Ferromagnetic Exchange Coupling between Fe Phthalocyanine and

Article

Ferromagnetic Exchange Coupling between Fe Phthalocyanine and Ni(111) Surface mediated by the Extended States of Graphene Andrea Candini, Valerio Bellini, David Klar, Valdis Corradini, Roberto Biagi, Valentina De Renzi, Kurt Kummer, Nicholas B. Brookes, Umberto del Pennino, Heiko Wende, and Marco Affronte J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5041663 • Publication Date (Web): 09 Jul 2014 Downloaded from http://pubs.acs.org on July 15, 2014

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The Journal of Physical Chemistry

Ferromagnetic Exchange Coupling between Fe Phthalocyanine and Ni(111) Surface mediated by the Extended States of Graphene Andrea Candini*,1, Valerio Bellini1, David Klar2, Valdis Corradini1, Roberto Biagi1,3, Valentina de Renzi1,3, Kurt Kummer4, Nicholas B. Brookes4, Umberto del Pennino1,3, Heiko Wende2, and Marco Affronte1,3 1

Centro S3, Istituto Nanoscienze – CNR, Via Campi 213/a, 41125 Modena, Italy

2

Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of

Duisburg-Essen, Lotharstraße 1, D-47048 Duisburg, Germany 3

Dipartimento di scienze Fisiche Informatiche e Matematiche, Università di Modena e Reggio

Emilia, Via Campi 213/a, 41125 Modena, Italy 4

European Synchrotron Radiation Facility (ESRF), Rue Jules Horowitz 6, 38043 Grenoble,

France

ACS Paragon Plus Environment

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ABSTRACT The interface spin coupling mechanism is studied in a hybrid structure made of Fe phthalocyanine molecules sublimed in ultra high vacuum on graphene grown on the magnetic substrate Ni(111). By using synchrotron x-ray magnetic circular dichroism, the field dependent magnetization of the isolated FePc molecules and of the Ni substrate has been measured at low temperature (8K). Along with density functional theory calculations, the role of the graphene interlayer in transmitting the magnetic coupling is addressed. Both experiments and theory show a ferromagnetic coupling between the molecules and the substrate which is weakened by the insertion of graphene. DFT calculations indicate that the key role is played by the π orbitals of graphene, which hybridize with the underlying magnetic Ni, giving rise to a sizeable spin polarized continuum at the molecular interface. The resulting overlap with the Fe orbitals favors a direct coupling of ferromagnetic nature, as evidenced by our spin density distribution plots. KEYWORDS spin interface, magnetic molecules, graphene, X-ray magnetic circular dichroism.


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TEXT INTRODUCTION Spin effects at the interface of hybrid systems made of inorganic magnetic electrodes and organic molecules are currently at the heart of molecular spintronics. While spin interface effects are commonly investigated in 2D structures, the key mechanisms can be naturally scaled down to the level of a single molecule1,2. In this rapidly growing field, metal-organic complexes, comprising an organic group such as phthalocyanines and porphyrins and a metallic center such as a transition metal or a rare earth ion, represent a prototypical case due to their versatility and high stability. Particularly appealing is the possibility to control the spin state of such molecules by adsorption on opportune substrates or by doping3-11. Recently, considerable effort has been focused on paramagnetic molecules such as metal-phthalocyanines or porphyrins deposited on graphene grown on different type of substrates12-24. On one hand, molecular doping can be used to tune the electrical properties of the graphene layer19, while the presence of graphene can drive molecular self-assembling12,15-17 and it can be used as a tool to switch the molecular spin state13. Since the interaction is mainly of van der Waals type, graphene can be used as a decoupling layer to efficiently preserve the electrical and structural properties of the molecular systems. Nevertheless, it has been reported that magnetic interaction can be transmitted through it20-24. Here, we report a combined experimental and theoretical study on Fe phthalocyanine (FePc) molecules deposited on bare Ni(111) and on graphene/Ni(111) substrates. A sizeable ferromagnetic exchange interaction is observed between the molecules and the substrate, with and without the graphene interlayer. By analyzing the molecular orbitals involed in the transmission of the magnetic interaction, our work evidences the role of extended states of graphene in the (de)coupling mechanism.


