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Cluster surface interactions: small Fe clusters driven nonmagnetic on graphite
Chemical Physics Letters 392 (2004) 498–502 www.elsevier.com/locate/cplett
Cluster surface interactions: small Fe clusters driven nonmagnetic on graphite K. Fauth a
a,b,*
, S. Gold b, M. Heßler a, N. Schneider a, G. Sch€ utz
b
Physikalisches Institut, EP IV, Am Hubland, Bayerische Julius-Maximilians-Universit€at W€urzburg, D-97074 W€urzburg, Germany b Max Planck Institut f€ur Metallforschung, Heisenbergstraße 3, D-70569 Stuttgart, Germany Received 6 May 2004; in final form 30 May 2004 Available online 19 June 2004
Abstract We present a study of the changes in the magnetic and electronic properties of small, deposited Fe clusters upon exposure to the graphite surface. The clusters exhibit strong X-ray magnetic circular dichroism (XMCD) at the L3 edge while matrix isolated in a thin Ar film. XMCD and photoemission experiments show that the clusters are driven into a nonmagnetic state by the interaction to graphite. Our results support earlier calculations for adatoms and dimers and extend their validity to larger cluster sizes. They also provide a basis for an understanding of the magnetic properties of carbon encapsulated transition metal particles. Ó 2004 Elsevier B.V. All rights reserved.
In recent years, considerable progress was achieved in the development of diverse novel magnetic materials using magnetic nanoparticles as their building blocks. Magnetic nanoparticles can be found in ferrofluids and recording media, they are used to form magnetoresistive devices, soft magnetic materials as well as high performance permanent magnets [1,2]. The present interest in such materials is due to the novel properties of nanoscale systems and aims at making use of the mesoscopic behavior in specifically designed materials. Due to their high surface to volume ratio, small particles will inevitably undergo modifications in their electronic structure and magnetic properties when incorporated into composite materials. Acquiring detailed knowledge on the interfacial interactions is therefore prerequisite to specific material design. Enormous progress in this direction was achieved in the last years for quasi twodimensional magnetic materials, originally boosted by the discovery of the giant magnetoresistance effect in layered composites of magnetic and nonmagnetic metals [3]. However, due to the complexity of many of the particle composites relevant for applications, progress in
*
Corresponding author. Fax: +499318884921. E-mail address: [email protected] (K. Fauth).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.06.004
the performance of these materials is still mostly achieved on an empirical basis. Transition metal clusters on surfaces may serve as model systems to study such interactions in detail. The deposition from a particle beam under soft landing conditions [4,5] offers precise control over the size distributions generated, down to atom by atom resolution in the limit of very small clusters. Their intrinsic magnetic properties have been studied in the molecular beam for more than a decade now by making use of the Stern Gerlach deflection technique [6–10]. These experiments have revealed that small Fe, Co and Ni clusters possess enhanced magnetizations compared to the respective bulk materials when formed of less than 500–700 atoms. Interestingly, recent magnetic studies on moderately size selected FeN (N 300) clusters deposited on graphite [11] have not fully reproduced these strong magnetization enhancements. Likewise, carbon encapsulation, while being an efficient way to protect transition metal particles from oxidation, generally leads to cluster ensembles displaying reduced magnetizations at small particle sizes [12,13]. In this Letter, we present an X-ray magnetic circular dichroism (XMCD) and photoelectron spectroscopy study of the magnetic and electronic properties of small iron clusters focussing on the changes that occur upon
K. Fauth et al. / Chemical Physics Letters 392 (2004) 498–502
N=20
25
30
35
FeN
cluster mass flow (a. u.)
