Graphene-Induced Magnetic Anisotropy of a Two-Dimensional Iron

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Letter

Graphene Induced Magnetic Anisotropy of a Two-Dimensional Iron-Phthalocyanine Network Simone Lisi, Pierluigi Gargiani, Mattia Scardamaglia, Nicholas B. Brookes, Violetta Sessi, Carlo Mariani, and Maria Grazia Betti J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b00260 • Publication Date (Web): 13 Apr 2015 Downloaded from http://pubs.acs.org on April 17, 2015

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The Journal of Physical Chemistry Letters

Graphene Induced Magnetic Anisotropy of a Two-Dimensional Iron-Phthalocyanine Network Simone Lisi,† Pierluigi Gargiani,† Mattia Scardamaglia,† Nicholas B. Brookes,‡ Violetta Sessi,‡ Carlo Mariani,† and Maria Grazia Betti∗,† Dipartimento di Fisica, Università di Roma “La Sapienza”, Piazzale A. Moro 5, I-00185 Roma, Italy, and European Synchrotron Radiation Facility - ESRF CS40220, 38043 Grenoble Cedex 9, France E-mail: [email protected]

∗

To whom correspondence should be addressed Dipartimento di Fisica, Università di Roma “La Sapienza”, Piazzale A. Moro 5, I-00185 Roma, Italy ‡ European Synchrotron Radiation Facility - ESRF CS40220, 38043 Grenoble Cedex 9, France †

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molecule 5 or by coupling with the underlying substrate, 7 due to hybridization or mixing between molecular and surface metal states. 8,9 On the other hand, electronic state coupling between the molecule and the metal substrate may be avoided or reduced, while maintaining a suitable support for ordered growth, by using an appropriate buffer layer like graphene. 10 Highly ordered graphene single-sheet can be grown on transition metal surfaces, obtaining a natural moiré superstructure whose corrugation depends on the metal-C interaction, 11–14 which can be used as template for the planar growth of regular molecular arrays. 10,15–17 A prototype class of organometallic molecules is constituted by metal-phthalocyanines (M-C32 H16 N8 , MPc), square-shaped planar molecules with a central metal ion, four pyrrole rings and four benzene rings. 18 MPcs self-assemble on supported graphene, producing ordered planar arrays at the monolayer coverage. 15–17,19 Among magnetic MPcs, FePc presents a spin state S=1 associated to the orbital and spin configuration of the d-states, involving orbitals with both in-plane and out-of-plane orientation with respect to the molecular plane. 20–22 Thus, it is an exemplary metal-organic system to test and tailor its magnetic properties, by lowering the symmetry to a two-dimensional (2D) array after adsorption on graphene. We have recently shown that graphene grown on Ir acts as a buffer layer decoupling a flat-lying single-layer (SL) of FePc from the underlying iridium metal surface, only slightly affecting the electronic states of the molecular layer and inducing a small electron doping of the graphene Dirac cone. 10 Despite this weak effect on the electronic properties, the FePc SL presents a higher adsorption energy on graphene/Ir than on highly-oriented pyrolitic-graphite (HOPG). 23,24 This relatively high adsorption energy ensures the formation of ordered 2D arrays, as recently shown for CoPc on graphene on Ir 17 and FePc on graphene on Ru. 15 In this letter, we present a study of the spin and orbital configuration of a single-layer of FePc adsorbed on graphene/Ir, by means of X-ray magnetic circular dichroism (XMCD) measured at the L2,3 edges of the central Fe ion. The FePc layers grow flat on the gently rippled moiré of Gr/Ir(111), as recently reported by analyzing the linear dichroic signal at the N-K egde, 10 even though a few degree tilt may occur increasing the film thickness. 10,25

