Self-Assembly and Superexchange Coupling of Magnetic Molecules

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Self-Assembly and Superexchange Coupling of Magnetic Molecules on Oxygen-Reconstructed Ferromagnetic Thin Film )

€ckerlin,† Timur K. Kim,†,§ Kathrin Mu €ller,†, Dorota Chylarecka,† Christian Wa ‡ ‡ ,† Frithjof Nolting, Armin Kleibert, Nirmalya Ballav,* and Thomas A. Jung*,† †

Laboratory for Micro- and Nanotechnology, Paul Scherrer Institute, CH 5232, Switzerland, and ‡Swiss Light Source, Paul Scherrer Institute, CH 5232, Switzerland

ABSTRACT We report on the distinctive molecular assembly and exchange coupling of paramagnetic manganese(III) tetraphenylporphyrin chloride (MnTPPCl) molecules on a metallic cobalt (Co(001)) and on an oxygen-reconstructed cobalt (O/Co(001)) substrate, the latter substrate being prepared by surfactant-mediated growth. For MnTPPCl, a ferromagnetic (FM) exchange coupling to Co(001) and an antiferromagnetic (AFM) exchange coupling to O/Co(001) were identified. The random adsorption of MnTPPCl on Co(001) is turned into a self-assembled and well-ordered 2D molecular domains on O/Co(001). Different oxidation states for Mn ions are found to exist on different substrates. The here presented spectromicroscopy-correlation approach, involving X-ray magnetic circular dichroism (XMCD) spectroscopy, X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM), demonstrates its strength to identify the mechanism involved in organic spintronic interfaces. SECTION Surfaces, Interfaces, Catalysis

O

rganic molecules exhibiting magnetic properties have received considerable research attention since the discovery of their long-range ferromagnetic interaction in 1987.1-3 In the meantime, magnetic molecules have been investigated in different architectures at interfaces, as single molecular adsorbates or adlayer islands or thin films on ferromagnetic substrates,4-8 and also in other cases as self-assembled single molecule magnets on suitable substrates.9-11 All of these experimental approaches not only offer important model systems to study the properties of magnetic molecules and layers at interfaces but also provide an advancement toward molecular spintronics and technological applications.12,13 In a previous study on manganese(III) tetraphenylporphyrin chloride (MnTPPCl, Scheme 1), we reported on the observation of ferromagnetic (FM) coupling between these molecules and a premagnetized metallic cobalt (Co(001)) substrate.4 Subsequently, Wende and coworkers investigated the magnetic coupling on different ferromagnetic substrates (e.g., Co and Ni) for iron(III) octaethylporphyrin chloride (FeOEPCl).5 Recently, these authors discovered an antiferromagnetic (AFM) coupling of FeOEPCl to oxygen-covered Co and Ni substrates,14 whereas the issues of the exact molecular species involved in the coupling as well as the structure of the magnetic molecular layer have not yet been addressed. To draw conclusions on these issues, we discuss the assembly of magnetic MnTPPCl molecules and their FM and AFM coupling to clean and oxygen-covered

r 2010 American Chemical Society

Co substrates in this letter. This is undertaken on the basis of complementary techniques that provide clear evidence for the identification of the chemical structure and bonding as well as for the exchange coupling in a tunable molecular spintronic model interface. MnTPPCl molecules were evaporated in ultrahigh vacuum onto thin films of metallic and oxygen-covered Co on Cu(001) substrates that have been kept in remanence at room temperature. The oxygen-covered Co substrate was obtained by oxygen-surfactant-mediated growth.15 In the following, the two samples terminated either by atomically clean Co or by oxygen-covered Co will be referred to as Co(001) or O/Co(001), respectively. The magnetization of such prepared interfaces of MnTPPCl and Co(001) and O/Co(001) was studied by element specific X-ray magnetic circular dichroism (XMCD)16 measurements at the L-edge for both Co and Mn species separately. The oxidation state of the central metal atom of the molecular adlayers as well as the chemical bonding to the substrate was studied by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). The structure of the substrate and the MnTPPCl adlayer for different systems were studied by scanning tunneling microscopy

Received Date: February 23, 2010 Accepted Date: March 31, 2010 Published on Web Date: April 14, 2010

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DOI: 10.1021/jz100253c |J. Phys. Chem. Lett. 2010, 1, 1408–1413

