High Spin Polarization at Ferromagnetic Metal–Organic Interfaces: A

Letter pubs.acs.org/JPCL

High Spin Polarization at Ferromagnetic Metal−Organic Interfaces: A Generic Property Fatima Djeghloul,†,⊥ Manuel Gruber,†,‡,# Etienne Urbain,† Dimitra Xenioti,† Loic Joly,† Samy Boukari,† Jacek Arabski,† Hervé Bulou,† Fabrice Scheurer,† François Bertran,¶ Patrick Le Fèvre,¶ Amina Taleb-Ibrahimi,¶ Wulf Wulfhekel,‡,§ Guillaume Garreau,∥ Samar Hajjar-Garreau,∥ Patrick Wetzel,∥ Mebarek Alouani,† Eric Beaurepaire,† Martin Bowen,† and Wolfgang Weber*,† †

Institut de Physique et de Chimie des Matériaux de Strasbourg (IPCMS), Université de Strasbourg, CNRS UMR 7504, 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France ‡ Physikalisches Institut, Karlsruhe Institute of Technology, Wolfgang-Gaede-Strasse 1, 76131 Karlsruhe, Germany ¶ Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, 91192 Gif-sur-Yvette, France § Institute of Nanotechnology, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany ∥ Institut de Science des Matériaux de Mulhouse, CNRS-UMR 7361, Université de Haute-Alsace, 68057 Mulhouse, France S Supporting Information *

ABSTRACT: A high spin polarization of states around the Fermi level, EF, at room temperature has been measured in the past at the interface between a few molecular candidates and the ferromagnetic metal Co. Is this promising property for spintronics limited to these candidates? Previous reports suggested that certain conditions, such as strong ferromagnetism, i.e., a fully occupied spin-up d band of the ferromagnet, or the presence of π bonds on the molecule, i.e., molecular conjugation, needed to be met. What rules govern the presence of this property? We have performed spin-resolved photoemission spectroscopy measurements on a variety of such interfaces. We find that this property is robust against changes to the molecule and ferromagnetic metal’s electronic properties, including the aforementioned conditions. This affirms the generality of highly spin-polarized states at the interface between a ferromagnetic metal and a molecule and augurs bright prospects toward integrating these interfaces within organic spintronic devices.

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he spin-dependent properties of ferromagnetic metal− organic (MO) interfaces have recently received considerable attention,1−9 not only because of their potential integration within hybrid and multifunctional devices, but also because these properties reflect novel fundamental phenomena that occur upon forming the interface. Ferromagnetic coupling between a molecule’s transition-metal site and the ferromagnet1,2 reflects a spin-polarized charge transfer between metal and molecule that is induced by the interface’s chemical bonds.5,10 This can alter the magnetism of the interface’s ferromagnetic layer7 or even promote magnetism when a nonmagnetic metal such as Cu is used.11 Thus, the magnetic impact on the interface’s metallic layer due to forming the interface is quite general. These chemical bonds can also cause the interface’s molecular layer to become metallic10,12 and to exhibit interface states (ISs) close to the Fermi level EF.13−18 If such ISs are strongly spin-polarized, as observed by spin-resolved photoemission spectroscopy for phthalocyanine (Pc) molecules on Co(001)19 and amorphous carbon (a-C) on Co(001),20 they can significantly enhance the efficiency of spin injection across these interfaces and thus the performance of organic spintronic devices. Is this property present solely for ISs that arise from combining the ferromagnetic metal Co with Pc19 or a-C?20 © 2016 American Chemical Society

The goal of the present study is to determine whether rules exist to predict the appearance of these highly spin-polarized ISs, or whether this property is general to such interfaces. The molecules studied in this work are (see insets in Figures 1 and 2) (a) 1,10-phenanthroline (“phen”; C12H8N2), (b) Mnphthalocyanine (MnPc; C32H16N8Mn), (c) C60, and (d) the alkane pentacontane (“penta”; C50H102). We address the following questions: (1) Do conjugated molecules other than Pc molecules and aC on Co(001) also show such behavior? (2) Does the presence or absence of highly spin-polarized ISs depend on the choice of the ferromagnetic substrate? We will compare the cases of MnPc deposited on fcc Co(001) and bcc Fe(001). (3) What is the role of the molecular geometry? Do highly spin-polarized ISs also appear for nonplanar molecules? We will compare the cases of planar MnPc and fullerene C60 deposited on Co(001). Received: May 23, 2016 Accepted: June 6, 2016 Published: June 6, 2016 2310