their exposure to the graphite surface. Our XMCD and complementary photoemission measurements demonstrate that Fe clusters, consisting of 100 atoms and less, are driven into a nonmagnetic state by the substrate interaction. X-ray magnetic circular dichroism (XMCD) at the L2;3 edges of the 3d transition metals has developed into a powerful method to investigate their magnetic properties [14,15]. Making use of the helicity dependence of the resonant absorption cross-section of circular polarized X-rays, it is inherently element and symmetry sensitive and offers a high sensitivity, reaching far down in the submonolayer regime [16,17]. These characteristics make XMCD ideally suited to investigate highly diluted samples such as deposited clusters at low coverages. Fe cluster cations were generated and deposited in situ at low kinetic energies with a portable, UHV compatible laser vaporization cluster source and deposition apparatus the details of which will be published elsewhere [18]. The cluster source is equipped with an ABB-Extrel quadrupole mass filter configurable to produce mass selected cluster ion beams up to 9000 amu. It is attached to a superconducting split coil magnet system which delivers the necessary applied magnetic fields of up to 2 T. The whole experimental setup was fitted to the PM3 monochromator for circular polarized soft X-rays at the BESSY synchrotron facility [19]. For the present XMCD experiments the degree of circular polarization was set to 76% and the spectral resolution to about 0.6 eV. Additionally, photoemission experiments were carried out at the U49 undulator beam line at BESSY-II. In this case, our cluster source was attached to a standard photoelectron spectrometer chamber. For the experiments described in the following, the mass resolving capabilities of the cluster source were used only to ascertain the cluster beam to be free of contaminants (Fig. 1). The cluster samples under investigation in the present study, however, were generated with very moderate mass selection only. This was achieved by operating the quadrupole rods and an octopole assembly further downstream as high and low mass pass filters, respectively. We deliberately admit FeN clusters in the range 7 < N < 100 to land on the substrate, which all show large magnetic moments of about 3 lB per atom in the gas phase [7]. The deposition process is monitored by measuring the cluster ion current on the deposition substrate. By varying the low mass cutoff value, we could evaluate the volume weighted mean cluster size to amount to 65–70 atoms per cluster. The total amount of deposited cluster material is kept below 0.05 monolayers to prevent statistical aggregation during cluster deposition and the subsequent thermal cycle (see below). After deposition, the sample is regularly inspected with respect to the
499
1000
1500
+
2000
cluster mass (amu) Fig. 1. Portion of a Fe cluster cation mass spectrum exiting the ion deposition optics. The bare mass spectrum was weighted with the cluster mass as to represent the relative volume fractions and thus contributions to the absorption and photoemission spectra. Only pure FeN clusters are contained in the molecular beam.
presence of contaminants either by X-ray absorption or by photoemission. In no cases could we detect traces of oxygen, oxidized iron or other unwanted species. The substrate consists of highly oriented pyrolithic graphite (HOPG), cleaved in situ and held within the magnet chamber at low temperature (T 14 K). Prior to cluster deposition, the HOPG substrate is exposed to 10 Langmuirs of argon. The resulting argon film effectively prevents the clusters from fragmentation and penetration towards the HOPG surface upon impact [4,20]. XMCD measurements are then performed by recording the sample drain current (total electron yield, TEY) as a function of X-ray wavelength in the presence of an applied magnetic field, oriented parallel or antiparallel with respect to the light propagation. Photon energy scans are measured at constant photon helicity. Spectra of opposite magnetization directions are obtained by reversing the magnetic field polarity at every wavelength setting. The simultaneous measurement of the electron yield signal from a gold mesh serves as a reference for data normalization. In Fig. 2a we show Fe L3 absorption spectra, measured on clusters embedded in an Ar thin film as described above. The spectra were measured at applied magnetic fields of 0.5 T, which proved sufficient to saturate the cluster samples. Strong magnetic contrast is observed between the spectra of opposite magnetization directions. While defering the detailed magnetic analysis of the Fe L2;3 -XMCD spectra to a separate publication [21], we note that the method of rare gas isolation gives us the unique opportunity of investigating the quasi intrinsic properties of transition metal clusters in a condensed phase. The average magnetic moment per atom, resulting from a sum rule [22,23] analysis are in rather good agreement with those obtained from the Stern Gerlach experiments [7].
(a)
K. Fauth et al. / Chemical Physics Letters 392 (2004) 498–502
6
B ↑↑ kph B ↑↓ kph
absorption (a.u.)
5 4 3 2 1 0 705
710
715
720
photon energy (eV)
(b)
4
B ↑↑ kph B ↑↓ kph
absorption (a.u.)
3
2
1
0
705
710
715
720
photon energy (eV)
Fig. 2. (a) L3 absorption spectra from Ar matrix isolated Fe clusters the applied field being oriented parallel (d) and antiparallel (s) with respect to the light propagation direction. (b) Same spectra as before, but after removal of the Ar layer.
