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Induced magnetic ordering in a molecular monolayer
Chemical Physics Letters 411 (2005) 214–220 www.elsevier.com/locate/cplett
Induced magnetic ordering in a molecular monolayer A. Scheybal a, T. Ramsvik
a,b,*
, R. Bertschinger a, M. Putero
a,c
, F. Nolting b, T.A. Jung
a
a
c
Laboratory for Micro- and Nanostructures, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland b Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland L2MP-CNRS UMR6137, Faculte´ des Sciences de St. Je´roˆme, Case 142, 13397 Marseille Cedex 20, France Received 18 October 2004; in final form 2 June 2005 Available online 1 July 2005
Abstract Chemically tunable molecular magnets exhibit new and attractive properties, particularly in the field of spintronics. In this work, the magnetic interaction between a magnetized thin film cobalt substrate and adsorbed manganese(III)–tetraphenylporphyrin chloride (MnTPPCl) molecules has been studied by X-ray magnetic circular dichroism (XMCD). For MnTPPCl submonolayer coverages circular dichroism is observed at the Mn LIII,II-edges. From temperature dependent studies and element-specific hysteresis curves, it is concluded that a net magnetization is induced on the complexed Mn in the adsorbed molecule. To our knowledge, this is the first clear evidence that exchange coupling between a large organic adsorbate and a ferromagnetic substrate are observable by XMCD. 2005 Elsevier B.V. All rights reserved.
Surface and interface magnetism quickly evolved from basic research [1] into applications like giant magnetoresistance (GMR) read-heads and magnetic random-access memory [2]. These applications rely on so-called spin transport electronics or ÔspintronicsÕ, where the electron spin rather than the charge is employed to carry information [3]. Novel organic and semiconducting materials are suitable spintronic materials, e.g., Xiong et al. [4] showed that an organic semiconductor like tris(8-hydroxyquinoline) aluminum (Alq3) can be used to prepare an organic spin-valve exhibiting giant magnetoresistance. To advance towards versatile magnetic materials with decreasing domain and layer dimensions, the understanding of local magnetic coupling in molecular monolayers is of utmost importance. One open question is the coupling of electron spin when single molecules interact with magnetic substrates. Recently, spin-polarized metastable deexcitation spectroscopy (SPMDS) experiments *
Corresponding author. Fax: +41 22 76 78380. E-mail address: [email protected] (T. Ramsvik).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.06.017
on one monolayer thick Cu–phthalocyanine [5] and pentacene [6] films on magnetized Fe(1 0 0) showed a significant spin-asymmetry. Due to its chemical selectivity [7] and submonolayer sensitivity [8, this Letter], X-ray magnetic circular dichroism (XMCD) allows for the quantitative differentiation of surface and ad-layer magnetization as long as different magnetic species are chosen [9]. Using XMCD at the O K-edge, magnetic coupling between a non-magnetic ad-layer and a magnetic substrate was recently demonstrated for the the CO – Ni/Co system [10] where the angular momentum of the degenerate CO 2p* orbitals arises from the hybridization with the Ni/Co 3dxz/ 3dyz orbitals. In the work presented here manganese(III)–tetraphenylporphyrin chloride (MnTPPCl) was chosen. For this molecule the degree of coupling of the high-spin Mn 3d4 to molecular and substrate electronic states can be tuned through the variation of chemical substituents [11], i.e., through variation of the size and type of the molecular orbitals active in the hybridization with the electronic states of the substrate. MnTPPCl is also
A. Scheybal et al. / Chemical Physics Letters 411 (2005) 214–220
the parent compound of the [Mn(III)–porphyrin][TCNE] family of molecular magnets (TCNE = tetracyanoethylene) [12]. High-spin MnTPPCl with S = 2 in the ground state belongs to a class of porphyrins with four phenyl-based substituents at the mesoposition of the porphyrin macrocycle (Fig. 1a). A manganese atom is located in the center, surrounded by four basal pyrrole nitrogen atoms and an axial chloride ion [13]. Our work reports about Mn XMCD experiments to detect the molecular magnetic coupling to a ferromagnetic substrate layer. The measurements were performed at the Surfaces and Interfaces Microscopy (SIM) beamline [14] at the Swiss Light Source (SLS). An XMCD endstation, containing a cryogenic manipulator [15] and a bipolar electromagnet has been attached to the SIM beamline providing high brilliance X-ray light in the energy range 130–2000 eV from two elliptical twin undulators. The polarization is user selectable, with a close to 100% efficiency while using first order light. Total electron yield X-ray absorption spectra and hysteresis loops have been
215
obtained by recording the sample drain current as function of X-ray energy [16] using left and right circular polarized light. An equivalent method to measure XMCD spectra is to reverse the external magnetic field direction while keeping the polarization fixed [17]. Element-specific hysteresis loops [18] have been acquired by measuring the sample drain current as a function of the magnetic field with the X-ray photon energy tuned to the LIII and LII resonances and by switching the helicity of the light. Epitaxially oriented Au(1 1 1) films of 100 nm thickness have been deposited Ôex situÕ on mica at 613 K, with subsequent annealing to 813 K [19], and carbon cleaning by UV-ozone treatment [20]. ÔIn situÕ ultra high vacuum (UHV) cobalt and MnTPPCl evaporators as well as a dedicated shutter and masking system allowed for the subsequent preparation of up to four sample spots without breaking the vacuum. At all stages of the experiments and the preparation, oxygen K-edge spectra have been recorded to monitor the purity of the sample. Following a recent study about this growth system [19],
Fig. 1. Mn–porphyrin on a cobalt layer: sample preparation and initial characterization. (a) Chemical structure of the MnTPPCl molecule. A manganese atom is located in the center, surrounded by four basal pyrrole nitrogen atoms and an axial chloride ion [13]. The manganese atom is in a III+ high spin oxidation state with S = 2. (b) Three-layer sample generated by controlled hetero-epitaxial growth. In the experimental setup l+ and l symbolize right and left circular polarized light, respectively. Happlied denotes the in-plane orientation of the applied magnetic field. (c and d) Mn LIII,II- and N K-edge NEXAFS spectra, respectively, have been used to verify the stoichiometry and oxidation state of the MnTPPCl after deposition of 1– 6.5 ML onto the cobalt film. In both spectra the MnTPPCl coverage is 1 ML. The inset figure in (d) shows the corresponding N K-edge spectrum for multilayer (bulk-like) MnTPPCl. For all spectra the stepheights are scaled to unity. The incoming light was linear polarized with its major component perpendicular to the surface plane.