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(STM). Complementary structural information was obtained from low-energy electron diffraction (LEED) experiments. All complementary measurements have been carried out at room temperature. XMCD spectra recorded for MnTPPCl molecules adsorbed on Co(001) and O/Co(001) are shown in Figure 1. The data show the difference of two X-ray absorption spectra recorded with radiation of opposite circular polarization (circþ and circ-). The relative sign of the XMCD spectra (e.g., at L2 and L3 edges for Mn, Co) reveals information on the magnetic coupling between various spin systems as to FM and AFM.16 In the present study, the relative signs of the Mn and Co XMCD spectra (e.g., at L2 and L3 edges) support a FM coupling mechanism of MnTPPCl on Co(001) and an AFM coupling mechanism on O/Co(001), which was manifested in both magnetization directions of the underlying Co layer. (Only one direction is presented.) Such opposite spin alignment has recently been reported for the first time for FeOEPCl on similar substrates (O/Co(001) and O/Ni(001)).14 Our data show that these observations are not restricted to FeOEPCl but can be expected for similar organic molecules as well. The following relevant coupling mechanisms have been proposed: (i) an indirect FM coupling via N atoms of the molecule

on Co(001) and (ii) a superexchange mechanism via the oxygen atoms for the AFM coupling on O/Co(001) substrate.5,14 Notably, the exact atomic geometry of the substrate and the chemical/physical bonding of the adsorbed magnetic molecules is a crucial determinant for the system-specific coupling mechanisms between molecular spin systems and magnetic substrates, which has not been addressed so far in detail in the previous works. Toward a conclusion on the magnetic coupling mechanisms, this letter focuses on the correlation of adsorbate/ substrate chemical structure and bonding with the magnetic interaction determined from XMCD. The STM data indeed revealed remarkable differences in the step arrangement between the two substrates: On the O/Co(001) (Figure 2a), steps are well aligned with the [100] and [010] directions forming a rectangular pattern, whereas growth of Co(001) (Figure 2d) results in rounded islands and step edges with little or no correlation to the crystallographic directions. The LEED pattern of the O/Co(001) substrate (inset in Figure 2a) shows a c(2 2) oxygen superstructure as reported in the literature.15 Notably in our STM results, also the molecular assembly of MnTPPCl on c(2 2)-O/Co(001) (Figure 2b) was found to differ clearly from the assembly on atomically clean Co(001) substrate (Figure 2e). Whereas MnTPPCl molecules evaporated onto Co(001) were adsorbed in a random fashion, on O/Co(001), self-assembled and well-ordered molecular domains were formed. The observed molecular assembly corresponds well with previous results on the self-assembly of porphyrins on substrates with higher or lower reactivity.17-21 The porphyrin/substrate interaction, which is an important factor influencing the self-assembly as well as the magnetic coupling to the substrate, shall here first be discussed in more general physicochemical terms: It is the balance of molecule/ substrate and intermolecular interactions together with the corrugation of the surface interaction potential that influences the diffusion and subsequently determines the self-assembly at a given thermal energy (kT).17-26 In the specific case of

Scheme 1. MnTPPCl Molecule Used in the Present Study

Figure 1. XMCD spectra acquired on (a,b) Co(001) and (d,e) O/Co(001) substrates after deposition of MnTPPCl molecules; Co and Mn XAS Ledges for opposite helicities of incoming X-rays (red (circþ) and blue (circ-)), and their respective dichroic signals (black). Note that MnXMCD signals at the L3-edge on Co(001) and O/Co(001) are at (b) ∼639.90 and (e) ∼641.20 eV photon energy (marked by dotted black lines) reflecting the presence of Mn2þ and Mn3þ species on the respective substrate. (c,f) Mn L3-edge XAS spectra (gray) for the two reference samples MnO and Mn2O3, respectively.


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Figure 2. STM images: (a,b) O/Co(001) without and with MnTPPCl molecules, respectively. The black arrows in part a represent in-plane crystallographic directions that apply to all STM images; the blue and black arrows in part b indicate two mirror domains also observed in LEED. (d,e) Co(001) without and with MnTPPCl molecules, respectively. (c) Zoom into self-assembled molecules on O/Co(001) and (f) into two types of molecules on Co(001). Insets: LEED pattern taken (a) at 53.2 eV on O/Co(001) and (d) at 53.2 eV on Co(001). Acquisition parameters for the STM images (I, V): (a) 0.26 nA, -1.5 V; (b) 0.08 nA, -3.0 V; (d) 0.05 nA, -1.7 V; and (e) 0.12 nA, -2.3 V.