In a second preparative step, the Argon film is gently removed by allowing the sample to warm up to just above 50 K for a short period. The clusters, however, remain quantitatively on the sample [4], albeit now in contact with the graphite substrate. Fig 2b shows the Fe L3 absorption spectra measured under otherwise unchanged conditions. Obviously, the interaction to the graphite surface has caused the XMCD effect to disappear. This observation still holds, when the applied magnetic field is increased to 1.7 T. Another set of XMCD measurements was carried out with the surface normal of the HOPG crystal tilted by 50° with respect to the incoming synchrotron radiation, in order to detect in-plane components of the magnetization [15], if present. In both cases, with and without matrix isolation, the results were identical to those displayed in Fig. 2. These XMCD experiments demonstrate that the interaction to graphite has a dramatic impact on the magnetism of small iron clusters, reducing their net magnetization to zero. Preliminary results indicate, that Ni and Co clusters on HOPG show a similar behavior.
It is desirable to trace this observation back to the changes in the underlying electronic structure of the clusters. To this end, we compare in Fig. 3 the helicity averaged (nonmagnetic) L3 absorption spectra of the iron clusters both, within the Ar matrix and exposed to the graphite substrate. The experimental spectra were normalized with respect to the edge jump before removing the background along the procedure outlined in [14]. The most obvious change in the XAS spectra is a marked shift of the Fe L3 absorption maximum by about 1.2 eV to higher photon energies after removing the Ar film, which goes along with a reduction of the integrated absorption strength by 20%. Both, reduction and shift of spectral weight to higher excitation energies apply to the L2 edge as well. These results are very much in line with recent electronic structure calculations for mono- and diatomic 3d transition metal adsorbates on hexagonal carbon structures [24,25]. According to these calculations, the transition metal 4s-related density of states is strongly shifted upwards in energy due to the repulsive interaction with the carbon p orbitals. As a consequence of this shift, the 4s related states are positioned above the highest occupied molecular orbital, effectively increasing the 3d electron count. In the atomic limit, this results in a 3dnþ2 4s0 configuration for the transition metal adsorbates (i.e. 3d8 in the case of Fe), where it is understood, that hybridization may lead to considerable intermixing [24]. Hybridization was studied by Cini et al. [26] in terms of a model hamiltonian and found to account well for a similar shift of the L edge X-ray absorption in Pd clusters on graphite. In combination, these scenarios qualitatively account well for our observations on the absorption data in Fig. 3. The increased 3d occupation number in turn reduces the number of available final states for the resonant absorption Fe L edge and hence its integrated
4
absorption (a.u.)
500
in Ar on HOPG
3
2
1
0 705
710
715
720
photon energy (eV)
Fig. 3. Comparison of the Fe L3 absorption spectra of clusters with and without rare gas matrix isolation. The spectra were normalized to identical edge jumps before background subtraction.
K. Fauth et al. / Chemical Physics Letters 392 (2004) 498–502
intensity. The unoccupied Fe 3d states hybridize with Cp orbitals to form hybrid resonance states, whose center of gravity is shifted away from the Fermi level [26]. More elaborate electronic structure calculations for deposited iron clusters being unavailable at present, the preceding discussion cannot be raised beyond this purely qualitative level. It is interesting to note, however, that both, adatoms and dimers as well as artificial hexagonal Fe monolayers on Graphite [27] are predicted by theory to possess (ferro-) magnetic ground states. These results are clearly at variance with the present experiments. One could speculate, though, that the interaction with the hexagonal lattice of the graphite substrate might stabilize the hexagonal (hcp) phase of Fe. While known to be effectively paramagnetic, the existence of local magnetic moments in hcp Fe is still a matter of debate [28]. Based on XMCD alone, we cannot draw definite conclusions concerning the magnetic ground state beyond the fact that it does have zero net magnetization. While it seems unlikely that an increased d-band filling would result in antiferromagnetic order [29], we cannot completely rule out the possibility of other, more complex or noncollinear spin structures as alternatives to a purely paramagnetic state. We have addressed this issue by measuring the 3s