- Email: [email protected]
The Mn 2p core-level photoelectron spectrum of Pd–Mn bimetallic systems on Pd(1 0 0)
Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 7–14
The Mn 2p core-level photoelectron spectrum of Pd–Mn bimetallic systems on Pd(1 0 0) Anders Sandell a,∗ , Alexander J. Jaworowski b b
a Department of Physics, Uppsala University, P.O. Box 530, Uppsala S-75121, Sweden Department of Synchrotron Radiation Research, Lund University, P.O. Box 118, Lund S-22100, Sweden
Received 1 July 2003; received in revised form 31 October 2003; accepted 14 November 2003
Abstract It is shown that the Mn 2p spectra of Mn atoms in Pd–Mn bimetallic surface systems formed on Pd(1 0 0) exhibit a satellite, separated 1 eV from the main line, in addition to the 5 eV satellite previously observed for Mn-containing compounds and surface alloys. The main line and the low-energy satellite are both assigned to 2p5 3d6 final states whereas the 5 eV satellite is associated with 2p5 3d5 final states. The identification of the two low-binding energy peaks is based on an energy comparison with the Mn 2p → 3d X-ray absorption spectrum. The relative intensities of the two low-binding energy peaks furthermore vary depending on the Mn atomic environment, reflecting the changes in the properties of the Mn 3d band rather than the geometric position of the Mn atoms. For Mn coverages ≤1 monolayer, the highest degree of Mn 3d localization was found for structures in which the Mn atoms are spread uniformly over the surface. The observed changes in the Mn 2p spectra are discussed in terms of varying Mn–Pd interaction, exchange splitting and magnetic ordering. © 2003 Elsevier B.V. All rights reserved. Keywords: Surface alloy; Core-level photoelectron spectroscopy; Metal surface; Electron correlation; X-ray absorption spectroscopy
1. Introduction Transition metal atoms arranged in structures of reduced dimensionality often show electronic properties that are quite different from the stable bulk phases. For example, the d-electrons exhibit a much stronger correlation, which in turn can lead to strongly enhanced magnetic moments. This has been found to be the case for Mn in a monolayer (ML) on Ag(1 0 0), which shows a magnetic moment as large as 4µB , close to the Hund’s rule atomic 6 S5/2 ground state value, as opposed to the itinerant low-spin ground state formed in Mn metal [1,2]. A most relevant question is therefore how the chemical surrounding influences the properties of the d-levels and a particularly interesting class of systems in this respect is the surface alloys, since the added atoms are situated in a matrix of host atoms and the surface alloy as such is low dimensional. Core-level photoelectron spectroscopy (also denoted X-ray photoelectron spectroscopy (XPS)) offers atomic ∗ Corresponding author. Tel.: +46-18-471-35-48; fax: +46-18-471-35-24. E-mail address: [email protected] (A. Sandell).
0368-2048/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2003.11.005
selectivity due to the strong localization of the core hole. The core-level photoemission spectrum is furthermore very sensitive to changes in the valence electronic structure and these properties make the technique particularly useful for studies of alloys and compounds. The core-level photoemission spectrum, however, often consists of a multitude of different components. One effect that gives rise to additional lines is the strong correlation between the core hole and electrons in an open valence shell, which results in an atomic multiplet splitting. In addition, relaxation effects must be taken into account. Extra features can appear in the core-level spectrum as a consequence of the strong perturbation caused by the core ionization. In a simple picture, the relaxation process leads to valence excitations in the core-ionized state, i.e. they represent more highly excited core-ionized states. It can furthermore be shown that the presence of these so-called satellites (or shake-ups) is caused by the difference between the lowest energy configuration of the core-ionized final state (FS) and the ground state [3]. Consequently, the main line and the various satellites represent different electron configurations, and, in principle, each final state electron configuration can display multiplet splitting.
