studies on proximity effects, where a material in contact with the topological insulator induces the emergence of a new property at the interface.21,22

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METHODS Sample Growth. The growth of FexSey:Bi2Se3 thin films was performed using a customized dual MBE reactor, where source fluxes were provided by conventional standard effusion cells.31 Flat (111)-B GaAs surfaces were prepared in a III−V chamber and subsequently transferred to a second chamber, where the growth of FexSey:Bi2Se3 epilayers was then carried out at T = 300 °C with a Se/Bi beam equivalent pressure ratio higher than 6.5 and an Fe flux adjusted to adjust the Fe atomic content. The growth temperature and Bi/Se flux ratio were fixed at optimal values within the growth parameters interval, leading to high-quality Bi2Se3 epilayers. Such values were determined in the case of undoped Bi2Se3 epilayers by monitoring the crystalline quality by X-ray diffraction (XRD) and the electronic structure (Dirac cone fingerprint in angle-resolved photoemission spectra).31,32 In the case of Fe-doped thin films, the epitaxy of Bi2Se3 on GaAs(111) is preserved, as checked by in situ reflection high-energy electron diffraction and ex situ by XRD. Samples were capped with protective amorphous Se or Au/ZnSe layers in order to prevent contamination and oxidation prior to air exposure, depending on the kind of measurement planned. For angle-resolved photoemission spectroscopy, a specific procedure implying the use of a Se cap layer was used, allowing us to recover a clean surface under ultrahigh vacuum after a mild annealing step.32 Sample Analysis. X-ray diffraction measurements were carried out using a Rigaku SmartLab diffractometer. High-resolution transmission electron microscopy and energy-filtered transmission electron microscopy data were acquired using a JEOL JEM 2100F equipped with a field-emission gun operated at 200 kV and a Gatan GIF spectrometer. Sample thinning was performed by focused ion beam. Xray absorption near-edge spectroscopy and extended X-ray absorption fine structure data (in the Fe K-edge spectral range) were collected on the SAMBA beamline of Synchrotron SOLEIL (St. Aubin, France). The K fluorescence intensity was monitored by a pixelated germanium detector. Opting for fluorescence detection allowed us to probe the whole epilayers. The Fe content in Bi2Se3 epilayers was determined by fluorescence measurements and calibrated using the signal from singlecrystalline Fe epilayers with accurately known thickness. In the present work, we report results obtained for systems containing between 8 and 20% Fe. The percentage of Fe is defined as the ratio of the Fe atomic content in the layers over the Fe + Bi atomic content. Magnetic measurements were performed using an apparatus combining superconducting quantum interference device and vibrating sample magnetometer (Quantum Design MPMS SQUID-VSM). Angleresolved photoemission spectroscopy was performed on the Advanced PhotoEmission (APE) beamline of Elettra Sincrotrone (Trieste, Italy).

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We are grateful to the SOLEIL staff for smoothly running the facility. We acknowledge support from CAPES, FAPESP, and CNRS (2014/50149-3-FAPESP/CNRS). Part of this work is supported by Agence Nationale de la Recherche (project Supertramp, ANR-11-BS04-0019). The authors gratefully thank D. Troadec (IEMN, Lille, France) for sample thinning using focused ion beam, and J.-M. Guigner, IMPMC, CNRS-UPMC, for access to the TEM facilities. F

DOI: 10.1021/acsnano.5b06430 ACS Nano XXXX, XXX, XXX−XXX

Article

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