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A. Scheybal et al. / Chemical Physics Letters 411 (2005) 214–220
10 nm thick Co thin films have then been deposited Ôin situÕ onto the Au(1 1 1) substrate held at room temperature. Application of an external magnetic field of 125 mT parallel to the surface plane assured singledomain magnetization along the easy axis. Finally, sub- and multilayer coverages ranging from 0.15 to 6 ML MnTPPCl have been sublimed onto the cobalt film (Fig. 1b). A quartz-crystal microbalance (QCMB) was used to control the deposition rate and exposure. In one experimental run, a continuous MnTPPCl deposition was monitored by simultaneous NEXAFS to provide the calibration between the microbalance reading and the layer thickness obtained by integration of the Mn LIII,II peaks. To characterize the prepared multilayer sample consisting of MnTPPCl/Co/Au(1 1 1) layers on mica, NEXAFS Mn LIII,II- and N K-edge spectra are shown in Fig. 1c,d for 1 ML MnTPPCl. The Mn LIII,II NEXAFS spectrum (Fig. 1c) of a submonolayer of MnTPPCl is quite different from the corresponding six-monolayerspectrum (Fig. 4a). Comparison with [21,35] indicates that the molecules in direct contact with the cobalt substrate are in a MnII+ high-spin state. The dominant antibonding orbitals in the rather complex N K-edge spectrum of a multilayer of MnTPPCl (Fig. 1d) is persistent also in single monolayer films, confirming that the porphyrin macrocycle is intact after molecule deposition. This is confirmed by STM studies where submolecular resolution has been achieved for tetraphenylporphyrins [22,23]. X-ray photoemission spectroscopy (XPS) showed only a small shift of the C 1s binding energy between 1 and 8 ML MnTPPCl on cobalt (<0.4 eV) with no detectable change in the peak width, supporting the assumption of intact phenyl rings. The change in the oxidation state is tentatively assigned to the reaction
Absorption spectra at the cobalt and at the manganese LIII,II-edges were measured at room temperature and at 15 K for normal and grazing incidence of the X-ray beam. Fig. 2 shows the room temperature measurement recorded at ±17 from the surface plane. A clear XMCD is observed at both the Mn and Co LIII,II-edges, which is reversed with opposite grazing incidence with respect to the spin moments in the cobalt film. No XMCD was detected for both elements at normal incidence in remanence. Furthermore, angle dependence studies demonstrate a spin orientation parallel to the surface plane for both elements. This provides a key result: un-ambiguous evidence for magnetic ordering of the manganese ion within the MnTPPCl molecules. The equal signs of Co and Mn XMCD signals show that the two species are ferromagnetically coupled to each other. Cooling the sample down to 15 K [25] did not increase the dichroic signal. It is thus concluded that the magnetic saturation of the manganese ions has been reached at room temperature. This excludes magnetic dipole–dipole interaction as the coupling mechanism for the unreasonably high magnetic fields required to reach paramagnetic saturation at the molecular sites in spite of the elevated (room) temperature. By this evidence, the magnetic coupling between the adsorbed molecules and the cobalt substrate originates from exchange interaction. Within the constraints given by electric dipole transitions, the orbital [26] and spin [27] magnetic moments can be deduced from the experimental NEXAFS spectra using the following relations: R þ nh 4 LIII þLII ðl l Þ dx R morb ¼ ; ð1Þ cos h 3 LIII þLII ðlþ þ l Þ dx
MnTPPCl ! MnTPP þ Clad
mspin ¼
where Clad symbolizes chlorine adsorbed to the cobalt surface. The well known polarization dependence of the p* and r* antibonding states [24] with the orientation of the linear polarized light can be used to determine the orientation of the MnTPPCl molecules on the cobalt substrate [16]. For thick, bulk-like MnTPPCl layers, the r* resonance at 408 eV, oriented parallel to the N–Mn–N plane, is suppressed when the E-field has its major component directed perpendicular to the surface plane, while the p* resonances at 400 eV and 403 eV, oriented perpendicular to the N–Mn–N plane, are enhanced. It is thus clear that the molecules are adsorbed in near co-planar orientation on the substrate. In the monolayer case this effect is enlarged which suggests that in the multilayer the molecules are somewhat tilted. This observation was also made by Suzuki et al. [5] for the system Cu–phthalocyanine on Fe(1 0 0).