8
A. Sandell, A.J. Jaworowski / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 7–14
Changes in the spectral shape can therefore sometimes be attributed to varying relative intensity of the states related to the different electron configurations. More than one final state configuration typically appear when charge transfer (CT) screening takes place and, which is of particular interest here, the Mn 2p spectra for Mn/Ag(1 0 0), Mn/Ni(1 1 0) and Mn/Cu(1 0 0) have been interpreted in this way [4–6]. For these systems, a main line was found at a binding energy (BE) of 640 eV and a satellite at 645 eV. The 5 eV satellite was in all cases assigned to a 2p5 3d5 final state configuration whereas the main line was associated with a 2p5 3d6 final state configuration. In the study of the Mn/Ag(1 0 0) system, the intensity of the 5 eV satellite relative to the main peak displayed a significant coverage dependent behavior; the relative intensity of the satellite progressively increased as the coverage was decreased [4]. The interpretation offered was in terms of different screening responses upon core ionization: At high coverages, the Mn 3d–3d interaction increases and this leads to an increased intensity of the 2p5 3d6 final states. At low coverages the Mn 3d states become more localized, due to the small interaction with the Ag 4d and Ag 5sp states, which gives rise to an increased intensity of the 2p5 3d5 final states. Concerning effects of atomic multiplet splitting it has been found that these play an important role for the shape of the Mn 2p main line in various compounds. This has been observed for the Heusler alloys, which are ternary alloys of the generic form X2 YZ or XYZ, where Y is Mn and X is, for example, Fe, Co, Ni, Cu, Pd or Rh and Z = Al, Ga, Si, Sn, Sb and In. It has been suggested that the line width of the Mn 2p main line is dependent on the local magnetic moment of the Mn atoms, that is, effects due to exchange splitting are most important [7,8]. We have previously reported detailed studies of the electronic and geometric structure of Pd–Mn bimetallic systems formed on Pd(1 0 0) [9–11]. Using X-ray absorption spectroscopy (XAS) it was demonstrated that Mn atoms in a c(2 × 2) Pd–Mn surface alloy formed on Pd(1 0 0) have a very correlated electronic structure, similar to that of the Mn/Ag and Mn/Cu systems and ionic Mn compounds. This was also the case for a Mn monolayer on Pd(1 0 0) formed at 90 K. Using photoelectron spectroscopy of the valence levels and ab initio calculations it was demonstrated that both the monolayer and antiferromagnetic (2 × 2) alloy were c and we were also able to distinguish between different types of possible c(2 × 2) surface alloy structures. In this paper, we continue the investigation of the electronic structure in well-characterized Pd–Mn systems formed on Pd(1 0 0) by discussing the Mn 2p spectrum in detail. We find that the Mn 2p main line consists of two features that are separated by 1 eV in binding energy. The relative intensities of these states vary depending on the Mn atomic environment, reflecting the changes in the properties of the Mn 3d states rather than the geometric position of the Mn atoms. For Mn coverages ≤1 ML, the highest degree of Mn 3d localization was found for structures in which the
Mn atoms are spread uniformly over the surface. This is discussed in terms of varying Mn–Pd interaction, exchange splitting and magnetic ordering.
2. Experimental The core-level photoemission data were recorded at beamline I311 at MAX II, a third generation electron storage ring at the Swedish national synchrotron facility MAXLAB. I311 is a new undulator-based beamline for studies in the VUV and with soft X-rays [12]. The end station has two separate chambers for measurements and sample preparation, respectively. A large hemispherical electron energy analyzer of Scienta type is used to record the photoelectron spectra. The Mn 2p spectra were measured at a photon energy of 750 eV (unless otherwise stated) and with a total energy resolution of 0.6 eV. The Pd 3d5/2 spectra were recorded with a photon energy of 400 eV and a total resolution of 100 meV. All spectra were obtained at normal emission and the binding energy values are relative to the Fermi edge of the sample. The XAS spectrum was recorded at beamline D1011 at MAXLAB [13] by monitoring the intensity of the Mn Auger features in the kinetic energy interval 500–512 eV using an electron energy analyzer similar to the one at BL I311. The photon energy resolution was set to 0.2 eV. The spectra were recorded at 40◦ incidence relative to the surface normal and the photon energy was calibrated using first and second-order light from the monochromator. The Pd crystal was cleaned by sputtering with 2.5 kV Ar+ ions at 900 K, followed by a 1 min anneal at 1150 K. In addition, several oxygen treatments were used to remove residual carbon. During these, the oxygen pressure was kept at 2 × 10−8 mbar while the temperature was cycled between 400 and 900 K. After oxygen treatments the crystal was flashed to 1100 K. Surface cleanliness was checked by the Pd 3d and the C 1s core-levels. Manganese was evaporated from a flake (Goodfellow, purity 99.95%) firmly placed inside a resistively heated tungsten coil. The evaporation rate was monitored by the shape and intensity of the Mn 2p spectrum, which has previously been correlated to coverages obtained by a thickness monitor (quartz microbalance). However, an amount corresponding to 0.7 ML according to the thickness monitor is close to the amount for one monolayer as inferred from the integrated Mn 2p intensities. Thus, given the well-defined monolayer situation [10], it is likely that the 0.7 ML as read by the thickness monitor in fact corresponds to a slightly higher amount.