photoelectron spectrum on Fe clusters exposed to the HOPG surface, prepared in the same way as described above. The presence of finite atomic magnetic moments leads to 3s photoemission spectra displaying multiplet features due to the exchange interaction of the core hole final state with the partially filled 3d shell [30], typical level splittings being of the order of several electron volts. While doublet spectra have been reported for some paramagnetic Fe compounds, singlet 3s spectra are only observed with no local moments present in the initial (ground) state [31]. Since the clusters’ 3s photoelectron spectrum, as shown in Fig. 4, consists of one single line only, we can definitely conclude, that Fe clusters on graphite
501
are driven into a nonmagnetic state by the interaction to the substrate. Quite clearly, the size regime in which magnetic moments in the adsorbed clusters are totally quenched must be limited. Indeed, Edmonds and co-workers [11,32] have found magnetizations comparable to bulk Fe in clusters of 300 atoms and larger, when deposited onto HOPG. We can, however, now understand why these experiments did not reproduce the enhanced magnetizations observed in the gas phase. From our findings, it is very likely, that a magnetically dead layer is formed at the cluster graphite interface. Based on these assumptions, we have performed a preliminary analysis of magnetization data obtained from graphite encapsulated Co nanoparticles [33] and obtain good agreement assuming a two monolayer thick magnetically dead surface layer and a core of bulklike magnetization. Clearly, the formation of magnetically dead interface layers will impose severe limitations to the usefulness of carbon encapsulation in producing strongly magnetized nanoscale particles. In summary, we have presented a detailed in situ analysis of the interaction of small Fe clusters with the graphite surface. While the clusters exhibit strong L2;3 XMCD when matrix isolated in an Ar thin film, the magnetic contrast in X-ray absorption is entirely lost, when the substrate interaction is switched on. Changes in spectral position and line shape of the resonant L3 absorption indicate the reorganization of the clusters’ electronic level system, in agreement with previous predictions for transition metal adatoms and dimers on graphite. 3s – photoelectron spectra show, that the ground state of the exposed Fe clusters is indeed nonmagnetic. Whether this observation is related to the formation of hcp iron clusters on HOPG deserves careful consideration in future experiments. On the basis of our present results and considering other experimental findings on magnetic nanoparticles we are led to believe that the presence of graphite-like carbon is detrimental to nanoscale magnetic materials formed from the late 3d transition metals. It is a pleasure to acknowledge E. Goering and M. Noll for initial assistance with the XMCD experiment, O. Rader and W. Gudat for the permission to use their photoelectron spectrometer and the BESSY staff for their continuous support. We also thank J.-M. Bonard for communicating experimental results prior to publication.
References
Fig. 4. Fe 3s photoelectron spectrum of a cluster sample prepared on HOPG as described in the text. The photon energy is hx ¼ 180 eV. The single peaked spectrum indicates the absence of atomic magnetic moments in the iron clusters.
[1] [2] [3] [4] [5]
R.H. Kodama, J. Magn. Magn. Mater. 200 (1999) 359. G.C. Hadjipanayis, J. Magn. Magn. Mater. 200 (1999) 373. P. Gr€ unberg et al., Phys. Rev. Lett. 57 (1986) 2442. K. Bromann et al., Science 274 (1997) 956. J.T. Lau, A. Achleitner, W. Wurth, Chem. Phys. Lett. 317 (2000) 269.
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[6] D.M. Cox et al., Phys. Rev. B 32 (1985) 7290. [7] I.M.L. Billas, A. Ch^atelain, W.A. deHeer, Science 256 (1994) 1682. [8] S.E. Apsel, J.W. Emmert, J. Deng, L.A. Bloomfield, Phys. Rev. Lett. 76 (1996) 1441. [9] M.B. Knickelbein, Phys. Rev. Lett. 86 (2001) 5255. [10] M.B. Knickelbein, Chem. Phys. Lett. 353 (2002) 221. [11] C. Binns, Surf. Sci. Rep. 44 (2001) 1, and references therein. [12] Y. Saito, J. Ma, J. Nakashima, M. Masuda, Z. Phys. D 40 (1997) 170. [13] J.J. Host et al., J. Appl. Phys. 83 (1998) 793. [14] C.T. Chen et al., Phys. Rev. Lett. 75 (1995) 152. [15] J. St€ ohr, J. Magn. Magn. Mater. 200 (1999) 470. [16] O. Rader et al., Europhys. Lett. 46 (1999) 231. [17] P. Gambardella et al., Phys. Rev. Lett. 88 (2002) 47 202. [18] A similar setup is described in: U. Heiz, F. Vanolli, L. Trento, W.D. Schneider, Rev. Sci. Instrum. 68 (1997) 1986. [19] H. Petersen, M. Willmann, F. Sch€afers, W. Gudat, Nucl. Instrum. Meth. Phys. Res. A 333 (1993) 594. [20] H.-P. Cheng, U. Landman, Science 260 (1993) 1304.