where morb and mspin are the orbital and spin magnetic moments in units of lB/atom, respectively, nh is the number of holes of the 3d orbital in the specific transition metal atom, and h is the angle between the incoming light and the surface plane. The LIII and LII indices refer to the integration range. ÆTzæ is the expectation value of the intra-atomic magnetic dipole operator and ÆSz is equal to half of mspin in Hartree atomic units. In these equations the linear polarized spectra, l0(x), has been replaced by [l+(x) + l(x)]/2. ÆTzæ provides a measure of the anisotropy of the field of the spins when the charge cloud is distorted, either by the spin–orbit interaction or by crystal-field effects [28]. The presented spin magnetic moment ms here, can be considered as an effec-
nh cos R h R 6 LIII ðlþ l Þ dx 4 LIII þLII ðlþ l Þ dx R ðlþ þ l Þ dx LIII þLII 1 7hT z i ; ð2Þ 1þ 2hS z i
A. Scheybal et al. / Chemical Physics Letters 411 (2005) 214–220
CD [%]
Isample/I0
a
2.0
µ+
Co LIII,II
b
2.0
µ-
1.5
1.0
0.5
0.5
0.0 30 0 -30
µ-
0.0 30 0 -30
µ+ - µ-
µ+ - µ770 780 790 800 810 Photon Energy [eV]
770 780 790 800 810 Photon Energy [eV]
µ+ µ-
µ+ µ-
hν Isample/I0
c
0.17 Mn L III,II
hν µ+ µ-
0.16
µ+
Co LIII,II
1.5
1.0
217
d
0.17
µ+
Mn LIII,II
µ-
0.16 CD [%]
0.15 10 0 -10
µ+ - µ630
0.15 20 0 -20
640 650 660 670 Photon Energy [eV]
+
-
µ -µ 630
640 650 660 670 Photon Energy [eV]
Fig. 2. Confirmation of magnetic ordering in MnTPPCl at room temperature. The two upper figures show Co LIII,II-edge NEXAFS and XMCD spectra (a and b) and the two lower show Mn LIII,II-edge NEXAFS and XMCD spectra (c and d) for two grazing incidence angles, +17 and 17, measured from the surface plane as indicated in the inset cartoons. Isample and I0 are the measured sample drain current and the incident photon flux, respectively. All the spectra are taken in remanent field. The XMCD signals are presented as percentage of highest resonance peak, relative to preedge. The change in sign for the two geometries verifies the validity of the XMCD signal.
tive moment composed of the sum of the isotropic spin moment mspin and the angle-dependent intra-atomic dipole moment [9]. NEXAFS spectra of 3d transition elements in the first half of the periodic system show a significant overlap of LIII- and LII-edge spectral features (jj-mixing) due to the high degree of 3d-electron-2p-core–hole interactions, thus making the use of spin sum rules doubtful [29]. Since Mn is on the threshold, the validity of the sum rules has been questioned. Correction factors for Mn ranging from 1.0 to 1.5 have been proposed [30,31]. Since deduction of such correction factors are dependent on the exact environment of the elements, uncorrected spin magnetic moments are presented in this Letter. The results should consequently be regarded as lower limits. Element-specific hysteresis curves have been extracted from XMCD spectra recorded as a function of the applied field. Results for Mn and Co for 0.75 ML MnTPPCl on cobalt recorded at room temperature are shown in Fig. 3. The resemblance of the two hysteresis curves is striking, which confirms exchange coupling as the primary source for the induced magnetism in MnTPPCl. At room temperature, the Mn ion within the MnTPPCl complex is fully saturated and coupled to the Co magnetization.
As seen in Eqs. (1) and (2), the number of holes (nh) in the Mn 3d band must be known to acquire correct spin and orbital magnetic moments. This is not straightforward for this particular system since the oxidation state of manganese in MnTPPCl varies depending on its degree of contact with the cobalt substrate (see Fig. 4a). However, it is known that the sum of the intensities under the LIII and LII resonances is proportional to the number of holes [9] when properly normalized to the number of atoms.1 Fig. 4b shows a plot of the ratio between the area under LIII and LII and the stepheight2 (left vertical axis), with an estimate of the number of holes (right vertical axis). It is assumed that the number of holes is close to 5, i.e., oxidation state MnII+, at extreme submonolayer coverage, while in the multilayer case the number corresponding to MnIII+, 6, is approached.3
1
It is here assumed that MnTPPCl keeps it fourfold symmetry (C4) normal to the surface plane also after adsorption. 2 Stepheight is defined as the difference in intensity between post- and preedge after background subtraction. 3 The form of the curve deviates somewhat from the predicated form, i.e., a flat characteristic up to 1 ML. However, this is not surprising taking into consideration the roughness of the prepared sample.
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A. Scheybal et al. / Chemical Physics Letters 411 (2005) 214–220
a
b 0.004
2
Mn
Magnetization [a.u.]
Co 1
0.002
0
0.000
-1
-0.002
-2
-100
-50
0
50
-0.004
100
-100
Magnetic Field [mT]
-50
0
50
100
Magnetic Field [mT]
Mn LIII.II
8
70 60
4
~0.75 ML 0 640 650 660 Photon Energy [eV]
670
40
5.5
30 20
5.0 0
Positive Inc. Angle Negative Inc. Angle
1.2
1
2
3
4
5
6
7
Coverage [ML] d
0.8
0.4
0.4
0.0 -0.4
Positive Inc. Angle Negative Inc. Angle
1.2
0.8 mL [µB]
*
mS [µB]
50
10 630
c
6.0
II
~6 ML
b
III
Intensity [a.u.]
12
Number of holes (nh)
a
(IL + IL ) / Stepheight
Fig. 3. Element-specific hysteresis curves for the cobalt LIII,II-edges (a) and the manganese LIII,II-edges (b) for 0.75 ML MnTPPCl on cobalt recorded at 25 from the surface plane. The coercivity- and saturation-field are found to be identical for the two elements, 7 and 10 mT, respectively. Hence, the magnetic properties of the MnTPPCl molecules mirror those in cobalt substrate. This is expected for the case that exchange interaction is the primary cause of the induced magnetism and further confirms the exchange interaction as the origin for the observed dichroism in MnTPPCl.