DOI: 10.1021/acs.jpclett.6b01112 J. Phys. Chem. Lett. 2016, 7, 2310−2315

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The Journal of Physical Chemistry Letters

using horizontally polarized photons of 20 eV energy impinging on the sample at 45°. We emphasize that no beam damage to the molecules could be evidenced. Photoelectrons were acquired in normal emission geometry. The energy resolution is 130 meV. The spin polarization was measured using a Mott detector, which exploits the left−right asymmetry A of electron scattering due to spin−orbit coupling.21 The effective Sherman factor S of the Mott detector is 0.12. To eliminate any experimental asymmetry (e.g., due to a misalignment of the electron beam with respect to the Mott detector), photoemission spectra for opposite magnetization directions were measured. The spin polarization component Pn perpendicular to the scattering plane is given by Pn = A/S. The spin-up and spin-down intensity spectra were obtained as follows: I I Iup = 2 (1 + Pn) and Idown = 2 (1 − Pn) with I the spinintegrated photoemission signal. To extract the spin-resolved photoemission signal (difference spectrum) that comes both from ISs, as well as from molecular layers atop the interface, we used a subtraction procedure that takes into account the molecule-induced attenuation of the Co or Fe substrate photoemission signal. This procedure and its justification are presented in great detail in refs 19 and 20 as well as in the Supporting Information of this Letter. Prior to deposition, the single crystalline Cu(001) or Pt(001) substrate was cleaned by several cycles of Ar-ion sputtering and annealing at 800 K. Then, a ferromagnetic film of face-centered cubic (fcc) Co or body-centered cubic (bcc) Fe was deposited onto the Cu or Pt substrate, respectively, at room temperature from a rod heated by electron beam bombardment. While the Co/Cu(001) system has been extensively investigated in the past (see for instance refs 22−25), there is only little work on Fe/Pt(001).26 Whereas Co films on Cu(001) are magnetized in-plane for all thicknesses, Fe films on Pt(001) exhibit a reorientation transition of the magnetization from out-of-plane to in-plane for a thickness of 2.2 ML.26 In both cases, films of 15 ML thickness were deposited so that the magnetization of all ferromagnetic films lies in-plane. In the second step of the sample preparation, organic molecules were deposited by radiative heating onto the ferromagnetic film at room temperature. The evaporation rate was controlled by a quartz microbalance. The thicknesses of the molecular layers were determined by Auger electron spectroscopy.27 All molecules were still intact after deposition, as expected regarding the deposition of MnPc and C60 on Fe or other transition metals (see for instance refs 13 and 19 for Pc and 28 and 29 for C60). The phenanthroline molecules on Co(001) were obtained by depositing the spin-crossover complex Fe(phen)2(NCS)2.30 Scanning tunneling microscopy (STM) studies showed that, while the molecular complexes remain intact when deposited onto Cu(111)31 or Cu(001) surfaces,32,33 a large portion of the complexes, which are composed of two phenanthroline groups, are broken predominantly into single phenanthroline molecules when deposited onto Co(001) at least in the first molecular layer (not shown). This molecular degradation when depositing onto Co compared to Cu is likely due to the former’s much stronger reactivity. Because there is only very little work on heavy alkanes deposited onto transition-metal surfaces, we performed STM, X-ray photoelectron spectroscopy (XPS), as well as ultraviolet photoelectron spectroscopy (UPS) to characterize the geometrical, electronic, and chemical structure of the pentacon-

Figure 1. Spin-resolved photoemission spectra as a function of the binding energy for 15 ML Co/Cu(001) (top panel); those after subtraction of a suitably normalized pure Co spectrum (“Co”) for 1.1 ML phenanthroline/Co (middle panel) and for 1.4 ML pentacontane/ Co (bottom panel). The photon energy is 20 eV. The insets show the corresponding molecules (hydrogen atoms are not shown).

Figure 2. As in Figure 1 with spectra for 15 ML Fe/Pt(001) (top panel), difference spectra for 1.6 ML MnPc/Fe (middle panel) and for 1 ML C60/Fe (bottom panel).