[21] K. Fauth, S. Gold, M. Hessler, G. Sch€ utz (to be published). [22] B.T. Thole, P. Carra, F. Sette, G. van der Laan, Phys. Rev. Lett. 68 (1992) 1943. [23] P. Carra, B.T. Thole, M. Altarelli, X. Wang, PRL 70 (1993) 694. [24] D.M. Duffy, J.A. Blackman, Phys. Rev. B 58 (1998) 7443. [25] R. Pandey, B.K. Rao, P. Jena, J.M. Newsam, Chem. Phys. Lett. 321 (2000) 142. [26] M. Cini et al., Phys. Rev. B 41 (1990) 5685. [27] P. Kr€ uger, A. Rakotomahevitra, J.C. Parlebas, D. Demangeat, Phys. Rev. B 57 (1998) 5276. [28] G. Steinle-Neumann, R. Cohen, L. Stixrude, J. Phys. Condens. Mat. 16 (2004) S1109, and references therein. [29] R.C. O’Handley, Modern Magnetic Materials, Wiley Interscience, New York, 1999. [30] S. H€ ufner, Photoemission Spectroscopy, third edn., Springer, Berlin, Heidelberg, New York, 2003. [31] J.F. van Acker et al., Phys. Rev. B 37 (1988) 6827. [32] K.W. Edmonds et al., Phys. Rev. B 60 (1999) 472. [33] J.-M. Bonard et al., Chem. Phys. Lett. 343 (2001) 251.
Cluster surface interactions: small Fe clusters driven nonmagnetic on graphite K. Fauth a
a,b,*
, S. Gold b, M. Heßler a, N. Schneider a, G. Sch€ utz
b
Physikalisches Institut, EP IV, Am Hubland, Bayerische Julius-Maximilians-Universit€at W€urzburg, D-97074 W€urzburg, Germany b Max Planck Institut f€ur Metallforschung, Heisenbergstraße 3, D-70569 Stuttgart, Germany Received 6 May 2004; in final form 30 May 2004 Available online 19 June 2004
Abstract We present a study of the changes in the magnetic and electronic properties of small, deposited Fe clusters upon exposure to the graphite surface. The clusters exhibit strong X-ray magnetic circular dichroism (XMCD) at the L3 edge while matrix isolated in a thin Ar film. XMCD and photoemission experiments show that the clusters are driven into a nonmagnetic state by the interaction to graphite. Our results support earlier calculations for adatoms and dimers and extend their validity to larger cluster sizes. They also provide a basis for an understanding of the magnetic properties of carbon encapsulated transition metal particles. Ó 2004 Elsevier B.V. All rights reserved.
In recent years, considerable progress was achieved in the development of diverse novel magnetic materials using magnetic nanoparticles as their building blocks. Magnetic nanoparticles can be found in ferrofluids and recording media, they are used to form magnetoresistive devices, soft magnetic materials as well as high performance permanent magnets [1,2]. The present interest in such materials is due to the novel properties of nanoscale systems and aims at making use of the mesoscopic behavior in specifically designed materials. Due to their high surface to volume ratio, small particles will inevitably undergo modifications in their electronic structure and magnetic properties when incorporated into composite materials. Acquiring detailed knowledge on the interfacial interactions is therefore prerequisite to specific material design. Enormous progress in this direction was achieved in the last years for quasi twodimensional magnetic materials, originally boosted by the discovery of the giant magnetoresistance effect in layered composites of magnetic and nonmagnetic metals [3]. However, due to the complexity of many of the particle composites relevant for applications, progress in
*
Corresponding author. Fax: +499318884921. E-mail address: [email protected] (K. Fauth).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.06.004
the performance of these materials is still mostly achieved on an empirical basis. Transition metal clusters on surfaces may serve as model systems to study such interactions in detail. The deposition from a particle beam under soft landing conditions [4,5] offers precise control over the size distributions generated, down to atom by atom resolution in the limit of very small clusters. Their intrinsic magnetic properties have been studied in the molecular beam for more than a decade now by making use of the Stern Gerlach deflection technique [6–10]. These experiments have revealed that small Fe, Co and Ni clusters possess enhanced magnetizations compared to the respective bulk materials when formed of less than 500–700 atoms. Interestingly, recent magnetic studies on moderately size selected FeN (N 300) clusters deposited on graphite [11] have not fully reproduced these strong magnetization enhancements. Likewise, carbon encapsulation, while being an efficient way to protect transition metal particles from oxidation, generally leads to cluster ensembles displaying reduced magnetizations at small particle sizes [12,13]. In this Letter, we present an X-ray magnetic circular dichroism (XMCD) and photoelectron spectroscopy study of the magnetic and electronic properties of small iron clusters focussing on the changes that occur upon
K. Fauth et al. / Chemical Physics Letters 392 (2004) 498–502
N=20
25
30
35
FeN
cluster mass flow (a. u.)