Figure 3. (a) LEED pattern of ∼1 ML MnTPPCl on O/Co(001) recorded at ∼28.4 eV; circles indicate c(2 2)-O reconstruction; (b) simulated LEED pattern for MnTPPCl on O/Co(001) with respect to O reconstruction; two mirror domains were observed (black and blue arrows). (c) Tentative model of molecular arrangement on O/Co(001) surface derived from LEED simulation (molecular atoms: red = C; blue = Mn; green = Cl; cyan = N; violet = H). (d) Cl2p XPS spectra recorded on Co(001) and O/Co(001) substrates after deposition of ∼1 ML MnTPPCl. Note that Cl2p XPS spectrum is doublet (2p3/2 and 2p1/2) in nature because of a spin-orbit coupling of ∼1.6 eV (see also Supporting Information), and fittings of raw spectral data (black) allowed us to identify only one Cl species (blue) on the O/Co(001) surface and two Cl species (blue and red) on the Co(001) surface.

related to the coulomb potential affecting both molecular orientation and assembly. MnTPPCl also contains an electric dipole (Mnδþ-Clδ ), which by itself is affected by the electronic interaction with the substrate and contributes to a “dipoledipole” interaction in the specific case of the more chemically inert Coδþ-Oδ surface dipole layer, therefore enhancing the formation of self-assembled molecular domains of MnTPPCl on O/Co(001) substrate. The assembly of molecules on O/Co(001) is confirmed by the characteristic LEED pattern shown in Figure 3a. In contrast, no molecular LEED pattern could be observed for 2D arrangement of molecules on Co(001) substrate. To draw conclusions on the recorded LEED pattern, we performed a simulation of the molecular arrangement with respect to O atoms (compare Figure 3a,b,c). Two mirror domains, at the angle of 22° with respect to each other, can be identified on the STM image (Figure 2b), that is, a 2D chirality. The formation of such self-assembled 2D chiral domains of nonchiral MnTPPCl molecules (Figure SI1a of

porphyrin adsorption, stronger interaction has been observed on metallic substrates than on ultrathin insulators,17 the latter substrate possibly providing an analogue to the O/Co(001) surface. Therefore, the disordered arrangement of MnTPPCl on Co(001) provides evidence of a considerable potential diffusion barrier that prevents the self-assembly during and after the deposition process. Specific to MnTPPCl is the broken symmetry caused by the axial Cl-Mn bond, which also introduces an electric dipole. The self-assembly of dipolecontaining molecules with axial symmetry on more or less reactive substrates has been studied for the case of subphthalocyaninato chloride (SubPcCl).23-25 For SubPcCl, the issues of molecular orientation and self-assembly have been identified by a combined STM and XPS study, which revealed the presence of a chemical bond between the molecule and the Ag substrate, which is not observed in the case of the Cu substrate. On vacancy islands in an ionic crystal substrate, the same molecules are oriented toward the polar pockets fitting the negative Cl polarity best.17 These processes have been


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the Supporting Information) is well known in the context of surface molecular assembly; it has been observed for the assembly of nonchiral cobalt(II) tetraphenylporphyrin (CoTPP) on Au(111),26 where a balance of “molecule-molecule” and “molecule-substrate” interactions played the decisive role in determining the final surface configuration.19-22 For molecules with C4 symmetry, for example, MnTPPCl on a substrate with C4 symmetry, two mirror domains of assembly are expected to exist, which is consistently observed in the LEED and STM data for MnTPPCl molecules on the O/Co(001) substrate. The mirror domains are rotated by (11° with respect to the mirror planes, which are in the [110] and [-110] directions. By comparison of the LEED data with the simulated pattern (Figure 3a,b), the molecular lattice with respect to the reported c(2 2)-O structure can be expressed in matrix notation as [2.5, ( 0.5; - 0.5, 2.5], which indicates the relative position of MnTPPCl molecules with respect to the surface O atoms, as depicted in Figure 3c. The mirror domains are marked by black and blue arrows in the STM image (Figure 2b) and in the simulated LEED pattern (Figure 3b). The molecular lattice constant derived from the LEED simulation gives a value of ∼1.27 nm, which is in good agreement with the Mn-Mn spacing measured in STM image: ∼1.20 nm (compare Figure 2c and Figure 3c). One can further note that this commensurate configuration allows every molecule to match identical sites on the surface (most likely on top of an oxygen atom, as discussed below). Notably, we have also observed the self-assembly of MnTPPCl molecules on Ag(111) surface, and the size of the molecular unit cell for MnTPPCl/Ag(111) was found to be ∼1.24 nm, which is close to the size of the unit cell of MnTPPCl on O/Co. (See also Figure SI1b of the Supporting Information.) Interestingly, the size of the identified molecular unit cell in self-assembled domains for similar metalloporphyrin molecules was found to be larger (e.g., for CoTPP/Ag(111): ∼1.40 nm18,26) than that for both MnTPPCl/O/Co(001) or MnTPPCl/Ag(111) systems. One reason for such differences in the unit cells can lie in the molecular arrangement; in particluar, the orientation of the phenyl rings (planar vs upright) and the competition of intermolecular interaction (mostly van der Waals interaction and dipole/dipole interaction) versus molecule/substrate interaction can lead to different packing densities of similar molecules. Furthermore, the typical molecule/substrate interaction as discussed above can also play an important role in the observed high surface molecular packing. Not only does the assembly of molecules look different on the two different substrates but also the appearance of the molecule itself is an important feature here. On the O/Co(001) (Figure 2b), all molecules look identical and appear with a depression in the center (“four-lobe” geometry). In contrast, on Co(001) (Figure 2d), one can clearly distinguish two different molecular appearances: with “four-lobes” and with “five-lobes” (protrusion in the center; see Figure 2f). These two molecular appearances visible in the STM images may result from one of the following possibilities: (i) different orientation of the Cl, either pointing down (toward the substrate) or up (toward vacuum), (ii) removal of Cl in some molecules, either during the evaporation process or (iii) due to