0.0 -0.4 -0.8
-0.8
Mn LIII.II
-1.2 0
1
2
3
4
5
6
7
Coverage [ML]
Mn LIII.II
-1.2 0
1
2
3
4
5
6
7
Coverage [ML]
Fig. 4. MnTPPCl coverage dependence. The coverage is here represented in unit monolayers (ML). (a) Mn L-edge NEXAFS spectra of 6 and 0.75 ML MnTPPCl on Co/Au(1 1 1). The spectra are scaled to equal stepheights and offset to improve comparability. (b) Ratio of area and stepheight (left vertical axis) and corresponding number of holes in Mn (right vertical axis) as function of coverage (square symbols). ILIII and ILII symbolize the integrated area under LIII and LII, respectively. The solid line represents a fit of the experimental data. (c and d) Measured spin (left) and orbital (right) moments as function of coverage for opposite incidence angles. ms stands for an effective spin magnetic moment which is the sum of the isotropic spin moment (ms) and the intra-atomic dipole moment [9]. In (c) the estimated error bars generated by the spectral analysis (background and double-step function subtraction, integration limits) are indicated. A significant spin magnetic moment is clearly seen at low coverages, where the sign correctly reflects the opposite coupling of the angular momentum of the photons with the in-plane spin in the manganese atom. The orbital magnetic moment (d) is found to be small and scatters around zero (see text). The error bars are omitted in (d) since systematic errors in this case dominate over statistical errors.
By using the sum rules (Eqs. (1) and (2)) quantitative information of the spin and orbital magnetic moment has been derived in Fig. 4c,d. An increase in the spin magnetic moment is clearly associated with the decreas-
ing monolayer coverage. At 0.15 ML MnTPPCl, the lowest coverage measured in this study, a spin magnetic moment of (1.0 ± 0.3) lB was found. For comparison, to obtain the same magnetization at room temperature by
A. Scheybal et al. / Chemical Physics Letters 411 (2005) 214–220
treating the MnTPPCl molecules as pure paramagnets, an effective field of 60 T must be applied. High-spin MnII+ in MnTPPCl has five unpaired electrons, giving a spin moment of maximum 5.0 lB, i.e., considerably higher than the maximum magnetic moment measured here. It is expected that the exchange coupling is crucially dependent on the molecule– substrate interaction at the particular adsorption site, i.e., the morphology of the cobalt substrate [19]. If, at different sites, the exchange coupling occurs through a particular set of orbitals, the magnetic moment would be reduced. Furthermore the ground state of MnTPPCl is perhaps no longer completely high-spin, but mixed with low-spin states. Preferential, thus stronger adsorption of tetraphenylporphyrins at step and kink sites has been observed on Cu(1 0 0), Au(1 1 0) and Ag(1 1 1) surfaces. The observed, rowlike growth of Co multilayers facilitates different adsorption geometries. The large, 1.3 nm [13] dimension of the MnTPPCl molecule and the planar p-orbitals facilitate co-planar adsorption on metallic substrates and the Mn–Cl moiety may form a chemical bond into the substrate, in some analogy to sub-phthalocyanine [32]. Partial molecular stacking, which is often observed for planar porphyrins, may further reduce the effective magnetic moment. This would also explain the continuous increase of the measured effective spin magnetic moment with coverages below 1 ML, as seen in Fig. 4c. Bulk MnTPPCl is in a spin-only state (S = 2) since the orbital magnetic moment is effectively quenched by the surrounding ligand field [33]. Since the porphyrin macrocycle, and hence the ligand field, stays intact after adsorption it is expected that the orbital moment remains small. Fig. 4d confirms this assumption. For the whole coverage range the orbital magnetic moment scatters around zero at the present accuracy level. This work provides clear evidence for a strong magnetic coupling between MnTPPCl, in a molecular monolayer geometry, and a cobalt substrate. A number of consistent surface magnetic experiments confirm this coupling to originate from exchange interaction. Further new and interesting questions arise: what type of exchange coupling induces the spin magnetic ordering in MnTPPCl? In an unperturbed case, the Mn atom would be 0.35 nm above an atomically flat cobalt substrate. This is too far for direct exchange coupling through orbital overlap between Co 3d and Mn 3d. Thus, future experiments should be performed to address the following scenarios: 1. Superexchange through the chlorine atom. 2. Indirect exchange coupling through delocalized electrons on the phenyl rings. 3. Direct exchange coupling, either by distortion of the phenyl substituents and/or selective adsorption at steps or kinks.
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Due to the change in the oxidation state it is suspected that the chlorine atom is removed upon adsorption. If this is the case, point 1 remains difficult to support. Furthermore, both Mn LIII,II- and N K-edge NEXAFS spectra suggest that the molecules maintain a high degree of electron localization. Therefore, direct exchange coupling seems to be the most probable cause among the three listed. Such coupling has been reported previously between submonolayer Mn and ferromagnetic transition metals [34,30], i.e., where Mn is in direct contact with the substrate. In conclusion, spin magnetic moments of up to (1.0 ± 0.3) lB have been induced on the manganese site in manganese(III)–tetraphenylporphyrin chloride through exchange coupling with a cobalt substrate. To the best of our knowledge, this is the first time that an exchange coupling between a large organic adsorbate and a ferromagnetic substrate has been demonstrated by XMCD.
Acknowledgements Funding by the Swiss National Science Foundation DIV II and IV and the NCCR on Nanoscale Science were of key importance for this work. R. Schelldorfer, J. Rothe and R. Betemps provided expert advice for the engineering of the experimental infrastructure and during the beamtime sessions. P. Morf is gratefully acknowledged for establishing the Au(1 1 1) thin film preparation and providing the samples, L. Kovacs for sample characterization, C. Vanoni and P. Morf are honourably mentioned for their excellent assistance during the beamtime sessions. E. Goering is acknowledged for extended fruitful and interesting discussion about surface magnetism. L. Ramoino, S. Berner and M. von Arx from the Nanolab at the University of Basel are gratefully acknowledged for their kind assistance with the implementation of the molecular deposition procedure and for providing access to their XPS/UPS equipment. R.B., A.S. and T.J. gratefully acknowledge interesting and fruitful discussions with Ph. Aebi, University of Neuchatel. Part of this work was performed at the Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland.