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suring the XMCD signal across the L2,3 absorption edges. The XMCD signals for a FePc-SL and FePc- thick film (TF), taken at 0◦ and 70◦ incidence angles between the surface normal and the photon beam direction, respectively, are shown in Figure 1 (raw data reported in the Supporting Information). By comparing the FePc SL dichroic signals 26 with those of the FePc TF, we observe a comparable XMCD magnitude for the SL data taken close to grazing incidence, while a strong reduction of the dichroism is present for the data taken at normal incidence. An estimation of the orbital and spin effective moment per 3d hole number nh for the FePc-SL and FePc-TF on Gr/Ir is reported in Figure 1 (right panel). The values of the orbital f (µL ) and effective spin (µef S ) moments of the FePc SL with respect to the TF are obtained

applying the XMCD sum rules 27,28 to the background subtracted SL and TF spectra. The separation of the L3 and L2 XAS adsorption edge is crucial to properly evaluate the effective spin moment by means of the sum rules. In the case of Fe, a discrepancy of about 10% between the computed expectation value and the sum rules one has been found, 29 thus we attribute a consistent uncertainty to the extrapolated effective spin magnetic moments. f For both FePc TF and SL, the effective spin (µef S ) moment follows the same angular

dependence and comparable amplitudes, ranging from zero (at normal incidence) to a few tenths of µB (at grazing incidence), with the TF presenting a higher value than the SL, probably due the molecular arrangement in the TF. It is worth noting that the effective spin magnetic moment is substantially different from the spin magnetic moment, due to the contribution of the magnetic dipole operator 30 occurring in the sum rules. The similar f behavior of the angular dependence for the µef S , in the SL with respect to the TF, suggests

a negligible difference due to the quadrupolar contributions in the two systems. The orbital moment (µL ), strictly related to the magnetocrystalline anisotropy via the spin-orbit (LS) coupling, 31 is strongly quenched at the normal photon incidence for the SL, while preserving the same value of the TF at grazing incidence. The angular dependence of the magnetic moment of this FePc TF is in agreement with that of FePc TF grown on other substrates,

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with similar planar geometry. 7,32 The FePc TF presents a pronounced anisotropy only in the effective spin component of the magnetic moment, while for the FePc SL on graphene/Ir a huge increase of orbital-associated magnetic anisotropy is observed, such as to bring into light the emerging of a magnetic easy axis parallel to the graphene surface. The ratio between easy and hard axis (for the orbital moment µL ) is estimated to increase from 1.6 measured on the molecular thin film to 5.6 for the FePc SL. In order to unveil the origin of the observed anisotropy and whether it is associated to the proximity of the graphene layer, we performed an XMCD experiment on FePc on Gr/Ir(111) as a function of coverage, at 0◦ and 70◦ photon beam incidence, as reported in Figure 2.

µ70° L

Fe L3 edge

-

e

0.00

µ0°L n

vs

e-

4 SL

x2

0.15

-0.04

-0.08

0.5 SL 1 SL 2 SL 4 SL

0°

70°

0.10

2 SL

H

H

0.5 SL

n

0.05 n

1 SL

-0.12

0.00 706 708 710 712 714 706 708 710 712 714 Photon Energy (eV)

0.5 1 2 4 Thickness (SL)

Figure 2: (Color online) L3 Circularly polarized XMCD data for FePc on Gr/Ir(111), for different coverages; data collected at 0◦ (left panel) and 70◦ (central panel) of impinging photons direction with respect to the surface normal. In the right panel, the orbital magnetic moment difference between the 70◦ and the 0◦ incidence angle configurations as a function of coverage. The magnetic response along the hard magnetization axis (0◦ ) shows an almost identical lineshape for coverages ranging from the sub-SL regime up to 4 SL, as shown by the XMCD signals (Figure 2, left panel). On the other hand, there is a maximum dichroism at grazing 6


µ70° -µ0° per nh (µB) L L

XMCD (arb. units)