3. Results 3.1. Geometric structure of Pd–Mn bimetallic systems on Pd(1 0 0) We will start by going through the different Pd–Mn systems that can be formed on Pd(1 0 0), focusing on the
A. Sandell, A.J. Jaworowski / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 7–14
Pd 3d5/2
Pd(100)-c(2x2)-Mn
Tdep = 90 K
I
hν = 400 eV
Intensity (arb. units)
Intensity (arb. units)
Mn 2p B
S
+1 ML Mn
+0.3 ML Mn
2p1/2
660
Clean Pd(100)
337
9
336
335
334
2p3/2
655 650 645 640 Binding Energy(eV)
635
Fig. 2. Extended Mn 2p photoelectron spectrum for Pd(1 0 0)–c(2×2)–Mn obtained after deposition of 0.7–1.0 ML Mn at room temperature followed by annealing to 570 K for 3 min.
Binding Energy(eV)
Fig. 1. Pd 3d5/2 spectra for clean Pd(1 0 0) and after deposition of increasing amounts of Mn at a substrate temperature of 90 K. The three peaks are due to pure Pd(1 0 0) surface (S), bulk Pd (B) and interface Pd atoms (I), respectively.
geometric properties for Mn coverages up to one monolayer. Fig. 1 shows the coverage dependent behavior of the Pd 3d5/2 line upon deposition of Mn on Pd(1 0 0) at 90 K. The bottom spectrum shows the spectrum for clean Pd(1 0 0) and the surface (S) and bulk (B) components are clearly resolved [14]. After deposition of 0.3 ML Mn at 90 K, it can be seen that the component due to clean Pd surface (S) has vanished and is now replaced by an interface peak (I). The increased BE of the interface peak is an effect of the bonding to Mn [10]. At the monolayer point, the interface peak has shifted more towards higher BE due to the increased Pd–Mn coordination and, at this temperature, the Mn monolayer covers the surface without intermixing with the Pd atoms [10]. A most interesting result is furthermore that the clean surface component has vanished already at 0.3 ML, meaning that at this point all atoms in the outermost Pd layer are directly coordinated to Mn. From this follows that the Mn atoms must be distributed uniformly over the surface, occupying highly coordinated sites (most probably four-fold hollow sites). The behavior at higher temperatures has been discussed extensively in previous work [9–11,15,16]. To summarize, incorporation of Mn atoms occurs at 300 K, forming a c(2 × 2) alloy, and a dramatically improved ordering of the alloy is obtained upon annealing to 570 K. At a Mn coverage of 0.7–1.0 ML, the c(2 × 2) reconstruction covers the whole surface [9–11]. Based on a comparison of the experimental and calculated electronic structures, it was proposed that the annealed alloy is two-layer thick, each of the layers consisting of Pd and Mn atoms arranged in a checkerboard pattern [10]. At submonolayer coverage (0.3 ML), it was
demonstrated that the surface consists of c(2 × 2) Pd–Mn alloy islands separated by (1 × 1) regions consisting of pure Pd [11].Thus, from the above findings it stands clear that by changing the Mn coverage and the temperature, it is possible to vary the distribution of the Mn atoms in the outermost layers in the Pd(1 0 0) host. 3.2. The Mn 2p X-ray photoelectron spectrum: observation of new fine structure The Mn 2p XPS spectrum for the c(2 × 2) alloy formed on Pd(1 0 0) is shown in Fig. 2. The alloy was prepared from an estimated amount of 0.7 ML Mn, which gives a surface fully covered by a c(2 × 2) Pd–Mn layer and, in addition, a sub-surface Pd–Mn alloy layer [10]. The spectrum covers both the 2p3/2 and 2p1/2 components and upon close inspection we see that there are basically two regions with features associated with each spin–orbit level. Focussing on the states associated with 2p3/2 ionization, there are structures between 639–641 eV and a broad feature at 645 eV. The general appearance is very similar to the spectra found for the related Mn/Ag(1 0 0), Mn/Ni(1 1 0) and Cu(1 0 0)–Mn–c(2×2) systems [4–6]. The Mn 2p spectra for these systems typically display two peaks, interpreted as a main line at a BE of about 640 eV and a feature at about 645 eV, assigned to a correlation satellite. Interestingly, the Mn 2p core-level spectrum shown in Fig. 2 also reveals fine structure in that there are two peaks in the main peak BE region around 640 eV. Such a doublet structure has not been observed for the Mn/Cu, Mn/Ni and Mn/Ag systems. One reason is simply that the earlier spectra are of such a quality that a splitting of this magnitude is difficult to discern. There is also the possibility that there are differences in the geometrical structure. For example, the fully developed Pd–Mn c(2 × 2) structure contains
10
A. Sandell, A.J. Jaworowski / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 7–14
Intensity (arb. units)
0.3 ML
0.7 ML
570 K
570 K
300 K
300 K 90 K
646
90 K
644
642
640
638
Binding Energy (eV)
646
644
642
640
638
Binding Energy (eV)
Fig. 3. Mn 2p3/2 spectra for two different Mn coverages prepared at three different temperatures.