their exposure to the graphite surface. Our XMCD and complementary photoemission measurements demonstrate that Fe clusters, consisting of 100 atoms and less, are driven into a nonmagnetic state by the substrate interaction. X-ray magnetic circular dichroism (XMCD) at the L2;3 edges of the 3d transition metals has developed into a powerful method to investigate their magnetic properties [14,15]. Making use of the helicity dependence of the resonant absorption cross-section of circular polarized X-rays, it is inherently element and symmetry sensitive and offers a high sensitivity, reaching far down in the submonolayer regime [16,17]. These characteristics make XMCD ideally suited to investigate highly diluted samples such as deposited clusters at low coverages. Fe cluster cations were generated and deposited in situ at low kinetic energies with a portable, UHV compatible laser vaporization cluster source and deposition apparatus the details of which will be published elsewhere [18]. The cluster source is equipped with an ABB-Extrel quadrupole mass filter configurable to produce mass selected cluster ion beams up to 9000 amu. It is attached to a superconducting split coil magnet system which delivers the necessary applied magnetic fields of up to 2 T. The whole experimental setup was fitted to the PM3 monochromator for circular polarized soft X-rays at the BESSY synchrotron facility [19]. For the present XMCD experiments the degree of circular polarization was set to 76% and the spectral resolution to about 0.6 eV. Additionally, photoemission experiments were carried out at the U49 undulator beam line at BESSY-II. In this case, our cluster source was attached to a standard photoelectron spectrometer chamber. For the experiments described in the following, the mass resolving capabilities of the cluster source were used only to ascertain the cluster beam to be free of contaminants (Fig. 1). The cluster samples under investigation in the present study, however, were generated with very moderate mass selection only. This was achieved by operating the quadrupole rods and an octopole assembly further downstream as high and low mass pass filters, respectively. We deliberately admit FeN clusters in the range 7 < N < 100 to land on the substrate, which all show large magnetic moments of about 3 lB per atom in the gas phase [7]. The deposition process is monitored by measuring the cluster ion current on the deposition substrate. By varying the low mass cutoff value, we could evaluate the volume weighted mean cluster size to amount to 65–70 atoms per cluster. The total amount of deposited cluster material is kept below 0.05 monolayers to prevent statistical aggregation during cluster deposition and the subsequent thermal cycle (see below). After deposition, the sample is regularly inspected with respect to the
499
1000
1500
+
2000
cluster mass (amu) Fig. 1. Portion of a Fe cluster cation mass spectrum exiting the ion deposition optics. The bare mass spectrum was weighted with the cluster mass as to represent the relative volume fractions and thus contributions to the absorption and photoemission spectra. Only pure FeN clusters are contained in the molecular beam.