(4) What is the role of the π-electrons in conjugated molecules in the formation of spin-polarized ISs? Does an organic molecule without π-electrons behave similarly? We will compare the cases of MnPc on Co(001) with pentacontane on Co(001). To answer the above questions, we have undertaken thorough spin-resolved photoemission experiments at room temperature on the Cassiopée beamline at Synchrotron Soleil 2311

DOI: 10.1021/acs.jpclett.6b01112 J. Phys. Chem. Lett. 2016, 7, 2310−2315

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shows how the presence of d states in both spin subchannels at EF does not prevent highly spin-polarized ISs from appearing around EF. We note, however, that the line width of the IS on Fe is twice as large as that obtained on Co. We thus infer a stronger hybridization between the molecules and Fe, compared to the case of Co. Interestingly, a similar evolution in the line width of the IS was observed in pure metallic interface systems when going from Pd/Co(0001) to Pd/Fe(110).35 In our earlier studies on Pc/Co19 and on a-C/Co20 as well as in the present study on phenanthroline/Co and MnPc/Fe, we deal with molecules that are planar, such that all carbon atoms of a molecule may contribute to the hybridization. This situation, however, could be different in the case of nonplanar molecules that have a strongly reduced contact surface with the substrate. In fact, only 5−6 C atoms out of the 60 in the fullerene C60 are in direct contact with the substrate surface. Consequently, we expect a weaker interfacial signal compared to planar organic molecules. Figure 2 (bottom panel) shows the spin-resolved difference spectra of 1 ML C60/Fe(001). A spectral feature appears around 0.4 eV binding energy that does not exist in thick C60 films and is therefore identified as an IS. Compared to the spectral features observed for planar organic molecules, this IS is relatively weak in intensity, in agreement with our expectations. In contrast, measurements of C60 on Co(001) (not shown, see the Supporting Information) show that any significant changes to the raw photoemission intensity around EF upon molecular adsorption merely reflect an attenuation in electronic density. This means that the IS has a weak density of states. Because Co is more to the right than Fe in the periodic system, the reduction in hybridization strength between Co with C atoms compared to the case of Fe is intuitively expected, in agreement with our previous discussion of IS line widths. Interestingly, the spin polarization of the IS of C60/Fe close to EF is negative in sign. This is also observed for MnPc molecules on the same substrate Fe(001). We note that, for C60/Fe, the spin-up feature cannot be identified because it is probably hidden by the strong increase in spectral intensity at higher energies due to the highest occupied molecular orbital (HOMO) level. In the Pc/Fe case, the spin-up feature is much weaker compared to the spin-down feature. If we suppose a similar situation in the case of C60, the presence of the strong HOMO level might make it difficult to identify the interfacial spin-up feature. The HOMO level at about 1.9 eV binding energy shows only a small negative spin polarization. This proves that the majority of the C atoms, which are responsible for this state, are only weakly polarized by the ferromagnetic Fe substrate. This is understandable: because the majority of the C atoms are not in direct contact with the Fe substrate, the average induced spin polarization per C atom must be small. We note that a small but significant dichroic signal has been observed in X-ray magnetic circular dichroism measurements at the C K edge of C60/Fe, indicating a nonvanishing magnetic moment on the C60−Fe hybridized orbitals.36 For all systems studied up to now, we find that the interfacial spin polarization at the vicinity of EF is opposed to that of the ferromagnetic substrate. In fact, the ISs close to EF of molecules on Co(001) are positively polarized while the spin polarization of Co is negative in this energy range [see refs 19 and 20 and Figure 1 (top and middle panels)]. The opposite situation is found with the Fe(001) substrate (see Figure 2). Such a