a chemical reaction upon adsorption onto the Co(001) substrate. To rule out molecular decomposition during the evaporation process, we have recorded UV-vis spectra (Figure SI2 of the Supporting Information) for MnTPPCl molecules in dimethylsulfoxide (a) as received from the manufacturer, (b) as residual left in the evaporator after the XMCD measurements, and (c) as separately sublimed. All UV-vis spectra show the same spectral features, which provide significant evidence against molecular decomposition of MnTPPCl during the evaporation. To assess the possibility (iii) we have recorded XAS spectra (Figure 1b,e) and compared them with oxidation states of reference compounds: MnO27 and Mn2O328 powders supported on an indium foil (Figure 1c,f). Interestingly, MnTPPCl (∼ 1 ML) on Co(001) revealed the presence of mostly Mn2þ ions after adsorption on the Co(001) substrate. In contrast, on the O/Co(001) substrate, the majority of Mn ions was found to be in Mn3þ oxidation state. In line with our previously discussed XAS results,4 the change of the oxidation state from 3þ to 2þ, upon adsorption of MnTPPCl molecule onto the Co(001) substrate, can be attributed to the removal of Cl from the molecule. Considering the chemical reactivity of an atomically clean bare metal surface, the probability of such a decomposition reaction occuring on the Co(001) substrate should be much higher than that on the O/Co(001). XPS data indicate the presence of Cl on both substrates, which in a first glance seems to contradict the removal of Cl. Because of the considerable reactivity of atomically clean Co(001), which may play a decisive role in the catalysis of this process, the Cl possibly remains bound to the substrate even after dissociation from the molecule. This interpretation is supported by Cl2p XPS spectra (Figure 3d), which exhibit different binding energies and spectral features on Co(001) and O/Co(001) substrates. In line with the previously reported data on Cl2p XPS binding energy, the principal peak observed on both Co(001) and O/Co(001) substrates at ∼198.5 eV (2p3/2) can be related to Mn-Cl or Co-Cl bonding configuration.29,30 Note that the Co-Cl bonding configuration could also arise from the chemical “polarization”31 effect after adsorption of MnTPPCl molecule on the Co(001) substrate with Cl atom facing toward the substrate. We tentatively assign the additional Cl2p XPS peak observed on Co(001) substrate at ∼200.0 eV (2p3/2) as differently bound Cl species formed after the partial molecular decomposition of MnTPPCl. Because Mn is found to be mostly in the Mn2þ oxidation state (Figure 1b,c) on Co(001), suggesting the removal of Cl from MnTPPCl, it is improbable that the two doublets in the respective Cl2p XPS spectrum correspond to the two different molecular arrangements visible in the STM image (Figure 2f). In our XMCD experiments, MnTPPCl molecules were found to couple ferromagnetically to Co(001) and antiferromagnetically to O/Co(001) substrate (Figure 1). As described above, two different coupling mechanisms are possible: indirect exchange (through N atoms of the molecule) on Co(001) and superexchange (through O atoms) on O/Co(001), respectively, as derived from DFTþU calculations.5,14,32,33 In the following, we discuss the magnetic coupling between the molecule and the substrate, first for the case of the metallic

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coupling to the substrate on both, the magnetic (Co(001)) layer, and the oxygen adlayer (O/Co(001)). This is despite the characteristically different adsorbate-substrate interaction of the different electronic configuration of the central metal ion of the molecules in the studied cases. Moreover, the self-assembly of magnetic molecules on the oxygen-covered ferromagnetic substrate provides a possible route toward uniform spintronic layers. Notably, the spectromicroscopy correlation approach, here involving XMCD, XPS, and STM, demonstrates its strengths to identify the mechanisms involved in organic spintronic interfaces: Spectromicroscopy correlation, in combination with different adatom layer species (S, F, Cl, as known from bulk AFM materials), and magnetic molecules is expected to provide a toolbox toward the engineering of well-defined single spin systems to be addressed individually, for example, by spinpolarized STM.