and bulk components of clean Pd(1 0 0) [14]. It is therefore unlikely that the existence of a doublet structure is due to two chemically shifted components, such as a sub-surface and a surface peak. How general is this effect? We have also prepared other Mn-containing systems. In Fig. 5, we compare the disperse and complete annealed c(2×2) surface alloys (b and c) with a thick Mn film grown at 300 K (a) and a thin oxide film formed by oxidation and subsequent annealing of a surface completely covered by the c(2 × 2) Pd–Mn alloy (d). Fine structure in the main line region is found for both the thick film and the thin oxide. In the case of the thick film, it is possible that the split is due to a surface core-level shift Mn 2p3/2 0.7 ML Full c(2x2) surface
Intensity (arb. units)
sub-surface as well as surface Mn atoms [10], whereas the Mn atoms in the c(2 × 2) Mn–Cu alloy on Cu(1 0 0) are all situated within the surface layer [17,18]. The doublet structure found for the Pd–Mn c(2×2) system could therefore be the result of having both sub-surface and surface Mn atoms, i.e. atoms that are chemically inequivalent. In order to study the effect of different geometrical positions of the Mn atoms we utilize our ability to change the amount and distribution of Mn atoms within the topmost layers. In Fig. 3, we have collected Mn 2p3/2 spectra for two different Mn coverages, 0.3 and 0.7 ML, measured at three different temperatures. It is very hard to discern a coverage dependent effect when comparing 0.3 and 0.7 ML deposited at 90 K, and, most importantly, the 90 K situations both has doublet structure even though Mn atoms situated on the Pd(1 0 0) surface is the only type of species present. The Mn 2p spectra for the annealed situations are however clearly different, which can be due to the fact that these systems contain chemically inequivalent atoms, i.e. sub-surface and surface atoms. The simultaneous presence of surface and sub-surface guest atoms can in favorable cases be revealed by recording spectra using different photon energies. This changes the kinetic energy of the photoelectrons and hence the surface sensitivity. In addition, kinetic energy dependent diffraction effects may occur which can lead to intensity variations between peaks due to atoms in different geometrical positions. Consequently, changing the photon energy is a very sensitive method for the identification of chemically shifted peaks. Fig. 4 shows Mn 2p spectra recorded at three photon energies for 0.1 and 0.7 ML of Mn. Both systems are the ones obtained after annealing. For each coverage, the changes in spectral shape are very small and using a simple two-component curve fitting, we find that the intensity ratio between the two peaks vary by less than 10%. This variation is much smaller than that found for, e.g. the surface
hν = 770 eV hν = 750 eV hν = 730 eV 0.1 ML c(2x2) islands hν = 770 eV hν = 750 eV hν = 730 eV
641
640 639 Binding Energy (eV)
638
Fig. 4. Mn 2p3/2 spectra, recorded at three different photon energies, for two different Mn coverages prepared by deposition at room temperature followed by annealing to 570 K.
A. Sandell, A.J. Jaworowski / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 7–14
11
Pd(100)-c(2x2)-Mn
Mn 2p3/2
hν = 750 eV
Intensity (arb. units)
Intensity (arb. units)
(d)
(c)
(b)
XAS
XPS
(a)
638
640
642
644
Energy (eV)
644
642 640 638 Binding Energy (eV)
Fig. 5. Mn 2p3/2 spectra for four different systems: (a) a thick Mn film deposited on Pd(1 0 0) at room temperature; (b) 0.3 ML Mn deposited at room temperature and annealed to 570 K; (c) 0.7 ML Mn deposited at room temperature and annealed to 570 K; (d) a MnOx film on Pd(1 0 0), obtained by oxidation of situation (c).