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incidence, i.e. along the easy magnetization axis, upon completion of the first FePc SL on Gr/Ir(111), followed by a decrease of the values at higher layer thicknesses (Figure 2, central panel), and eventually the data recover the bulk-like values for FePc at thin-film formation (200 Å). The orbital magnetic moment difference between the 70◦ and the 0◦ incidence angle configurations (Figure 2, right panel) shows how the maximum magnetic anisotropy is achieved when the molecules are in direct contact with the Gr sheet, confirming the emerging of an easy magnetization direction parallel to the substrate. It is worth noting that the XMCD dichroism and the orbital magnetic moment in the 0.5 SL system are slightly lower that the full SL one. These minor differences might be ascribed to preferential adsorption sites at low molecular density. The FePc single-layer on graphene/Ir, albeit does not present any orbital hybridization or state mixing either with graphene or with the underlying metal surface, 10 does show an enhanced magnetic anisotropy with respect to the FePc TF. The multiplet redistribution in the Fe L2,3 edge can play a relevant role in determining the magnetic state, that is the magnetic anisotropy. In order to unravel such contribution, we present in Figure 3 a detail of the L3 near-edge absorption for the FePc SL and TF. Both the SL and TF present the same absorption peak sequence, as discussed in the literature. 8,32,33 In brief, the intense absorption peak at 707.3 eV photon energy (707.2 eV), for the TF (SL), for the 70◦ data, is associated to the a1g single empty state, while the lower energy peak at 706.6 eV and the higher lying structures at 708.9 eV and 709.7 eV mainly attributed to the eg state of dzy and dxz character. 7,32 The 0◦ /70◦ dichroism reflects the flat orientation of the molecules in both phases (SL and TF). The SL data presents a slightly narrower lineshape and a different relative intensity of the a1g -related resonance (in the normal incidence configuration) with respect to the TF, an almost negligible energy shift of the same feature (better seen in the grazing incidence configuration), but not any major difference that could account for a significant change in the states sequence or hybridization, at contrast to the case of FePc/Au(110) where the SL

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presents a strongly different absorption features, in particular a quenching of the a1g peak. 8 Multiplet splitting calculation has been recently performed to simulate the Fe-L3 absorption spectra in FePc revealing consistency with the D4h symmetry, 34 but the differences between the SL and the TF of FePc on Gr/Ir are too subtle to be dealt with, through a parameter optimization procedure. Different causes may contribute in establishing the observed enhancement of the magnetic anisotropy of FePc molecules in contact with graphene: (i) the broken symmetry in the adsorption process of the FePc molecular states; (ii) the re-hybridization of the d-orbitals; (iii) the 3d-electron localization and electron correlation effects. The variation of the magnetic response of FePc takes place in a few layers thickness range, with a maximum found at the completion of the SL. This excludes the magnetic anisotropy as due only to the twodimensional nature of the molecular film, and underlines the dominant role exerted by the presence of the graphene layer in breaking the symmetry. The reduction of symmetry from D4h to D2h , due to the induced level splitting, can produce an eg -a1g level mixing. 33 It is well known that the FePc interaction with Ir underlying graphene is quenched by the presence of the graphene buffer layer, 10 at variance with other metal substrates like Ni, where an interaction of the Fe orbitals with Ni states through the graphene layer has been established, 35 and magnetic coupling of porphyrin and phthalocyanine with Ni uderlying graphene has been recently measured. 36,37 Thus, the presence of graphene/Ir may produce a symmetry breaking slightly influencing the eg -a1g distribution, as actually observed in the associated resonances between 706.5 eV and 709 eV photon energies (Figure 3). The FePc ground state configuration cannot be simply inferred by the mere multiplet XAS sequence due to the almost degenerate low lying energy states contributions, and the symmetry of the ground state has been extensively debated. 38,39 However, a general consensus on the 3 Eg symmetry has been established 8 (a comprehensive discussion can be found in the work by Kuz’min et al. 40 and references therein). Magnetic anisotropy has been observed for Co films intercalated beneath graphene/Ir, 41,42