presence of contaminants either by X-ray absorption or by photoemission. In no cases could we detect traces of oxygen, oxidized iron or other unwanted species. The substrate consists of highly oriented pyrolithic graphite (HOPG), cleaved in situ and held within the magnet chamber at low temperature (T 14 K). Prior to cluster deposition, the HOPG substrate is exposed to 10 Langmuirs of argon. The resulting argon film effectively prevents the clusters from fragmentation and penetration towards the HOPG surface upon impact [4,20]. XMCD measurements are then performed by recording the sample drain current (total electron yield, TEY) as a function of X-ray wavelength in the presence of an applied magnetic field, oriented parallel or antiparallel with respect to the light propagation. Photon energy scans are measured at constant photon helicity. Spectra of opposite magnetization directions are obtained by reversing the magnetic field polarity at every wavelength setting. The simultaneous measurement of the electron yield signal from a gold mesh serves as a reference for data normalization. In Fig. 2a we show Fe L3 absorption spectra, measured on clusters embedded in an Ar thin film as described above. The spectra were measured at applied magnetic fields of 0.5 T, which proved sufficient to saturate the cluster samples. Strong magnetic contrast is observed between the spectra of opposite magnetization directions. While defering the detailed magnetic analysis of the Fe L2;3 -XMCD spectra to a separate publication [21], we note that the method of rare gas isolation gives us the unique opportunity of investigating the quasi intrinsic properties of transition metal clusters in a condensed phase. The average magnetic moment per atom, resulting from a sum rule [22,23] analysis are in rather good agreement with those obtained from the Stern Gerlach experiments [7].
(a)
K. Fauth et al. / Chemical Physics Letters 392 (2004) 498–502
6
B ↑↑ kph B ↑↓ kph
absorption (a.u.)
5 4 3 2 1 0 705
710
715
720
photon energy (eV)
(b)
4
B ↑↑ kph B ↑↓ kph
absorption (a.u.)
3
2
1
0
705
710
715
720
photon energy (eV)
Fig. 2. (a) L3 absorption spectra from Ar matrix isolated Fe clusters the applied field being oriented parallel (d) and antiparallel (s) with respect to the light propagation direction. (b) Same spectra as before, but after removal of the Ar layer.
In a second preparative step, the Argon film is gently removed by allowing the sample to warm up to just above 50 K for a short period. The clusters, however, remain quantitatively on the sample [4], albeit now in contact with the graphite substrate. Fig 2b shows the Fe L3 absorption spectra measured under otherwise unchanged conditions. Obviously, the interaction to the graphite surface has caused the XMCD effect to disappear. This observation still holds, when the applied magnetic field is increased to 1.7 T. Another set of XMCD measurements was carried out with the surface normal of the HOPG crystal tilted by 50° with respect to the incoming synchrotron radiation, in order to detect in-plane components of the magnetization [15], if present. In both cases, with and without matrix isolation, the results were identical to those displayed in Fig. 2. These XMCD experiments demonstrate that the interaction to graphite has a dramatic impact on the magnetism of small iron clusters, reducing their net magnetization to zero. Preliminary results indicate, that Ni and Co clusters on HOPG show a similar behavior.
It is desirable to trace this observation back to the changes in the underlying electronic structure of the clusters. To this end, we compare in Fig. 3 the helicity averaged (nonmagnetic) L3 absorption spectra of the iron clusters both, within the Ar matrix and exposed to the graphite substrate. The experimental spectra were normalized with respect to the edge jump before removing the background along the procedure outlined in [14]. The most obvious change in the XAS spectra is a marked shift of the Fe L3 absorption maximum by about 1.2 eV to higher photon energies after removing the Ar film, which goes along with a reduction of the integrated absorption strength by 20%. Both, reduction and shift of spectral weight to higher excitation energies apply to the L2 edge as well. These results are very much in line with recent electronic structure calculations for mono- and diatomic 3d transition metal adsorbates on hexagonal carbon structures [24,25]. According to these calculations, the transition metal 4s-related density of states is strongly shifted upwards in energy due to the repulsive interaction with the carbon p orbitals. As a consequence of this shift, the 4s related states are positioned above the highest occupied molecular orbital, effectively increasing the 3d electron count. In the atomic limit, this results in a 3dnþ2 4s0 configuration for the transition metal adsorbates (i.e. 3d8 in the case of Fe), where it is understood, that hybridization may lead to considerable intermixing [24]. Hybridization was studied by Cini et al. [26] in terms of a model hamiltonian and found to account well for a similar shift of the L edge X-ray absorption in Pd clusters on graphite. In combination, these scenarios qualitatively account well for our observations on the absorption data in Fig. 3. The increased 3d occupation number in turn reduces the number of available final states for the resonant absorption Fe L edge and hence its integrated
4
absorption (a.u.)