tane−Co interface (see the Supporting Information). STM images show that pentacontane molecules completely wet the Co surface, leading to the completion of the first monolayer with intact flat-lying molecules whose C−C−C planes are parallel to the Co surface. The latter point is confirmed by UPS measurements (see the Supporting Information). More precisely, the monolayer of pentacontane consists of randomly distributed molecules that present multiple geometries of inplane adsorption. This disordered structure is radically distinct from the well-organized alkane monolayer as formed on noble metal surfaces such as Cu (see the Supporting Information). These observations suggest, as expected, a stronger molecule− substrate interaction in the case of Co compared to noble metal substrates. To investigate the hybridization state of the carbon atoms that form the interface with Co, we analyzed the C1s core level peak as a function of pentacontane coverage. We find that most (≥70%) carbon atoms within the first monolayer are sp3hybridized. The remaining carbon atoms are either in a carbidic C−Co (≤20%) or in a C−O (≤10%) environment. The carbidic contribution is most probably due to the reactivity of the Co surface, which can partly dehydrogenate the alkane and thus promote a strong C−Co binding of those dehydrogenated carbon atoms.34 Note that our molecular dynamics calculations (not shown) predict the presence of a certain dehydrogenation. The essential point of the XPS measurements within our context, however, is the fact that there is no indication of sp2hybridized carbon atoms and that the electronic structure is dominated by sp3 hybridized C atoms. The latter point is further corroborated by UPS measurements. Indeed, the UPS spectrum of the pentacontane monolayer exhibits for binding energies above 5 eV the same main features as UPS spectra for thicker layers (see the Supporting Information). We thus conclude that the pentacontane molecules in direct contact with the Co substrate preserve the sp3-governed electronic structure of alkanes. Turning now to spin-polarized photoemission results, we first discuss additional examples of conjugated molecules, the role of the ferromagnetic metallic substrate, and the molecular geometry before examining the role of sp3-hybridization in forming the interface and driving its spin polarization. Figure 1 (middle) shows the spin-resolved difference spectra of 1.1 ML phenanthroline/Co(001). A comparison with the spectra of MnPc or H2Pc on Co(001)19 shows that the spin-polarized ISs appear at essentially the same binding energies. As for Pc/Co, a clear energy gap in the spin-down channel appears while a spinup structure appears in the vicinity of EF. This proves that the details of the molecular structure do not drive the appearance of highly spin-polarized ISs. To check the role of the ferromagnetic substrate in IS formation, we replaced fcc-Co with bcc-Fe films. In contrast to fcc-Co, bcc-Fe is a weak ferromagnet; that is, its spin-up d-band is not completely filled. As a consequence, spin-up Fe d-states are available at the Fermi energy. We thus test whether the absence of spin-up d-states at the Fermi energy in the case of fcc-Co plays a crucial role in obtaining highly spin-polarized ISs as argued before.19 Figure 2 (middle panel) shows the spinresolved electron distribution curves after subtraction of a suitably normalized pure Fe spectrum from that of 1.6 ML MnPc/Fe(001). We observe the presence of two ISs: a spindown state at about 0.6 eV binding energy and a spin-up state at about 1.1 eV binding energy. Consequently, the interfacial spin polarization close to the Fermi level is almost −100%. This 2312

DOI: 10.1021/acs.jpclett.6b01112 J. Phys. Chem. Lett. 2016, 7, 2310−2315

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properties of each constituent drive the formation of highly spin-polarized interface states at the Fermi level, EF. We find that this property, initially reported for the conjugated molecules MnPc and H2Pc deposited onto Co,19 also appears for the conjugated molecule phenanthroline on Co. Switching from MnPc/Co to MnPc/Fe maintains a high amplitude of spin polarization of the IS around EF but switches its sign, i.e., tracks the spin polarization of the ferromagnet’s d bands at EF. This shows that a full spin-polarization of these d states at EF is not required as previously thought.19 Switching to the nonplanar molecule C60 reduces the hybridization strength while maintaining the highly spin-polarized IS around EF. This is also the case when switching to the nonconjugated pentacontane molecule, whose sp3 hybridization we find is maintained at the Co interface through auxiliary spectroscopic experiments. This shows that hybridization between a molecule’s π bonds with p character and the ferromagnet’s d orbitals is not crucial here as was inferred when examining the amorphous C/Co system.20 Rather, our results suggest that both in-plane and out-of-plane molecular bonds contribute to the IS, in agreement with previous reports,10,40 to yield a high spin polarization around EF. This should stimulate future calculations on systems that consist of only σ electrons, in an experimental context dominated by planar molecules with large π conjugation. Taken together, our results provide compelling evidence that the room-temperature high spin polarization of Fermi level states at the interface between a 3d ferromagnetic metal and molecules is a very general phenomenon, in a fashion similar to hybridization-induced changes to the metal’s magnetization, whether it is initially ferromagnetic7 or not.11 The hybrizidation that promotes it overshadows the nominal electronic structure of the molecule, the choice of which in turn becomes secondary from a fundamental aspect. This likely precludes a chemical tuning of the IS properties at the molecular level.10 On the other hand, this flexibility when choosing ferromagnetic metal and molecular candidates to form such interfaces augurs bright prospects toward deploying these interfaces in organic spintronic devices from materials science and technological perspectives.