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and for Co atoms on graphene/Pt, 43 but in these cases there is a strong hybridization and re-distribution of the metal levels due to the interaction of the d-metal states with the πelectrons. All spectral evidences of FePc adsorption on graphene/Ir bring to light a weak interaction, with a minor molecular macrocycle deformation on the moiré template, 10 thus a net charge donation and/or mixing of the graphene-molecular states can be ruled out, excluding the electronic state mixing as the cause of the enhanced anisotropy. We can also exclude a dominant contribution due to the molecular coordination of the FePc molecules in the 2D assembled array, which does not noticeably alter the molecular geometry, as shown by the Fe L2,3 absorption lineshape, at contrast with the heavily altered lineshape measured for FePc/graphene/Ni. 25,37 First-principles calculations have recently shown that variations of the order of a few % of Fe-N bond distance in Fe-porphyrin, drastically alter the spinstate. 44 Although the FePc molecular single-layer basically presents an unaltered geometry with respect to a purely symmetric molecule, we cannot exclude slight modifications, which reflect in the slightly modified L3 lineshape presented in Figure 3. Electron localization effects may play a role. In the L2,3 absorption spectra for the SL FePc with respect to those in the FePc TF, 10 there is a narrowing of the multiplet peaks related to the d-molecular levels, even presenting the same sequence of absorption structures, for the FePc-SL in contact with graphene. The XAS peaks sequence could be addressed by means of of multiplet calculation methods, to infer the ground state and the magnetic properties of the system. This approach has been exploited to address relevant charge transfer and configurational mixing effects in more interacting systems. 34 However, in the present case only slight intensity differences are found, and such slight effects cannot be properly accounted for by a parametric calculation. In fact, not any relevant FePc doping and charge transfer effects take place from the Ir(111) surface, due to the almost free standing nature of the Gr sheet on Ir(111). Furthermore, the presence of graphene on one of the faces of the 2D FePc-SL breaks the strictly D4h symmetry. These tiny geometrical and localization effects do not fundamentally alter the sequence and energy position of the multiplet absorption peaks, but can be

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considered as the source of the enhanced magnetic anisotropy at the SL coverage for the iron-phthalocyanine network on graphene/Ir. Graphene presents the double effect of acting as a decoupling buffer layer and of enhancing the magnetic anisotropy of the magnetic organometallic molecules. New routes can be explored with magnetic layers below graphene for tailoring the spinpolarization transfer and magnetic order at higher temperatures, once an almost free standing and partially decoupling graphene sheet could be preserved.

Experimental Methods X-Ray absorption measurements were carried out at the ID08 beam line of the ESRF in Grenoble. X-ray absorption spectra (XAS) at the Fe L2,3 absorption edges were recorded in the total electron yield mode for different incidence angles (from 0◦ to 70◦ ) between the graphene/Ir(111) surface normal and the photon beam direction, in order to study the magnetic anisotropy. Magnetic fields (B=± 5 T) were applied collinear to the circularly polarized X-Ray incident beam direction, as shown in the inset of Figure 1, and XMCD data were taken at a sample temperature of 8 K. The background correction to the Fe L2,3 absorption spectra and the detail of the intensity analysis to evaluate the dichroic signal are described by Gargiani et al. 7 The graphene/Ir(111) sample was prepared in-situ by repeated cycles of C2 H4 dosing followed by annealing up to 1300 K, ensuring a well long-range ordered graphene layer, whose structural quality has been checked by means of Scanning Tunneling Microscopy and Low Energy Electron Diffraction. FePc molecules have been deposited as detailed in Ref., 10 from submonolayer coverage to a thick film of about 200 Å.

Acknowledgments We thank the ID08 beamline staff at the ESRF synchrotron radiation laboratory in Grenoble. The Sapienza Università di Roma is acknowledged for funding. This work was supported by the MIUR project PRIN "GRAPH", contract N. 20105ZZTSE. 11



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Supporting Information Available In the Supporting Information section, we report the raw XAS data, at the Fe L2,3 absorption edge, used to extrapolate the XMCD signals of Figure 1. This information is available free of charge via the Internet at http://pubs.acs.org/.

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Graphene-Induced Magnetic Anisotropy of a Two-Dimensional Iron

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