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EXPERIMENTAL AND THEORETICAL METHODS Experiments were carried out at the ID8 beamline of the European Synchrotron Radiation Facility in Grenoble, France. The graphene/Ni(111) system was prepared ex situ in our laboratory UHV-chamber with a base pressure of 5.0 x 10-10 mbar. A Ni(111) single crystal was used as the substrate. Before the graphene growth, the surface was cleaned by repeated cycles of Ar+ sputtering and annealing (Temperature = 800 °C for 5 minutes). The quality and the cleanness of the surface was checked by Low Energy Electron Diffraction (LEED) and X-ray Photoemission Spectroscopy (XPS). The freshly prepared Ni(111) surface was then heated and stabilized at 500 °C. Ordered graphene overlayers were prepared by introducing propene gas (C3H6) in the chamber at a pressure 2×10−7 mbar for 15 min, following procedures established in literature25,26. The quality, homogeneity and cleanliness of the prepared graphene/Ni(111) system were checked by LEED, XPS and STM. The procedure was repeated until no evidence of carbide phase was detected by XPS. The graphene/Ni(111) system is very stable after air exposition. We have verified that even few weeks after the preparation, in a sample kept in ambient conditions, we can easily re-obtain a clean system showing the same characteristics of the pristine one, by heating in UHV at 500 °C for 5 minutes to desorb impurities. This stability has been already observed and reported by other groups27 also on graphene on different metals, such as Cu(111)28. Further details on the graphene growth and XPS and LEED characterizations can be found in the Supporting Information. After insertion in the ID08 experimental chamber and the recovering of the clean graphene/Ni(111) system, a 0.4 monolayer of FePc molecules was evaporated after long degassing of the powders, at a base pressure of 1.0 x 10-9 mbar. The thickness of the molecular film was monitored in situ through a calibrated quartz microbalance.


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Circularly and linearly polarized XAS measurements at the Fe and Ni L2,3 and N K absorption edges were performed in total electron yield mode. For all the measurements, the base pressure in the measurement chamber was 1.0 x 10-10 mbar and the temperature was 8 K. As the magnetic easy axis of the Ni(111) single crystal is along the plane, the external magnetic field B (always parallel to the incident photon beam) was applied at grazing incidence, with an angle θ with the sample surface of 70°. To avoid sample degradation induced by the synchrotron radiation, we employed a reduced beam intensity and carefully monitor the XAS spectra. We also frequently changed the sample position in order to constantly expose a non-irradiated part of the sample to the beam. The XMCD spectra are defined as the difference between the XAS spectra taken with the helicity of the incident photon antiparallel (I-) and parallel (I+) to the external field, normalized by the height of the XAS edge.

DFT calculations have been performed using the VASP code29 using GGA-PBE exchangecorrelation potential30 and including van der Waals interactions in the semi-empirical method of Grimme31. The FePc/Ni(111) and FePc/Graphene/Ni(111) systems have been simulated with supercells consisting of a Ni lattice with 8x8 in-plane periodicity; the Ni slab is composed of three (five) monolayers in the presence (absence) of graphene (a sketch the relaxed structures without and with the graphene layer have been depicted in Figure 1, upper panels), and sufficient vacuum space in the perpendicular direction is considered in order to avoid replicas interaction. In the case of graphene/Ni slab, graphene is adsorbed (if not differently written) in a top-fcc stacking, which we found to be energetically more favorable than the top-bridge one24. A mesh of 2x2 K-points have been considered for the 2D-Brillouin zone integration. The chosen Ni lattice constant is 3.49 Å, and the atomic coordinates of the FePc molecule and of the topmost


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substrate layer (Ni or graphene), have been relaxed until forces were less than 2 meV/Å. In order to include (static) correlation effects in the d-orbitals of Fe, we performed the calculations in presence of a +U Hubbard-like term, included in the simplified (rotationally invariant) approach to LSDA+U introduced by Dudarev et al.32; we have chosen U=4eV (J=0eV), values in line with the ones already used in the literature 33.

Figure 1. (a,b): Schematic view of the system under investigation. The beam incidence angle theta (θ) is 70° (grazing incidence).(c,d): Linearly polarized X-Ray absorption spectra at the N K edge after background subtraction for the FePc molecules directly on Ni(111)(c) and on graphene/Ni(111)(d).

RESULTS


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Experimental results In Figure 1 we present a schematic view of the system under study. In order to assess the structural configuration of our molecular film, we measured the linearly polarized X-ray Linear Dichroism (XLD) at the N K edge, shown in the lower panel of Figure 1. We notice clear π* and σ* resonances with strong polarization dependence at 397 eV and 405 eV, respectively. In agreement with other reports5,11 on similar molecules adsorbed on surfaces, this indicates that the molecules lie flat and parallel to the surface. It is also evident that the π* absorption peaks are sharper when the molecules are deposited on the graphene covered Ni surface, resembling the spectrum of the freestanding molecules34,35. This suggests a weaker electronic interaction of N atoms with the substrate when the graphene interlayer is present as it is similarly discussed in Ref.11. Next we focus on the analysis of the Fe L2,3 absorption edges, shown in the upper panels of Figure 2 for the FePc molecules deposited on Ni(111)(left) and graphene/Ni(111)(right). The Xray absorption spectrum (XAS) of the molecule deposited on bare Ni is very broad, pointing towards a strong Fe-Ni interaction, in agreement with what has been already reported35. The L3 edge becomes sharper upon the insertion of graphene, and some of the characteristic lineshape details of the spectra associated with the non-interacting molecules, although not yet complete34,36, starts to develop. This illustrates the role of graphene as a decoupling layer for the electronic interaction. In order to investigate the magnetic properties of our system, the corresponding X-ray Magnetic Circular Dichroism (XMCD) spectra are presented in the lower panels of Figure 2. Measurements have been taken under an external magnetic field of 5 Tesla and at the base


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temperature of 8 K. For both the FePc molecules on Ni(111) and on graphene/Ni(111) a XMCD signal of approximately the same peak intensity is observed, although the spectrum corresponding to the molecules directly deposited on the metal surface displays a slightly larger peak area on both the L2,3 edges. This likely reflects the different broadening level of the molecular characteristic features, due to the different interaction strength with and without the graphene layer.