behavior of the interfacial spin polarization has indeed been predicted for several conjugated molecules (cyclopentadienyl radical C5H5, benzene C6H6, and cyclooctatetraene C8H8) on Fe(110)5 and confirmed experimentally for H2Pc on Fe(110)5 and pentacene on Co(111). 37 It is believed that the hybridization of the ferromagnet d-states (preferably the dz2 orbital) with C pz atomic type orbitals, which originally form the π-orbitals, is at the origin of this behavior.5,37 This suggests that highly spin-polarized ISs might not be present in systems without π-electrons. To determine whether the highly spin-polarized ISs require π-electrons, we study the molecule pentacontane which, as an alkane, does not possess π-electrons because the carbon atoms are sp3 hybridized. As discussed above, this sp3 hybridization also dominates the formation of the ISs with Co. Here again, our photoemission measurements reveal clear evidence for highly spin-polarized ISs at similar binding energies (see Figure 1, bottom panel) as those found for Pc and phenanthroline on Co(001). This proves that π-electrons in the isolated molecule are not, a priori, important when forming the highly polarized IS. How can sp3-hybridized alkanes promote ISs similarly to π conjugated molecules such as Pc, phenanthroline, and C60? This apparent contradiction arises from the argument19,20 that, in π-conjugated systems, the hybridization of the 3d metal’s d orbitals with the p orbitals of the atoms pointing across the interface promotes highly spin-polarized ISs. Although there are no π-electrons in alkanes, near edge X-ray absorption fine structure measurements on the alkane n-octane (C8H18) deposited on Ni(111) and Cu(111) showed the presence of ISs close to the Fermi level.38 It is therefore possible to obtain ISs despite the fully saturated carbon bonds in the isolated molecule. We briefly consider the origin of these ISs. Our XPS results reveal the presence of Co−C bonding. This is only possible through dehydrogenation. Yet Co−C bonding amounts to less than 20% of the ISs. Dehydrogenation alone can thus not explain the ISs because, otherwise, the ISs’ intensity measured in photoemission experiments would be much weaker than observed. We thus speculate that molecular distortion upon chemisorption and the ensuing charge transfer alters the bond lengths and geometries, leading to deviations from a purely sp3 description of the molecule’s carbon bonds as reported previously.39 We note that the pentacontane hybrid orbitals (of sp3 type) are also formed by p orbitals pointing in and out of the molecular plane, which in turn could form ISs similarly to those found for conjugated molecules. Furthermore, calculations of the CoPc/Fe(110)40 and MnPc/Co(001)10 interfaces show how the ferromagnet’s 3d states hybridize not only with molecular orbitals of π-character, but also of σ-character because the latter are significantly affected by the interaction with the ferromagnetic surface. These considerations underscore how the charge transfer, which involves the metal’s much larger density of states compared to that of the molecule, strongly alters the molecule’s initial orbital configuration upon forming the interface and its ensuing ISs. This in turn explains why the highly spin-polarized IS around EF, found at room temperature, depends so little on the choice of molecule and instead appears to be a generic feature of the interface between a ferromagnetic metal and a molecule. In conclusion, we performed spin-resolved photoemission spectroscopy experiments on a range of ferromagnetic metal− molecule interfaces to test whether/how the electronic

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01112. Additional figures and discussion of experimental procedures and results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses ⊥

F.D.: Université de Ferhat Abbas Sétif 1, Faculté de Technologie, Sétif, Algeria. # M.G.: IEAP, Christian-Albrechts-Universität zu Kiel, 24098 Kiel, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the CNRS, the Institut Carnot MICA’s “Spinterface” grant, from ANR Grant ANR-11-LABX-0058 NIE and from the Franco-German 2313

DOI: 10.1021/acs.jpclett.6b01112 J. Phys. Chem. Lett. 2016, 7, 2310−2315

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university. We thank the SOLEIL staff for technical assistance and insightful discussions. The DFT calculations were performed using HPC resources from GENCI-CINES Grant 2015-gem1100 and from Strasbourg HPC mesocenter.

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