500
in Ar on HOPG
3
2
1
0 705
710
715
720
photon energy (eV)
Fig. 3. Comparison of the Fe L3 absorption spectra of clusters with and without rare gas matrix isolation. The spectra were normalized to identical edge jumps before background subtraction.
K. Fauth et al. / Chemical Physics Letters 392 (2004) 498–502
intensity. The unoccupied Fe 3d states hybridize with Cp orbitals to form hybrid resonance states, whose center of gravity is shifted away from the Fermi level [26]. More elaborate electronic structure calculations for deposited iron clusters being unavailable at present, the preceding discussion cannot be raised beyond this purely qualitative level. It is interesting to note, however, that both, adatoms and dimers as well as artificial hexagonal Fe monolayers on Graphite [27] are predicted by theory to possess (ferro-) magnetic ground states. These results are clearly at variance with the present experiments. One could speculate, though, that the interaction with the hexagonal lattice of the graphite substrate might stabilize the hexagonal (hcp) phase of Fe. While known to be effectively paramagnetic, the existence of local magnetic moments in hcp Fe is still a matter of debate [28]. Based on XMCD alone, we cannot draw definite conclusions concerning the magnetic ground state beyond the fact that it does have zero net magnetization. While it seems unlikely that an increased d-band filling would result in antiferromagnetic order [29], we cannot completely rule out the possibility of other, more complex or noncollinear spin structures as alternatives to a purely paramagnetic state. We have addressed this issue by measuring the 3s photoelectron spectrum on Fe clusters exposed to the HOPG surface, prepared in the same way as described above. The presence of finite atomic magnetic moments leads to 3s photoemission spectra displaying multiplet features due to the exchange interaction of the core hole final state with the partially filled 3d shell [30], typical level splittings being of the order of several electron volts. While doublet spectra have been reported for some paramagnetic Fe compounds, singlet 3s spectra are only observed with no local moments present in the initial (ground) state [31]. Since the clusters’ 3s photoelectron spectrum, as shown in Fig. 4, consists of one single line only, we can definitely conclude, that Fe clusters on graphite
501
are driven into a nonmagnetic state by the interaction to the substrate. Quite clearly, the size regime in which magnetic moments in the adsorbed clusters are totally quenched must be limited. Indeed, Edmonds and co-workers [11,32] have found magnetizations comparable to bulk Fe in clusters of 300 atoms and larger, when deposited onto HOPG. We can, however, now understand why these experiments did not reproduce the enhanced magnetizations observed in the gas phase. From our findings, it is very likely, that a magnetically dead layer is formed at the cluster graphite interface. Based on these assumptions, we have performed a preliminary analysis of magnetization data obtained from graphite encapsulated Co nanoparticles [33] and obtain good agreement assuming a two monolayer thick magnetically dead surface layer and a core of bulklike magnetization. Clearly, the formation of magnetically dead interface layers will impose severe limitations to the usefulness of carbon encapsulation in producing strongly magnetized nanoscale particles. In summary, we have presented a detailed in situ analysis of the interaction of small Fe clusters with the graphite surface. While the clusters exhibit strong L2;3 XMCD when matrix isolated in an Ar thin film, the magnetic contrast in X-ray absorption is entirely lost, when the substrate interaction is switched on. Changes in spectral position and line shape of the resonant L3 absorption indicate the reorganization of the clusters’ electronic level system, in agreement with previous predictions for transition metal adatoms and dimers on graphite. 3s – photoelectron spectra show, that the ground state of the exposed Fe clusters is indeed nonmagnetic. Whether this observation is related to the formation of hcp iron clusters on HOPG deserves careful consideration in future experiments. On the basis of our present results and considering other experimental findings on magnetic nanoparticles we are led to believe that the presence of graphite-like carbon is detrimental to nanoscale magnetic materials formed from the late 3d transition metals. It is a pleasure to acknowledge E. Goering and M. Noll for initial assistance with the XMCD experiment, O. Rader and W. Gudat for the permission to use their photoelectron spectrometer and the BESSY staff for their continuous support. We also thank J.-M. Bonard for communicating experimental results prior to publication.
References
Fig. 4. Fe 3s photoelectron spectrum of a cluster sample prepared on HOPG as described in the text. The photon energy is hx ¼ 180 eV. The single peaked spectrum indicates the absence of atomic magnetic moments in the iron clusters.
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