Figure 2. Fe L2,3 unpolarized XAS (upper panels) and XMCD (bottom panels) spectra for FePc

molecules

after

background

subtraction

directly

on

Ni(111)(left)

and

on

graphene/Ni(111)(right).


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The magnetic properties of our system have been studied by measuring the XMCD signal as a function of the external magnetic field, similarly to a magnetization cycle M(B) measurement. In Figure 3, we plot the XMCD peak intensity at the L3 edge of Fe as a function of the external field for the molecules directly on Ni(111)(black squares) and on graphene/Ni(111)(red disks). When the molecules are on the bare Ni, the XMCD intensity is the same at 5 T and at 0.25 T, while we observed a decrease of signal when the graphene layer is inserted. Below 0.25 T the magnetization of the Ni(111) substrate gradually reduces to zero for B = 0 (see inset of Figure 3) with vanishing coercive field since the system breaks up into domains. As a consequence, no XMCD signal is found at zero magnetic field for the FePc molecule with or without graphene. For comparison, in Figure 3 we show the behavior expected for a paramagnetic spin S = 1 at the same temperature (green line), that is what is usually reported for the FePc molecule in thick film or when deposited on non-magnetic substrates6,36. The difference between the experimental data and the expected paramagnetic signal is due to the existence of a sizable magnetic exchange coupling between the molecules and the substrate5,20. We can quantify this coupling and reproduce the experimental behavior using a Brillouin function (BJ(B)) of the type MNi(B)*BJ(B+Bex), where Bex is the exchange field with the substrate and MNi(B) is the Ni(111) magnetization; we used J = 1, T = 8 K and we neglected anisotropic terms. Our fit yields a coupling of 0.5±0.1 meV when the molecules are deposited on graphene/Ni(111). When the molecule are adsorbed directly on the magnetic substrate, we can only deduce a lower bound for the coupling of 1.2 meV.


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Figure 3. Field-dependent XMCD intensity for FePc directly on Ni(111) and on graphene/Ni(111). Solid lines are the fitting curves following the model described in the text. For comparison, the curve corresponding to a paramagnetic signal is also shown in solid green. Inset: magnetization of the Ni(111) single crystal substrate. Theoretical Results In order to shed light on the FePc – substrate interaction we performed DFT simulations of a FePc molecule deposited on a Ni(111) substrate, with and without the graphene decoupling layer. As evidenced in References37,38, a graphene layer on Ni may have different stackings. Although top-fcc is the most abundant, top-bridge is also a possible stable solution (see Supporting Information for pictorial views). In the following we will show the results for both top-fcc and top-bridge stackings, although if not explicitly stated, the analysis is relative to the top-fcc case. When the FePc is placed directly on the Ni(111) surface it covalently adsorbs and the equilibrium average FePc-Ni distance is ≈ 2.2 Å, while, in presence of the graphene layer, the molecule-substrate interaction weakens, leading to a larger adsorption distance of ≈ 3.1 Å (for a


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pictorial view, see the sketches in Figure 1). In the former case, some deviation from the molecular in-plane position occurs for the Hydrogen atoms and the N atoms in direct contact with Fe; upon structural relaxation, the interaction with Ni is also strong enough to shift the molecule to some amount from the initial hollow adsorption site, and the Fe ion turns to sit in between a hollow and a bridge sites (see Supporting Information for top views of the relaxed structures). On the other hand, in presence of graphene, the planarity of the molecule remains intact, and the hollow adsorption site is preserved. In both cases the Fe ion attains a spin S = 1 (mFe = 2.00 µB), in contrast to what has been observed for a Fe porphyrine on graphene/Ni(111) for the same adsorption site, where the spin of the molecule varies from S = 1 to S = 222. When the FePc molecule is in direct contact with Ni, charge transfer between the molecule and the substrate leads to sensible modifications in the spin moments of some of the Ni atoms below the molecule (from 0.68 µB to ≈ 0.30-0.40 µB), while the variation in the spin-polarization of the organic part of the molecule is very limited, and the net transferred polarization from the substrate is of small entity (moments of ±0.01-0.02 µB are induced on some C atoms). We have calculated for both systems the exchange energy Eex defined as the difference between the total energy for antiparallel and parallel orientations of the Fe and Ni moments, i.e. Eex = EAP - EP. The favored magnetic coupling of the FePc molecule with the Ni substrate is (as expected) ferromagnetic, and Eex attains a value of 73 meV. Note that for the case of FePc adsorbed on a Co(001) surface, the value of 229 meV was reported11. The presence of the graphene layer reduces sensibly the coupling to Eex = 14 meV. Interestingly, the sign of the interaction is preserved, in agreement with the experimental evidence. Our calculations performed for the topbridge stacking show that molecule-substrate coupling is also ferromagnetic and Eex amounts to 6 meV. The larger value of the calculated exchange coupling as compared to experiments could