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Induced magnetic ordering in a molecular monolayer A. Scheybal a, T. Ramsvik
a,b,*
, R. Bertschinger a, M. Putero
a,c
, F. Nolting b, T.A. Jung
a
a
c
Laboratory for Micro- and Nanostructures, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland b Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland L2MP-CNRS UMR6137, Faculte´ des Sciences de St. Je´roˆme, Case 142, 13397 Marseille Cedex 20, France Received 18 October 2004; in final form 2 June 2005 Available online 1 July 2005
Abstract Chemically tunable molecular magnets exhibit new and attractive properties, particularly in the field of spintronics. In this work, the magnetic interaction between a magnetized thin film cobalt substrate and adsorbed manganese(III)–tetraphenylporphyrin chloride (MnTPPCl) molecules has been studied by X-ray magnetic circular dichroism (XMCD). For MnTPPCl submonolayer coverages circular dichroism is observed at the Mn LIII,II-edges. From temperature dependent studies and element-specific hysteresis curves, it is concluded that a net magnetization is induced on the complexed Mn in the adsorbed molecule. To our knowledge, this is the first clear evidence that exchange coupling between a large organic adsorbate and a ferromagnetic substrate are observable by XMCD. 2005 Elsevier B.V. All rights reserved.
Surface and interface magnetism quickly evolved from basic research [1] into applications like giant magnetoresistance (GMR) read-heads and magnetic random-access memory [2]. These applications rely on so-called spin transport electronics or ÔspintronicsÕ, where the electron spin rather than the charge is employed to carry information [3]. Novel organic and semiconducting materials are suitable spintronic materials, e.g., Xiong et al. [4] showed that an organic semiconductor like tris(8-hydroxyquinoline) aluminum (Alq3) can be used to prepare an organic spin-valve exhibiting giant magnetoresistance. To advance towards versatile magnetic materials with decreasing domain and layer dimensions, the understanding of local magnetic coupling in molecular monolayers is of utmost importance. One open question is the coupling of electron spin when single molecules interact with magnetic substrates. Recently, spin-polarized metastable deexcitation spectroscopy (SPMDS) experiments *
Corresponding author. Fax: +41 22 76 78380. E-mail address: [email protected] (T. Ramsvik).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.06.017
on one monolayer thick Cu–phthalocyanine [5] and pentacene [6] films on magnetized Fe(1 0 0) showed a significant spin-asymmetry. Due to its chemical selectivity [7] and submonolayer sensitivity [8, this Letter], X-ray magnetic circular dichroism (XMCD) allows for the quantitative differentiation of surface and ad-layer magnetization as long as different magnetic species are chosen [9]. Using XMCD at the O K-edge, magnetic coupling between a non-magnetic ad-layer and a magnetic substrate was recently demonstrated for the the CO – Ni/Co system [10] where the angular momentum of the degenerate CO 2p* orbitals arises from the hybridization with the Ni/Co 3dxz/ 3dyz orbitals. In the work presented here manganese(III)–tetraphenylporphyrin chloride (MnTPPCl) was chosen. For this molecule the degree of coupling of the high-spin Mn 3d4 to molecular and substrate electronic states can be tuned through the variation of chemical substituents [11], i.e., through variation of the size and type of the molecular orbitals active in the hybridization with the electronic states of the substrate. MnTPPCl is also
A. Scheybal et al. / Chemical Physics Letters 411 (2005) 214–220
the parent compound of the [Mn(III)–porphyrin][TCNE] family of molecular magnets (TCNE = tetracyanoethylene) [12]. High-spin MnTPPCl with S = 2 in the ground state belongs to a class of porphyrins with four phenyl-based substituents at the mesoposition of the porphyrin macrocycle (Fig. 1a). A manganese atom is located in the center, surrounded by four basal pyrrole nitrogen atoms and an axial chloride ion [13]. Our work reports about Mn XMCD experiments to detect the molecular magnetic coupling to a ferromagnetic substrate layer. The measurements were performed at the Surfaces and Interfaces Microscopy (SIM) beamline [14] at the Swiss Light Source (SLS). An XMCD endstation, containing a cryogenic manipulator [15] and a bipolar electromagnet has been attached to the SIM beamline providing high brilliance X-ray light in the energy range 130–2000 eV from two elliptical twin undulators. The polarization is user selectable, with a close to 100% efficiency while using first order light. Total electron yield X-ray absorption spectra and hysteresis loops have been
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obtained by recording the sample drain current as function of X-ray energy [16] using left and right circular polarized light. An equivalent method to measure XMCD spectra is to reverse the external magnetic field direction while keeping the polarization fixed [17]. Element-specific hysteresis loops [18] have been acquired by measuring the sample drain current as a function of the magnetic field with the X-ray photon energy tuned to the LIII and LII resonances and by switching the helicity of the light. Epitaxially oriented Au(1 1 1) films of 100 nm thickness have been deposited Ôex situÕ on mica at 613 K, with subsequent annealing to 813 K [19], and carbon cleaning by UV-ozone treatment [20]. ÔIn situÕ ultra high vacuum (UHV) cobalt and MnTPPCl evaporators as well as a dedicated shutter and masking system allowed for the subsequent preparation of up to four sample spots without breaking the vacuum. At all stages of the experiments and the preparation, oxygen K-edge spectra have been recorded to monitor the purity of the sample. Following a recent study about this growth system [19],
Fig. 1. Mn–porphyrin on a cobalt layer: sample preparation and initial characterization. (a) Chemical structure of the MnTPPCl molecule. A manganese atom is located in the center, surrounded by four basal pyrrole nitrogen atoms and an axial chloride ion [13]. The manganese atom is in a III+ high spin oxidation state with S = 2. (b) Three-layer sample generated by controlled hetero-epitaxial growth. In the experimental setup l+ and l symbolize right and left circular polarized light, respectively. Happlied denotes the in-plane orientation of the applied magnetic field. (c and d) Mn LIII,II- and N K-edge NEXAFS spectra, respectively, have been used to verify the stoichiometry and oxidation state of the MnTPPCl after deposition of 1– 6.5 ML onto the cobalt film. In both spectra the MnTPPCl coverage is 1 ML. The inset figure in (d) shows the corresponding N K-edge spectrum for multilayer (bulk-like) MnTPPCl. For all spectra the stepheights are scaled to unity. The incoming light was linear polarized with its major component perpendicular to the surface plane.