but this is not very likely, given the fact that the SCLS is expected to be smaller than 0.3 eV for Mn as estimated by the well-known method [19]. In the case of the oxide, we cannot exclude that the shoulder at 639 eV binding energy is due to the presence of Mn atoms that are not oxidized, although the oxidation of surface and near-surface atoms with a high tendency for oxidation is expected to be very efficient, as exemplified by the MnOx formation on the Cu(1 0 0) surface, starting from the c(2 × 2) Cu–Mn structure [20], and the growth of a thin Al2 O3 film on the Ni–Al(1 1 0) surface [21]. Consequently, at the present stage we can merely point out that the fine structure found for the Pd–Mn systems can also be present in pure Mn and thin MnOx films formed on metallic substrates. In addition, we have also previously presented Mn 2p spectra for a 1 ML Mn/1 ML Pd/W(1 1 0) sandwich and a two-layer thick c(2 × 2) Pd–Mn alloy on W(1 1 0) formed upon annealing of the sandwich [22]. The Mn 2p spectra for these two systems were nearly identical, exhibiting two leading structures, and the spectral shape is very similar to the spectrum for the c(2 × 2) alloy on Pd(1 0 0). To summarize, we conjecture that the position of the Mn atoms within the Pd matrix has very little effect on the spectral shape for a Mn coverage of 0.7–1 ML. However, forming an alloy by annealing of a Mn amount smaller than 0.7 ML results in an increase of the relative intensity of the sharp low BE component. Likewise, the Mn 2p spectrum for a thick Mn film formed on the Pd(1 0 0) surface shows an increased relative intensity of the low BE peak.
Fig. 6. Mn 2p3/2 photoelectron (XPS) and X-ray absorption (XAS) spectra for 0.7 ML Mn deposited at room temperature and annealed to 570 K. The common energy scale corresponds to the binding energy relative to the Fermi level (XPS) and the absolute photon energy (XAS).
3.3. Final state energetics It is important to be able to identify the final state electronic configurations and to deduce how they are related in energy. When discussing the energetics of core-ionized and core-excited final states, a comparison between the XAS and XPS spectrum has in many cases proven to be most useful [23]. Fig. 6 shows the Mn 2p3/2 XPS spectrum and the Mn 2p3/2 → 3d XAS spectrum on a common energy scale. The energy of the Mn 2p3/2 XPS peaks corresponds to the energy relative to the Fermi level, whereas the energy of the XAS resonance corresponds to the absolute photon energy needed for the Mn 2p3/2 → 3d core excitation. The valence electronic structure of the Mn atoms in the alloy shows intensity reaching down to the Fermi level [10]. It is therefore justified to assume that the XPS peak with lowest (binding) energy corresponds to a final state screened by an electron at the lowest energy state, i.e. at the Fermi level. The consequence of this is that the energy of the lowest-BE XPS peak pinpoints the position of the Fermi level in the XAS spectrum [23]. The XAS process is governed by the dipole selection rule and the XAS spectrum can furthermore to a very good approximation be viewed as valence states projected on the core-excited atom. The XAS spectrum for the c(2 × 2) Pd–Mn alloy show a strong resemblance to the spectrum for Mn2+ in a high spin 3d5 ground state, exhibiting the characteristic atomic multiplet structure [10]. One can therefore assume that the final state created by the 2p → 3d X-ray absorption process is 2p5 3d6 . It is seen in Fig. 6 that the energy regime of the XPS peaks of interest agrees with a 2p5 3d6 FS configuration. In fact, the position of the high BE
12
A. Sandell, A.J. Jaworowski / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 7–14
component agrees with the position of the XAS resonance, whereas the position of the low BE component falls on the XAS onset. Hence, the main line regime is associated with 2p5 3d6 final states, in agreement with the previous studies of Mn compounds and surface alloys [4–6,24,25]. To verify that the state at a binding energy of 645 eV is associated with 2p5 3d5 final states, as previously suggested, one can make a comparison with a recent XPS study of atomic Mn [26]. The final state observed in the Mn 2p spectrum is in this case definitely due to a 2p5 3d5 configuration since charge transfer screening is not possible. The binding energy of the feature at lowest BE given in this work is 650.0 eV versus the vacuum level. In order to compare with our spectra, which are referenced to the Fermi level, one must subtract the appropriate work function. For the c(2 × 2) alloy, the work function has been determined to be about 5 eV, a result obtained from the DFT calculations presented earlier [10,27]. From this follows that the 2p5 3d5 final state configuration is expected to occur at about 645 eV binding energy relative to the Fermi level, which agrees very well with the broad satellite observed in the Mn 2p spectrum for the c(2 × 2) alloy.