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be partly traced back to the well-known missing electronic correlation effects intrinsic in meanfield electronic structure calculations and it has been observed in previous similar studies20. The choice of U is also of moderate influence; we have performed test calculations for the top-fcc stacking by varying the value of U on the Co d orbital from 2eV to 5eV and Eex is found to vary from 22 meV to 11 meV. The sign of the coupling is therefore robust upon variation in chosen U value. The molecule-substrate interaction can be further discussed by analyzing the local density of states (LDOS). In Figure 4 we plot the orbital- and site-projected LDOS for the isolated FePc molecule (Fe-d in panel (a) and N-sp, C-sp in panel (b)), for the FePc/Graphene/Ni(111) (Fe-d in panel (c), N-sp, C-sp in panel (d), Ni-d of the top-most metal layer in panel (e) and C-sp of the graphene layer in panel (f)) and for the FePc/Ni(111) (Fe-d in panel (g), N-sp, C-sp in panel (h) and Ni-d of the top-most metal layer in panel (i)) systems. Under D4h crystal field, the d-orbitals of Fe splits into dxy (b2g), dxz+dyz (eg), dz2 (a1g) and dx2-y2 (b1g) symmetry (the x,y-axes system connects the Fe ion with the nearest N neighbor). The analysis of Figure 4(a) and Figure 4(b) shows that the Fe d orbitals hybridize with the sp states of C and N; this leads to further splitting of the energy level so that several peaks are observed in Figure 4(a). A small antiparallel moment of ≈0.02 µB is induced on the four N atom which bonds to Fe, while residual small polarization is present in the rest of the molecule. The LDOS of the isolated molecule is consistent with a 3

EgA electronic configuration, which is one of the configurations reported in the literature,

together with other ones39-41.


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Figure 4. Spin-resolved LDOS projection onto different orbitals of isolated FePc molecule [panels (a) and (b)], for the FePc/Graphene/Ni(111) [panels from (c) to (f)] and FePc/Ni(111) [panels from (g) to (i)] systems. In panels (a), (c) and (g) the projection onto Fe-d orbitals of different symmetries [b1g/dx2-y2 (black), eg/dxz+dyz (red), a1g/dz2 (orange) and b2g/dxy (green)] is given. Similarly in panels (b), (d) and (h) the projection onto N-sp (turquoise) and C-sp (magenta) is supplied. Panel (f) depict the sp-projected DOS onto the two different (by symmetry) C atoms of the graphene layer [top (dark brown) and fcc (light brown)] for the FePc/Graphene/Ni(111) system, while panels (e) and (i) shows the d-projected DOS of the Ni top-most metal layer [dxy+dx2-y2 (black), dxz+dyz (red) and dz2 (orange)] in presence and absence of the graphene layer, respectively.


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The adsorption of the FePc molecule on the graphene/Ni(111) substrate leads to hybrization between the out-of-plane orbitals eg (red) and a1g (orange), evident from the broadening and further splitting of the relative peaks in Figure 4(c). The graphene LDOS (Figure 4(f)) shows that top and fcc C atoms hybridize differently with the Ni orbitals below (Figure 4(e)), the former in the [-3.5 eV; -1.5 eV] energy window, the latter in the vicinity of the Fermi level42. Upon molecule adsorption, similar behaviour is observed for those states of the N and C atoms of the FePc (Figure 4(d)) that hybridize with the Fe d orbitals. When the molecule is placed in direct contact with the Ni surface, the interaction is so strong that the a1g (orange) peak of Fe broadens out and only some character in the majority channel around -4 eV from the Fermi level is retained (Figure 4(g)), where hybridization with the lower energy part of the Ni d states takes place (Figure 4(i)). The analysis of the LDOS at the N and C atoms of the FePc (Figure 4(h)) indicates that the π electron system of the organic part of the FePc interacts sensibly with the Ni states, so that for energies above -3 eV from EF, a continuum of states matching the continuum of the sp metal states below is observed. Additional information can be found by looking at the spin-density distribution close to the molecule-surface contact region for the FePc/Graphene/Ni(111) systems, in the two cases of parallel and antiparallel coupling between the Fe and Ni spin moments. By plotting the spin density three-dimensional iso-surfaces and zooming in the molecule-graphene contact region we are able to visualize the spin communication channels which form in the parallel/ferromagnetic coupling case (see Figure 5(a)). This is due to a preferred spin-up polarization of the extended π orbitals of the C atoms, which outstretches farther (light green lobes) towards the FePc molecule as compared to spin-down (light blue lobes). Conversely, such channel fades away in the antiparallel case (Figure 5(b)), because of the mismatch of the spin index between electrons


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belonging to the molecule and to the graphene layer. A similar matching is observed in the case when the graphene stacks in the top-bridge registry on Ni, see Figure 5(c) and (d), where the extended spin-polarized states above the graphene layer are particularly evident.