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10 nm thick Co thin films have then been deposited Ôin situÕ onto the Au(1 1 1) substrate held at room temperature. Application of an external magnetic field of 125 mT parallel to the surface plane assured singledomain magnetization along the easy axis. Finally, sub- and multilayer coverages ranging from 0.15 to 6 ML MnTPPCl have been sublimed onto the cobalt film (Fig. 1b). A quartz-crystal microbalance (QCMB) was used to control the deposition rate and exposure. In one experimental run, a continuous MnTPPCl deposition was monitored by simultaneous NEXAFS to provide the calibration between the microbalance reading and the layer thickness obtained by integration of the Mn LIII,II peaks. To characterize the prepared multilayer sample consisting of MnTPPCl/Co/Au(1 1 1) layers on mica, NEXAFS Mn LIII,II- and N K-edge spectra are shown in Fig. 1c,d for 1 ML MnTPPCl. The Mn LIII,II NEXAFS spectrum (Fig. 1c) of a submonolayer of MnTPPCl is quite different from the corresponding six-monolayerspectrum (Fig. 4a). Comparison with [21,35] indicates that the molecules in direct contact with the cobalt substrate are in a MnII+ high-spin state. The dominant antibonding orbitals in the rather complex N K-edge spectrum of a multilayer of MnTPPCl (Fig. 1d) is persistent also in single monolayer films, confirming that the porphyrin macrocycle is intact after molecule deposition. This is confirmed by STM studies where submolecular resolution has been achieved for tetraphenylporphyrins [22,23]. X-ray photoemission spectroscopy (XPS) showed only a small shift of the C 1s binding energy between 1 and 8 ML MnTPPCl on cobalt (<0.4 eV) with no detectable change in the peak width, supporting the assumption of intact phenyl rings. The change in the oxidation state is tentatively assigned to the reaction
Absorption spectra at the cobalt and at the manganese LIII,II-edges were measured at room temperature and at 15 K for normal and grazing incidence of the X-ray beam. Fig. 2 shows the room temperature measurement recorded at ±17 from the surface plane. A clear XMCD is observed at both the Mn and Co LIII,II-edges, which is reversed with opposite grazing incidence with respect to the spin moments in the cobalt film. No XMCD was detected for both elements at normal incidence in remanence. Furthermore, angle dependence studies demonstrate a spin orientation parallel to the surface plane for both elements. This provides a key result: un-ambiguous evidence for magnetic ordering of the manganese ion within the MnTPPCl molecules. The equal signs of Co and Mn XMCD signals show that the two species are ferromagnetically coupled to each other. Cooling the sample down to 15 K [25] did not increase the dichroic signal. It is thus concluded that the magnetic saturation of the manganese ions has been reached at room temperature. This excludes magnetic dipole–dipole interaction as the coupling mechanism for the unreasonably high magnetic fields required to reach paramagnetic saturation at the molecular sites in spite of the elevated (room) temperature. By this evidence, the magnetic coupling between the adsorbed molecules and the cobalt substrate originates from exchange interaction. Within the constraints given by electric dipole transitions, the orbital [26] and spin [27] magnetic moments can be deduced from the experimental NEXAFS spectra using the following relations: R þ nh 4 LIII þLII ðl l Þ dx R morb ¼ ; ð1Þ cos h 3 LIII þLII ðlþ þ l Þ dx
MnTPPCl ! MnTPP þ Clad
mspin ¼
where Clad symbolizes chlorine adsorbed to the cobalt surface. The well known polarization dependence of the p* and r* antibonding states [24] with the orientation of the linear polarized light can be used to determine the orientation of the MnTPPCl molecules on the cobalt substrate [16]. For thick, bulk-like MnTPPCl layers, the r* resonance at 408 eV, oriented parallel to the N–Mn–N plane, is suppressed when the E-field has its major component directed perpendicular to the surface plane, while the p* resonances at 400 eV and 403 eV, oriented perpendicular to the N–Mn–N plane, are enhanced. It is thus clear that the molecules are adsorbed in near co-planar orientation on the substrate. In the monolayer case this effect is enlarged which suggests that in the multilayer the molecules are somewhat tilted. This observation was also made by Suzuki et al. [5] for the system Cu–phthalocyanine on Fe(1 0 0).