4. Discussion From the above results it can be concluded that the presence of the doublet structure at 639–640 eV BE is not due to two chemically inequivalent species. Instead, the structures must be interpreted in terms of main line and satellite states. That the satellite at 1 eV higher BE is due to a 2p5 3d5 final state, in contrast to the 2p5 3d6 final state of the main line, is furthermore contradicted by the energy comparison with the Mn 2p spectrum for the free atom. In the following we will therefore assume that the structures at 639–640 eV binding energy are due to 2p5 3d6 final states and focus the discussion on effects due to non-local versus local screening, variations in the band structure, multiplet splitting and magnetic ordering. The presence of a low-energy satellite peak can be due to two different responses to the core ionization. This model has been employed in order to explain the Ni 2p spectrum of NiO, where a 1 eV split doublet is seen [28]. It was proposed that both peaks stem from final states of the same 3d-count. The low BE component (main line) was attributed to screening from O nearest neighbors and the process was written as 2p6 3dn → 2p6 3dn+1 L where L denotes a hole in ¯ was interpreted the oxygen valence band. The 1¯eV satellite 6 n 6 n+1 as due to a 2p 3d → 2p 3d L process where the extra ¯ d-electron is provided by neighboring NiO6 units. Thus, the term non-local screening was coined to describe the satellite process, as opposed to the local screening associated with the main line. From this follows that variations in the spectral shape may occur even though only one formal final state electron configuration is present.
The scheme with non-local and local screening mechanisms is very attractive, since it gives a very direct picture of the coverage dependent effects seen for the annealed situations (Fig. 3). Within this model, the low BE peak can be associated with a 2p5 3d6 L final state configuration, where L denotes a hole in the Pd valence band (“local screening”). The 1 eV satellite would then correspond to a final state in which screening is provided by neighboring MnPdx units, giving a 2p5 3d6 configuration (“non-local screening”). As previously mentioned, when decreasing the coverage from 0.7 ML down to about 0.3 ML, the situation after annealing changes from a fully reconstructed c(2 × 2) Pd–Mn surface to a surface with disperse c(2 × 2) Pd–Mn islands separated by (1 × 1) Pd parts. Consequently, the average number of neighboring MnPdx units is reduced and hence also the relative intensity of the 1 eV satellite, as seen in Fig. 3. This behavior is thus directly comparable to that found when comparing the Ni 2p spectra for bulk NiO with that of a NiO monolayer/MgO(1 0 0) and dilute Ni in MgO [28]. The processes above can also be discussed more in detail by considering the valence electronic structure, where one can envisage both pure Mn 3d states and Mn 3d states hybridized with Pd 4d states. Calculations show that the occupied valence band, which reaches down to the Fermi level, is rather broad due to mixing of Mn 3d majority spin states and Pd 4d states [10]. The unoccupied part of the DOS dominates by a sharp peak 1.5 eV above the Fermi level, which has a pronounced Mn 3d minority spin character [10]. Since the screening involves population of an initially unoccupied state at the core-ionized site, one must consider the unoccupied density-of-states. The energy cost for the promotion of an electron from the Fermi level to the empty state of strong Mn 3d character agrees in energy with the shift between the main line and the satellite, as confirmed by the direct comparison of the XPS and XAS spectra shown in Fig. 6. In this context, it is important to note that the effect of the Mn 2p hole on the position of the empty Mn 3d states is very small [10]. Consequently, in an attempt to merge the non-local/local screening picture with the band structure picture, one could, very qualitatively, say that local screening produces a FS typical of a metal, that is, the screening electron resides at the Fermi level, where Mn–Pd mixed states dominate the DOS, whereas non-local screening produces an atomic-like 2p5 3d6 FS, that is, the screening electron resides in a pure Mn level, a state which is situated at 1 eV higher binding energy. As previously noted, the presence of atomic multiplet splitting is also expected to affect the spectral shape. In a study of Co2 –Mn–Sn and Pd2 –Mn–Sn Heusler alloys, different shapes of the Mn 2p main line was found, and it was stated that this was partly due to different local magnetic moments, which gives rise to differences in the exchange splitting, and partly due to varying intensity of the sub-levels due to dipole matrix elements effects [7]. It was argued that the latter effect comes into play since the peaks
A. Sandell, A.J. Jaworowski / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 7–14