Figure 5. Three-dimensional contour plots of the spin density distribution at the FePc/Graphene/Ni(111) interface for top-fcc (a) and (b) and top-bridge (c) and (d) stacking; (a) and (c) are for the parallel, (b) and (d) for the antiparallel coupling between the Fe and Ni spin moments. Light green (light blue) color refers to an excess of spin up/majority (down/minority) electrons. An isovalue of ±0.0003 electrons has been considered. DISCUSSIONS To better understand the role of graphene and the peculiarities of FePc, it is worth to compare our results with what has been obtained on other systems. Our calculations show that the coupling between FePc and Ni remains ferromagnetic, even in presence of a graphene decoupling layer. While this behaviour is similar to the one of a Fe porphyrin on graphene/Ni22,


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in the case of a Co porphyrin, the insertion of a graphene layer is found to modify the sign of the coupling20,21,23, being due to the different orbitals involved in the spin communication between the molecule and the magnetic substrate. Another non- trivial finding is that the sign of the coupling is not influenced by the type of stacking of the graphene on the Ni substrate. In fact, previous calculations performed for a Co porphyrin on the same substrates, indicates a change in the sign of the coupling depending on whether the top-fcc or bridge-top stacking is considered, although the absolute value of the coupling were unusually large (a hundred of meVs) in both cases20. As could be inferred from the previously reported DOS analysis, a simple energy-spin matching scheme between the molecule and substrate state could not provide a comprehensive understanding of the type (ferro- cf. antiferromagnetic) coupling, as it was instead possible, for instance, in the case of a Cobaltocene molecule on graphene/Ni(111)24; this is due to the multiple Fe-d channels involved in the coupling, as opposed to Cobaltocene, where only one spinpolarized orbital belonging to the Co ion close to EF is responsible for the magnetic coupling. Finally it is interesting to observe that an oxygen interlayer usually promotes 180° superexchange bridges of mostly molecular/atomic character thus favoring antiferromagnetic interaction5,11. Conversely, graphene behaves as a continuous (one atom thick) layer, which becomes polarized due to the hybridization with the substrates and mediates the magnetic interaction by its extended orbitals.


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CONCLUSIONS The possibility to use a graphene layer to establish a magnetic exchange coupling between a magnetic surface and a molecule without strong chemical bonds has been reported only very recently20. Yet, this system is already considered very promising for the study and the development of spin coupling effect at the hybrid metal – organic interface20-24. So far most of the experimental work has been dedicated to systems displaying an antiferromagnetic coupling through graphene, whilst here we have reported the experimental evidence of a graphene mediated coupling of ferromagnetic character. Ab-initio DFT calculations indicate that the origin of this interaction is related to the magnetic hybridization of the graphene extended orbitals with the ones of the underlying Ni(111) substrate, combined with the specific orbitals of the d-metal ion of the molecule. Our findings show the possibility to effectively engineer the coupling interaction by a suitable choice of the molecular components. Beside a systematic investigation of different metal ions inside the organic molecule compound, further possibilities to effectively tune the interaction have already been demonstrated or proposed, such as molecular doping21, atomic intercalation at the graphene/metal interface24, changing the molecular film thickness23 or exploiting the atomic defects of graphene22. All these facts depict how graphene at the hybrid interface between molecules and substrate is a unique platform to study and engineer spin coupling.

FIGURES CAPTIONS


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Figure 1. (a,b): Schematic view of the system under investigation. The beam incidence angle theta (θ) is 70° (grazing incidence).(c,d): Linearly polarized X-Ray absorption spectra at the N K edge after background subtraction for the FePc molecules directly on Ni(111)(c) and on graphene/Ni(111)(d). Figure 2. Fe L2,3 unpolarized XAS (upper panels) and XMCD (bottom panels) spectra for FePc molecules after background subtraction directly on Ni(111)(left) and on graphene/Ni(111)(right). Figure 3. Field-dependent XMCD intensity for FePc directly on Ni(111) and on graphene/Ni(111). Solid lines are the fitting curves following the model described in the text. For comparison, the curve corresponding to a paramagnetic signal is also shown in solid green. Inset: magnetization of the Ni(111) single crystal substrate. Figure 4. Spin-resolved LDOS projection onto different orbitals of isolated FePc molecule [panels (a) and (b)], for the FePc/Graphene/Ni(111) [panels from (c) to (f)] and FePc/Ni(111) [panels from (g) to (i)] systems. In panels (a), (c) and (g) the projection onto Fe-d orbitals of different symmetries [b1g/dx2-y2 (black), eg/dxz+dyz (red), a1g/dz2 (orange) and b2g/dxy (green)] is given. Similarly in panels (b), (d) and (h) the projection onto N-sp (turquoise) and C-sp (magenta) is supplied. Panel (f) depict the sp-projected DOS onto the two different (by symmetry) C atoms of the graphene layer [top (dark brown) and fcc (light brown)] for the FePc/Graphene/Ni(111) system, while panels (e) and (i) shows the d-projected DOS of the Ni top-most metal layer [dxy+dx2-y2 (black), dxz+dyz (red) and dz2 (orange)] in presence and absence of the graphene layer, respectively. Figure 5. Three-dimensional contour plots of the spin density distribution at the FePc/Graphene/Ni(111) interface for top-fcc (a) and (b) and top-bridge (c) and (d) stacking; (a)