where morb and mspin are the orbital and spin magnetic moments in units of lB/atom, respectively, nh is the number of holes of the 3d orbital in the specific transition metal atom, and h is the angle between the incoming light and the surface plane. The LIII and LII indices refer to the integration range. ÆTzæ is the expectation value of the intra-atomic magnetic dipole operator and ÆSz is equal to half of mspin in Hartree atomic units. In these equations the linear polarized spectra, l0(x), has been replaced by [l+(x) + l(x)]/2. ÆTzæ provides a measure of the anisotropy of the field of the spins when the charge cloud is distorted, either by the spin–orbit interaction or by crystal-field effects [28]. The presented spin magnetic moment ms here, can be considered as an effec-
nh cos R h R 6 LIII ðlþ l Þ dx 4 LIII þLII ðlþ l Þ dx R ðlþ þ l Þ dx LIII þLII 1 7hT z i ; ð2Þ 1þ 2hS z i
A. Scheybal et al. / Chemical Physics Letters 411 (2005) 214–220
CD [%]
Isample/I0
a
2.0
µ+
Co LIII,II
b
2.0
µ-
1.5
1.0
0.5
0.5
0.0 30 0 -30
µ-
0.0 30 0 -30
µ+ - µ-
µ+ - µ770 780 790 800 810 Photon Energy [eV]
770 780 790 800 810 Photon Energy [eV]
µ+ µ-
µ+ µ-
hν Isample/I0
c
0.17 Mn L III,II
hν µ+ µ-
0.16
µ+
Co LIII,II
1.5
1.0
217
d
0.17
µ+
Mn LIII,II
µ-
0.16 CD [%]
0.15 10 0 -10
µ+ - µ630
0.15 20 0 -20
640 650 660 670 Photon Energy [eV]
+
-
µ -µ 630
640 650 660 670 Photon Energy [eV]
Fig. 2. Confirmation of magnetic ordering in MnTPPCl at room temperature. The two upper figures show Co LIII,II-edge NEXAFS and XMCD spectra (a and b) and the two lower show Mn LIII,II-edge NEXAFS and XMCD spectra (c and d) for two grazing incidence angles, +17 and 17, measured from the surface plane as indicated in the inset cartoons. Isample and I0 are the measured sample drain current and the incident photon flux, respectively. All the spectra are taken in remanent field. The XMCD signals are presented as percentage of highest resonance peak, relative to preedge. The change in sign for the two geometries verifies the validity of the XMCD signal.
tive moment composed of the sum of the isotropic spin moment mspin and the angle-dependent intra-atomic dipole moment [9]. NEXAFS spectra of 3d transition elements in the first half of the periodic system show a significant overlap of LIII- and LII-edge spectral features (jj-mixing) due to the high degree of 3d-electron-2p-core–hole interactions, thus making the use of spin sum rules doubtful [29]. Since Mn is on the threshold, the validity of the sum rules has been questioned. Correction factors for Mn ranging from 1.0 to 1.5 have been proposed [30,31]. Since deduction of such correction factors are dependent on the exact environment of the elements, uncorrected spin magnetic moments are presented in this Letter. The results should consequently be regarded as lower limits. Element-specific hysteresis curves have been extracted from XMCD spectra recorded as a function of the applied field. Results for Mn and Co for 0.75 ML MnTPPCl on cobalt recorded at room temperature are shown in Fig. 3. The resemblance of the two hysteresis curves is striking, which confirms exchange coupling as the primary source for the induced magnetism in MnTPPCl. At room temperature, the Mn ion within the MnTPPCl complex is fully saturated and coupled to the Co magnetization.
As seen in Eqs. (1) and (2), the number of holes (nh) in the Mn 3d band must be known to acquire correct spin and orbital magnetic moments. This is not straightforward for this particular system since the oxidation state of manganese in MnTPPCl varies depending on its degree of contact with the cobalt substrate (see Fig. 4a). However, it is known that the sum of the intensities under the LIII and LII resonances is proportional to the number of holes [9] when properly normalized to the number of atoms.1 Fig. 4b shows a plot of the ratio between the area under LIII and LII and the stepheight2 (left vertical axis), with an estimate of the number of holes (right vertical axis). It is assumed that the number of holes is close to 5, i.e., oxidation state MnII+, at extreme submonolayer coverage, while in the multilayer case the number corresponding to MnIII+, 6, is approached.3
1
It is here assumed that MnTPPCl keeps it fourfold symmetry (C4) normal to the surface plane also after adsorption. 2 Stepheight is defined as the difference in intensity between post- and preedge after background subtraction. 3 The form of the curve deviates somewhat from the predicated form, i.e., a flat characteristic up to 1 ML. However, this is not surprising taking into consideration the roughness of the prepared sample.
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a
b 0.004
2
Mn
Magnetization [a.u.]
Co 1
0.002
0
0.000
-1
-0.002
-2
-100
-50
0
50
-0.004
100
-100
Magnetic Field [mT]
-50
0
50
100
Magnetic Field [mT]
Mn LIII.II
8
70 60
4
~0.75 ML 0 640 650 660 Photon Energy [eV]
670
40
5.5
30 20
5.0 0
Positive Inc. Angle Negative Inc. Angle
1.2
1
2
3
4
5
6
7
Coverage [ML] d
0.8
0.4
0.4
0.0 -0.4
Positive Inc. Angle Negative Inc. Angle
1.2
0.8 mL [µB]
*
mS [µB]
50
10 630
c
6.0
II
~6 ML
b
III
Intensity [a.u.]
12
Number of holes (nh)
a
(IL + IL ) / Stepheight
Fig. 3. Element-specific hysteresis curves for the cobalt LIII,II-edges (a) and the manganese LIII,II-edges (b) for 0.75 ML MnTPPCl on cobalt recorded at 25 from the surface plane. The coercivity- and saturation-field are found to be identical for the two elements, 7 and 10 mT, respectively. Hence, the magnetic properties of the MnTPPCl molecules mirror those in cobalt substrate. This is expected for the case that exchange interaction is the primary cause of the induced magnetism and further confirms the exchange interaction as the origin for the observed dichroism in MnTPPCl.