can be described as direct transitions from Mn 2p states into final state d bands, i.e. it is a way to describe the screening process. Changes in spectral shape due to screening effects have been discussed above, but the exchange splitting has so far not been considered. This effect provides a possible origin to the varying widths of the 1 eV satellite as shown in Fig. 5. It can be observed that the width of the 1 eV satellite in the Mn 2p spectrum for the annealed 0.3 ML situation (Fig. 5b) is narrower than in the spectrum for the annealed 0.7 ML situation (Fig. 5c). This would suggest a smaller exchange splitting for the satellite final state in the case where Mn atoms are situated within disperse c(2 × 2) islands than for the satellite final state when Mn atoms form the fully developed c(2 × 2) alloy. A plausible reason for this is that a decreased hybridization with the surrounding Pd atoms increases the local magnetic moment. If this interpretation is correct, it furthermore implies that the local magnetic moment has increased even further for the oxide film, since this spectrum is considerably broader (Fig. 5d). To summarize the discussion so far, we can state that the Mn–Pd interaction for the disperse and fully developed c(2 × 2) situations are expected to be different and this in turn affects the correlation of the Mn 3d states. An increased correlation gives rise to an increased relative intensity and width of the 1 eV satellite, associated with an increased weight of atomic-like 2p5 3d6 FS and a concomitant increased exchange splitting. The relative intensity of the low BE peak thus shows a minimum at a Mn coverage of around 0.7–1.0 ML, indicating a maximized correlation of the Mn 3d states in this coverage regime. This finding is at variance with the Mn/Cu and Mn/Ag systems, for which a progressively increased localized character was noted with decreasing coverage. The reason for this discrepancy is likely to be caused by different Mn–host interaction strengths; due to their larger overlap in energy, the Mn 3d states are expected to interact more strongly with the Pd 4d states than with the Ag 4d states, prohibiting the localization effect found for the MnAg system. However, another important effect concerns magnetic ordering of the Mn atoms, which can have a strong effect on the localization of the Mn 3d states. It is well known that the magnetic ordering depend on the number of atoms having a magnetic moment and how they are distributed. For instance, if one has small, well-separated islands, the correlation between the islands is reduced as compared to a homogeneous surface and this leads to a lower degree of magnetic ordering. It is therefore not unlikely that the separate c(2 × 2) islands formed at 0.3 ML has a lower degree of magnetic ordering than the uniform alloy formed at 0.7 ML. Likewise, the overlayer formed by deposition of 0.3 ML Mn at 90 K is more uniform than the disperse c(2 × 2) islands since the atoms reside in the same layer and are uniformly distributed over the whole surface. A higher magnetic ordering increases the 3d localization and this is consistent with the finding that the uniform situations show a more localized behavior.
13
Turning finally to the temperature effects of the 0.3 ML situation, it is clear that the 90 K case has Mn situated on top of the Pd surface whereas the annealed situation features separate c(2 × 2) islands, with Mn at the surface as well as in the bulk. Clearly, the Mn–Pd interaction is expected to be stronger upon substitution and alloy formation. Moreover, we have previously stated that the presence of a doublet is not due to having chemically inequivalent Mn atoms. However, it is important to be aware of the fact that the relative intensities of the doublet peaks can vary depending on whether the Mn atom resides in the surface or bulk layer. In fact, calculations indicate that the Mn 3d DOS differ between the bulk and surface atoms; the 3d states of the bulk atoms are more strongly hydridized with the Pd 4d states [10]. Thus, it is feasible that the Mn 3d states of the bulk atoms are more delocalized than the 3d states of the surface atoms. This leads to a low BE component which is more pronounced for the bulk atoms than for the surface atoms, which may account for the 10% variation in relative intensity of the two peaks seen at different photoelectron kinetic energies.
5. Summary We have shown that the Mn 2p spectra of Mn atoms in Pd–Mn bimetallic surface systems formed on Pd(1 0 0) exhibit a satellite, separated 1 eV from the main line, in addition to the previously observed 5 eV satellite. The main line and the low-energy satellite are both assigned to 2p5 3d6 final states, whereas the 5 eV satellite is associated with 2p5 3d5 final states. The identification of the two low-binding energy peaks is based on an energy comparison with the 2p → 3d X-ray absorption spectrum. The relative intensities of the two low-binding energy peaks furthermore vary depending on the Mn atomic environment, reflecting the changes in the properties of the Mn 3d band rather than the geometric position of the Mn atoms. For Mn coverages ≤1 ML, the highest degree of Mn 3d localization was found for structures in which the Mn atoms are spread uniformly over the surface. The observed changes in the Mn 2p spectra can be attributed to varying Mn–Pd interaction, exchange splitting and magnetic ordering.
Acknowledgements The helpfulness of MAXLAB staff is gratefully acknowledged. This work has been financially supported by the Swedish Natural Science Research Council.
References [1] P. Schieffer, C. Krembel, M.C. Hanf, G. Gewinner, Phys. Rev. B 57 (1998) 1141.