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and (c) are for the parallel, (b) and (d) for the antiparallel coupling between the Fe and Ni spin moments. Light green (light blue) color refers to an excess of spin up/majority (down/minority) electrons. An isovalue of ±0.0003 electrons has been considered. ASSOCIATED CONTENT Supporting Information Available: Details on graphene preparation and LEED and XPS characterization; Details on density-functional calculations, including FePc adsorption sites, Graphene/Ni(111) stackings and Spin-density of FePc on Ni(111). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Andrea Candini Centro S3 Modena, Istituto Nanoscienze – CNR Via Campi 213/a, 41125 Modena, Italy E-mail: [email protected]

ACKNOWLEDGMENT This work has been supported by FIR grant RBFR13YKWX and PRIN grant 20105ZZTSE “GRAF” of the Italian Ministry for Research (MIUR) and by EU ICT-FET Proactive “MoQuaS” project N. 610449. We also acknowledge the European Synchrotron Radiation Facility (Project HE 3739) and we would like to thank the beamline staff for assistance in using beamline ID08. We acknowledge the CINECA center for granting the high-performance computing resources. We thank M. Schleberger for fruitful discussion and for providing the Ni substrates.


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REFERENCES 1

Bogani, L.; Wernsdorfer, W. Molecular Spintronics Using Single-Molecule Magnets. Nat.

Mater. 2008, 7, 179-186. 2

Dediu, V. A.; Hueso, L. E.; Taliani C. Spin Routes in Organic Semiconductors. Nat. Mater.

2009, 8, 707-716. 3

Scheybal, A.; Ramsvik, T.; Bertschinger, R.; Putero, M.; Nolting, F.; Jung, T. A. Induced

Magnetic Ordering in a Molecular Monolayer. Chem. Phys. Lett. 2005, 411, 214-220. 4

Wende, H.; Bernien, M.; Luo, J.; Sorg, C.; Ponpandian, N.; Kurde, J.; Miguel, J.; Piantek, M.;

Xu, X.; Eckhold, Ph.; Kuch, W.; Baberschke, K.; Panchmatia, P. M.; Sanyal, B.; Oppeneer, P. M.; Eriksson, O. Substrate-Induced Magnetic Ordering and Switching of Iron Porphyrin Molecules. Nat. Mater. 2007, 6, 516-520. 5

Bernien, M.; Miguel, J.; Weis, C.; Ali, Md. E.; Kurde, J.; Krumme, B.; Panchmatia, P. M.;

Sanyal, B.; Piantek, M.; Srivastava, P.; Baberschke, K.; Oppeneer, P. M.; Eriksson, O.; Kuch, W.; Wende, H. Tailoring the Nature of Magnetic Coupling of Fe-Porphyrin Molecules to Ferromagnetic Substrates. Phys. Rev. Lett. 2009, 102, 047202. 6

Javaid, S.; Bowen, M.; Boukari, S.; Joly, L.; Beaufrand, J.-B.; Chen, X.; Dappe, Y. J.;

Scheurer, F.; Kappler, J.-P.; Arabski, J.; Wulfhekel, W.; Alouani, M.; Beaurepaire, E. Impact on Interface Spin Polarization of Molecular Bonding to Metallic Surfaces. Phys. Rev. Lett. 2010, 105, 077201.


20

Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7

Miguel, J.; Hermanns, C. F.; Bernien, M.; Krüger, A.; Kuch, W. Reversible Manipulation of

the Magnetic Coupling of Single Molecular Spins in Fe-Porphyrins to a Ferromagnetic Substrate. J. Phys. Chem. Lett. 2011, 2, 1455. 8

Bhandary, S.; Brena, B.; Panchmatia, P. M.; Brumboiu, I.; Bernien, M.; Weis, C.; Krumme,

B.; Etz, C.; Kuch, W.; Wende, H.; Eriksson, O.; Sanyal, B. Manipulation of Spin State of Iron Porphyrin by Chemisorption on Magnetic Substrates. Phys. Rev. B 2013, 88, 024401. 9