0.0 -0.4 -0.8
-0.8
Mn LIII.II
-1.2 0
1
2
3
4
5
6
7
Coverage [ML]
Mn LIII.II
-1.2 0
1
2
3
4
5
6
7
Coverage [ML]
Fig. 4. MnTPPCl coverage dependence. The coverage is here represented in unit monolayers (ML). (a) Mn L-edge NEXAFS spectra of 6 and 0.75 ML MnTPPCl on Co/Au(1 1 1). The spectra are scaled to equal stepheights and offset to improve comparability. (b) Ratio of area and stepheight (left vertical axis) and corresponding number of holes in Mn (right vertical axis) as function of coverage (square symbols). ILIII and ILII symbolize the integrated area under LIII and LII, respectively. The solid line represents a fit of the experimental data. (c and d) Measured spin (left) and orbital (right) moments as function of coverage for opposite incidence angles. ms stands for an effective spin magnetic moment which is the sum of the isotropic spin moment (ms) and the intra-atomic dipole moment [9]. In (c) the estimated error bars generated by the spectral analysis (background and double-step function subtraction, integration limits) are indicated. A significant spin magnetic moment is clearly seen at low coverages, where the sign correctly reflects the opposite coupling of the angular momentum of the photons with the in-plane spin in the manganese atom. The orbital magnetic moment (d) is found to be small and scatters around zero (see text). The error bars are omitted in (d) since systematic errors in this case dominate over statistical errors.
By using the sum rules (Eqs. (1) and (2)) quantitative information of the spin and orbital magnetic moment has been derived in Fig. 4c,d. An increase in the spin magnetic moment is clearly associated with the decreas-
ing monolayer coverage. At 0.15 ML MnTPPCl, the lowest coverage measured in this study, a spin magnetic moment of (1.0 ± 0.3) lB was found. For comparison, to obtain the same magnetization at room temperature by
A. Scheybal et al. / Chemical Physics Letters 411 (2005) 214–220
treating the MnTPPCl molecules as pure paramagnets, an effective field of 60 T must be applied. High-spin MnII+ in MnTPPCl has five unpaired electrons, giving a spin moment of maximum 5.0 lB, i.e., considerably higher than the maximum magnetic moment measured here. It is expected that the exchange coupling is crucially dependent on the molecule– substrate interaction at the particular adsorption site, i.e., the morphology of the cobalt substrate [19]. If, at different sites, the exchange coupling occurs through a particular set of orbitals, the magnetic moment would be reduced. Furthermore the ground state of MnTPPCl is perhaps no longer completely high-spin, but mixed with low-spin states. Preferential, thus stronger adsorption of tetraphenylporphyrins at step and kink sites has been observed on Cu(1 0 0), Au(1 1 0) and Ag(1 1 1) surfaces. The observed, rowlike growth of Co multilayers facilitates different adsorption geometries. The large, 1.3 nm [13] dimension of the MnTPPCl molecule and the planar p-orbitals facilitate co-planar adsorption on metallic substrates and the Mn–Cl moiety may form a chemical bond into the substrate, in some analogy to sub-phthalocyanine [32]. Partial molecular stacking, which is often observed for planar porphyrins, may further reduce the effective magnetic moment. This would also explain the continuous increase of the measured effective spin magnetic moment with coverages below 1 ML, as seen in Fig. 4c. Bulk MnTPPCl is in a spin-only state (S = 2) since the orbital magnetic moment is effectively quenched by the surrounding ligand field [33]. Since the porphyrin macrocycle, and hence the ligand field, stays intact after adsorption it is expected that the orbital moment remains small. Fig. 4d confirms this assumption. For the whole coverage range the orbital magnetic moment scatters around zero at the present accuracy level. This work provides clear evidence for a strong magnetic coupling between MnTPPCl, in a molecular monolayer geometry, and a cobalt substrate. A number of consistent surface magnetic experiments confirm this coupling to originate from exchange interaction. Further new and interesting questions arise: what type of exchange coupling induces the spin magnetic ordering in MnTPPCl? In an unperturbed case, the Mn atom would be 0.35 nm above an atomically flat cobalt substrate. This is too far for direct exchange coupling through orbital overlap between Co 3d and Mn 3d. Thus, future experiments should be performed to address the following scenarios: 1. Superexchange through the chlorine atom. 2. Indirect exchange coupling through delocalized electrons on the phenyl rings. 3. Direct exchange coupling, either by distortion of the phenyl substituents and/or selective adsorption at steps or kinks.
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Due to the change in the oxidation state it is suspected that the chlorine atom is removed upon adsorption. If this is the case, point 1 remains difficult to support. Furthermore, both Mn LIII,II- and N K-edge NEXAFS spectra suggest that the molecules maintain a high degree of electron localization. Therefore, direct exchange coupling seems to be the most probable cause among the three listed. Such coupling has been reported previously between submonolayer Mn and ferromagnetic transition metals [34,30], i.e., where Mn is in direct contact with the substrate. In conclusion, spin magnetic moments of up to (1.0 ± 0.3) lB have been induced on the manganese site in manganese(III)–tetraphenylporphyrin chloride through exchange coupling with a cobalt substrate. To the best of our knowledge, this is the first time that an exchange coupling between a large organic adsorbate and a ferromagnetic substrate has been demonstrated by XMCD.
Acknowledgements Funding by the Swiss National Science Foundation DIV II and IV and the NCCR on Nanoscale Science were of key importance for this work. R. Schelldorfer, J. Rothe and R. Betemps provided expert advice for the engineering of the experimental infrastructure and during the beamtime sessions. P. Morf is gratefully acknowledged for establishing the Au(1 1 1) thin film preparation and providing the samples, L. Kovacs for sample characterization, C. Vanoni and P. Morf are honourably mentioned for their excellent assistance during the beamtime sessions. E. Goering is acknowledged for extended fruitful and interesting discussion about surface magnetism. L. Ramoino, S. Berner and M. von Arx from the Nanolab at the University of Basel are gratefully acknowledged for their kind assistance with the implementation of the molecular deposition procedure and for providing access to their XPS/UPS equipment. R.B., A.S. and T.J. gratefully acknowledge interesting and fruitful discussions with Ph. Aebi, University of Neuchatel. Part of this work was performed at the Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland.
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