and then for the oxygen covered substrate. The dominance of Mn2þ in the MnTPPCl adsorbed on the Co(001) substrate indicates a partial decomposition of Mn(III)TPPCl on the surface, that is, by dissociation of Cl. This dissociation correspondingly changes the spin magnetic moment of Mn from 4.90 to 5.92 μB (if Mn-porphyrin is assumed to remain on the surface in the high-spin configuration).14,31 Interestingly, on the metallic substrate, only the Mn(II)TPP fraction (as identified by the [Mn2þ] signal at ∼639.90 eV) contributes to the observed magnetization, which is clearly reflected by the XMCD (Figure 1b dotted line) signal at this photon energy. FM exchange coupling is rather associated with the CoMn bonding configuration than with the Co-Cl-Mn bonding configuration: in the latter case, AFM contributions are expected in the XMCD spectra because of the possibility of superexchange interaction via the Cl atoms. Such a signal is not observed in our experiments. On the oxygen-covered substrate, on the contrary, the predominance of [Mn3þ] signal at ∼641.20 eV (Figure 1e) in the Mn L3 XMCD suggests that the molecules stay intact during sublimation and also upon the adsorption process. The uniform appearance of MnTPPCl on this substrate in STM suggests unidirectional molecular orientation within the regular 2D domains formed by the selfassembly (Figure 2b). The observation of an AFM exchange coupling in this case suggests the following electronic configuration: Coδþ-Oδ -Mnδþ-Clδ . This assignment is supported by the expectation of Cl not effectively binding to oxygen and is also in agreement with the superexchange coupling (Co-O-Mn) mechanism. Furthermore, the appearance of a single Cl 2p3/2 XPS peak at ∼198.5 eV on O/Co(001) suggests that the Cl atoms are not directly bound to O atoms: in that case, one would expect a Cl 2p3/2 XPS peak at g200.0 eV,34 which is not observed in our spectra. Therefore, our XPS and XMCD data on MnTPPCl provide support for the assigned FM (via N atoms of the porphyrin ring) and AFM (via O atoms) exchange coupling mechanisms with the Co(001) and O/ Co(001) substrates, respectively. In conclusion, molecular assembly and magnetic ordering of MnTPPCl molecules have been studied upon deposition on Co(001) and O/Co(001). It was found that MnTPPCl molecules couple ferromagnetically to Co(001) but antiferromagnetically to O/Co(001) substrate. The coupling scheme is therefore similar to recent findings on FeOEPCl molecules. However, our STM observations, in conjunction with the XMCD spectra and XPS data, reveal important new issues: On the Co(001) substrate, molecules appear in two different forms and are randomly distributed across the surface. In contrast, on O/Co(001), molecules appear indistinguishable, which suggests unidirectional orientation in the self-assembled 2D domains. A significant fraction of molecules is characterized by the Mn2þ oxidation state after adsorption on Co(001), whereas Mn3þ is found almost exclusively on the O/Co(001) substrate. Hence, also in combination with UV-vis characterization of the sublimed compound, these data indicate a partial decomposition of MnTPPCl by removal of Cl upon adsorption on the metallic Co surface, whereas on O/Co(001), the uniform appearance and oxidation state provide concerted and hitherto significant evidence against such a molecular decomposition. Surprisingly, the molecules show a magnetic


SUPPORTING INFORMATION AVAILABLE Sample preparations and characterization details. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: thomas. [email protected]; [email protected].

Present Addresses: §

)

Leibniz Institute for Solid State and Materials Research, Dresden, D 01069, Germany. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973.

ACKNOWLEDGMENT Financial support from Swiss National

Science Foundation (SNSF), National Center of Competence in Research (NCCR), and Research Equipment founding (R’Equip), Switzerland is greatly acknowledged. Part of this work has been performed at the Swiss Light Source (SLS), Paul Scherrer Institute (PSI), Villigen, Switzerland. We thank Rolf Schelldorfer (PSI) and Andrea Steger (PSI) for technical support all throughout, Arantxa Fraile-Rodriguez (PSI) for the help during beamtimes at SLS, and Izabela Czekaj (PSI) for providing molecular schemes. We also thank Ernst Meyer (University of Basel) and Peter Oppeneer (Uppsala University) for interesting and helpful discussions.

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