Lodi Rizzini, A.; Krull, C.; Balashov, T.; Kavich, J. J.; Mugarza, A.; Miedema, P. S.; Thakur,

P. K.; Sessi, V.; Klyatskaya, S.; Ruben, M.; Stepanow, S; Gambardella, P. Coupling Single Molecule Magnets to Ferromagnetic Substrates. Phys. Rev. Lett. 2011, 107, 177205. 10

D. Klar, S. Klyatskaya, A. Candini, B. Krumme, K. Kummer, P. Ohresser, V. Corradini, V.

de Renzi, R. Biagi, L. Joly, J.-P. Kappler, U. del Pennino, M. Affronte, H. Wende, M. Ruben, Antiferromagnetic Coupling of TbPc2 Molecules to Ultrathin Ni and Co Films. Beilstein J. Nanotechnol. 2013, 4, 320. 11

Klar, D.; Brena, B.; Herper, H. C.; Bhandary, S.; Weis, C.; Krumme, B.; Schmitz-Antoniak,

C.; Sanyal, B.; Eriksson, O.; Wende, H. Oxygen-Tuned Magnetic Coupling of FePhthalocyanine Molecules to Ferromagnetic Co Films. Phys. Rev. B 2013, 88, 224424. 12

Mao, J.; Zhang, H.; Jiang, Y.; Pan, Y.; Gao, M.; Xiao, W.; Gao, H.-J. Tunability of

Supramolecular Kagome Lattices of Magnetic Phthalocyanines Using Graphene-Based Moiré Patterns as Templates. J. Am. Chem. Soc. 2009, 131, 14136-14137. 13

Bhandary, S.; Ghosh, S.; Herper, H.; Wende, H.; Eriksson, O.; Sanyal, B. Graphene as a

Reversible Spin Manipulator of Molecular Magnets. Phys. Rev. Lett. 2011, 107, 257202.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

14

Page 22 of 26

Candini, A.; Klyatskaya, S.; Ruben, M.; Wernsdorfer, W.; Affronte, M. Graphene Spintronic

Devices with Molecular Nanomagnets. Nano Lett. 2011, 11, 2634-2639. 15

Yang, K.; Xiao, W. D.; Jiang, Y. H.; Zhang, H. G.; Liu, L. W.; Mao, J. H.; Zhou, H. T.; Du,

S. X.; Gao, H.-J. Molecule–Substrate Coupling between Metal Phthalocyanines and Epitaxial Graphene Grown on Ru(0001) and Pt(111). J. Phys. Chem. C 2012, 116, 14052-14056. 16

Hämäläinen, S.K.; Stepanova, M.; Drost, R.; Liljeroth, P.; Lahtinen, J.; Sainio, J. Self-

Assembly of Cobalt-Phthalocyanine Molecules on Epitaxial Graphene on Ir(111). J. Phys. Chem. C 2012, 116, 20433-20437. 17

Dou, W.; Yang, Q.; Lee, C.-S. Anisotropic Film Growth of Iron-Phthalocyanine on

Graphene on a Ni(111) Substrate: Roles of Molecule-Substrate and Intermolecular Interaction. Appl. Phys. Lett. 2013, 102, 131606. 18

Uihlein, J.; Peisert, H.; Glaser, M.; Polek, M. ; Adler, H. ; Petraki, F.; Ovsyannikov, R.;

Bauer, M.; Chassé, T. Communication: Influence of Graphene Interlayers on the Interaction Between Cobalt Phthalocyanine and Ni(111). J. Chem. Phys. 2013, 138, 081101. 19

Scardamaglia, M.; Lisi, S.; Lizzit, S.; Baraldi, A.; Larciprete, R.; Mariani, C.; Betti, M. G.

Graphene-Induced Substrate Decoupling and Ideal Doping of a Self-Assembled IronPhthalocyanine Single Layer. J. Phys. Chem. C 2013, 117, 3019-3027. 20

Hermanns, C. F.; Tarafder, K.; Bernien, M.; Krüger, A.; Chang, Y.-M.; Oppeneer, P. M.;

Kuch, W. Magnetic Coupling of Porphyrin Molecules Through Graphene. Adv. Mater. 2013, 25, 3473-3477.


22

Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21

Hermanns, C. F.; Bernien, M.; Krüger, A.; Walter, W.; Chang, Y.-M.; Weschke, E.; Kuch,

W. Huge Magnetically Coupled Orbital Moments of Co Porphyrin Molecules and Their Control by CO Adsorption. Phys. Rev. B 2013, 88, 104420. 22

Bhandary,

S.;

Eriksson,

O.;

Sanyal,

B.

Defect

Controlled

Magnetism

in

FeP/Graphene/Ni(111). Sci. Rep. 2013, 3, 3405. 23

Klar, D.; Bhandary, S.; Candini, A.; Joly, L.; Ohresser, P.; Klyatskaya, S.; Schleberger, M.;

Ruben, M.; Affronte, M.; Eriksson, O.; Sanyal, B.; Wende, H. Field-Regulated Switching of the Magnetization of Co-Porphyrin on Graphene. Phys. Rev. B. 2014, 89, 144411. 24

Marocchi, S.; Ferriani, P.; Caffrey, N. M.; Manghi, F.; Heinze, S.; Bellini, V. Graphene-

Mediated Exchange Coupling Between a Molecular Spin and Magnetic Substrates. Phys. Rev. B. 2013, 88, 144407. 25

Dedkov, Y. S.; Fonin, M. Electronic and Magnetic Properties of the Graphene–Ferromagnet

Interface. New Journal of Physics 2010, 12, 125004. 26

Grüneis, A.; Kummer, K.; Vyalikh, D. Dynamics of Graphene Growth on a Metal Surface: a

Time-Dependent Photoemission Study. New Journal of Physics 2009, 11, 073050. 27

Dedkov, Y. S.; Fonin, M.; Laubschat, C. A Possible Source of Spin-Polarized Electrons: The

Inert Graphene/Ni(111) System. Appl. Phys. Lett. 2008, 92, 052506. 28

Walter, A. L.; Nie, S.; Bostwick, A.; Kim, K. S.; Moreschini, L.; Chang, Y. J.; Innocenti, D.;

Horn, K.; McCarty, K. F.; Rotenberg, E. Electronic Structure of Graphene on Single-Crystal Copper Substrates. Phys. Rev. B 2011, 84, 195443.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

29

Page 24 of 26

Kresse, G.; Furthmueller, J. Efficient Iterative Schemes for ab initio Total-Energy

Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169. 30

Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple.

Phys. Rev. Lett. 1996, 77, 3865. 31

Grimme, S. Semiempirical GGA-Type Density Functional Constructed With a Long-Range

Dispersion Correction. J. Comput. Chem. 2006, 27, 1787-1799. 32

Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Electron-

Energy-Loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+U Study. Phys. Rev. B 1998, 57, 1505. 33

Panchmatia, P. M.; Sanyal, B.; Oppeneer, P. M. GGA + U Modeling of Structural,

Electronic, and Magnetic Properties of Iron Porphyrin-Type Molecules. Chem. Phys. 2008, 343, 47-60. 34

Annese, E.; Fujii, J.; Vobornik, I.; Panaccione, G.; Rossi, G. Control of the Magnetism of

Cobalt Phthalocyanine by a Ferromagnetic Substrate. Phys. Rev. B 2011, 84, 174443. 35

Annese, E.; Casolari, F.; Fujii, J.; Rossi, G. Interface Magnetic Coupling of Fe-

Phthalocyanine Layers on a Ferromagnetic Surface. Phys. Rev. B 2013, 87, 054420. 36

Gargiani, P.; Rossi, G.; Biagi, R.; Corradini, V.; Pedio, M.; Fortuna, S.; Calzolari, A.;

Fabris, S.; Criginski Cezar, J.; Brookes, N. B.; Betti, M. G. Spin and Orbital Configuration of Metal Phthalocyanine Chains Assembled on the Au(110) Surface. Phys. Rev. B 2013, 87, 165407.


24

Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


37

Zhao, W.; Kozlov, S. M.; Hoefert, O.; Gotterbarm, K.; Lorenz, M. P. A.; Vines, F.; Papp,

Ch.; Goerling, A.; Steinrueck, H.-P. Graphene on Ni(111): Coexistence of Different Surface Structures. J. Phys. Chem. Lett. 2011, 2, 759-764. 38

Bianchini, F.; Patera, L. L.; Peressi, M.; Africh, C.; Comelli, G. Atomic Scale Identification

of Coexisting Graphene Structures on Ni(111). J. Phys. Chem. Lett. 2014, 5, 467-473. 39

Hu, J.; Wu, R. Control of the Magnetism and Magnetic Anisotropy of a Single-Molecule

Magnet with an Electric Field. Phys. Rev. Lett. 2013, 110, 097202. 40

Marom, N.; Kronik, L. Density Functional Theory of Transition Metal Phthalocyanines, II:

Electronic Structure of MnPc and FePc—Symmetry and Symmetry Breaking. Appl. Phys. A 2009, 95, 165. 41

Mugarza, A.; Robles, R.; Krull, C.; Korytár, R.; Lorente, N.; Gambardella, P. Electronic and

Magnetic Properties of Molecule-Metal Interfaces: Transition-Metal Phthalocyanines Adsorbed on Ag(100). Phys. Rev. B 2012, 85, 155437. 42

Karpan, V. M.; Giovannetti, G.; Khomyakov, P. A.; Talanana, M.; Starikov, A. A.;

Zwierzycki, M.; van den Brink, J.; Brocks, G.; Kelly, P. J. Graphite and Graphene as Perfect Spin Filters. Phys. Rev. Lett. 2007, 99, 176602.


25


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Ferromagnetic Exchange Coupling between Fe Phthalocyanine and

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