14
A. Sandell, A.J. Jaworowski / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 7–14
[2] P. Schieffer, C. Krembel, M.C. Hanf, G. Gewinner, Surf. Sci. 400 (1998) 95S. [3] N. Mårtensson, A. Nilsson, in: W. Eberhardt (Ed.), Applications of Synchrotron Radiation: High-Resolution Studies of Molecules and Molecular Adsorbates on Surfaces, Springer, Berlin, 1995. [4] P. Schieffer, C. Krembel, M.C. Hanf, G. Gewinner, J. Electron Spectrosc. Rel. Phenom. 104 (1999) 127. [5] O. Rader, T. Mizokawa, A. Fujimori, A. Kimura, Phys. Rev. B 64 (2001) 165414. [6] O. Rader, E. Vescovo, M. Wuttig, D.D. Sarma, S. Blügel, F.J. Himpsel, A. Kimura, K.S. An, T. Mizokawa, A. Fujimori, C. Carbone, Europhys. Lett. 39 (1997) 429. [7] Yu.M. Yarmoshenko, M.I. Katsnelson, E.I. Shreder, E.Z. Kurmaev, A. Slebarski, S. Plogmann, T. Schlathölter, J. Braun, M. Neumann, Eur. Phys. J. B 2 (1998) 1. [8] S. Plogmann, T. Schlathölter, J. Braun, M. Neumann, Yu.M. Yarmoshenko, M.V. Yablonskikh, E.I. Shreder, E.Z. Kurmaev, A. Wrona, A. Slebarski, Phys. Rev. B 60 (1999) 6428. [9] A.J. Jaworowski, S. Gray, M. Evans, P. Uvdal, A. Sandell, Phys. Rev. B 63 (2001) 125401. [10] A. Sandell, P.H. Andersson, E. Holmström, A.J. Jaworowski, L. Nordström, Phys. Rev. B 65 (2002) 035410. [11] A.J. Jaworowski, R. Ásmundsson, P. Uvdal, S.M. Gray, A. Sandell, Surf. Sci. 501 (2002) 83. [12] R. Nyholm, J.N. Andersen, U. Johansson, B.N. Jensen, I. Lindau, Nucl. Instrum. Meth. A 467–468 (2001) 520. [13] J.N. Andersen, O. Björneholm, A. Sandell, R. Nyholm, J. Forsell, L. Thånell, A. Nilsson, N. Mårtensson, Synchrotron Radiat. News 4 (1991) 15.
[14] R. Nyholm, M. Qvarford, J.N. Andersen, S.L. Sorensen, C. Wigren, J. Phys. Cond. Matter 4 (1992) 227. [15] D. Tian, S.C. Wu, F. Jona, P.M. Marcus, Solid State Commun. 70 (1989) 199. [16] D. Tian, R.F. Lin, F. Jona, P.M. Marcus, Solid State Commun. 74 (1990) 1017. [17] T. Flores, M. Hansen, M. Wuttig, Surf. Sci. 279 (1992) 251; M. Wuttig, et al., Surf. Sci. 292 (1993) 189; M. Wuttig, et al., Phys. Rev. B 53 (1996) 7551. [18] M. Wuttig, Y. Gauthier, S. Blügel, Phys. Rev. Lett. 70 (1993) 3619. [19] B. Johansson, N. Mårtensson, Phys. Rev. B 21 (1980) 4427. [20] P.J. Knight, R. Toomes, S.M. Driver, D.P. Woodruff, Surf. Sci. 418 (1998) 521. [21] R.M. Jaeger, H. Kuhlenbeck, H.-J. Freund, M. Wuttig, W. Hoffman, R. Franchy, H. Ibach, Surf. Sci. 259 (1991) 235. [22] A.J. Jaworowski, A. Sandell, Surf. Sci. 477 (2001) 141. [23] N. Mårtensson, A. Nilsson, J. Electron Spectrosc. Rel. Pheom. 75 (1995) 209. [24] A.J. Nelson, J.G. Reynolds, J.W. Roos, J. Vac. Sci. Technol. A 18 (2000) 1072. [25] K. Okada, A. Kotani, J. Phys. Soc. Jpn. 61 (1992) 4619. [26] Ph. Wernet, B. Sonntag, M. Martins, P. Glatzel, B. Obst, P. Zimmermann, Phys. Rev. A 63 (2001) 050702(R). [27] E. Holmström, private communication. [28] M.A. van Veenendal, G.A. Sawatzky, Phys. Rev. Lett. 70 (1993) 2459; D. Alders, F.C. Voogt, T. Hibma, G.A. Sawatzky, Phys. Rev. B 54 (1996) 7716.
The Mn 2p core-level photoelectron spectrum of Pd–Mn bimetallic systems on Pd(1 0 0) Anders Sandell a,∗ , Alexander J. Jaworowski b b
a Department of Physics, Uppsala University, P.O. Box 530, Uppsala S-75121, Sweden Department of Synchrotron Radiation Research, Lund University, P.O. Box 118, Lund S-22100, Sweden
Received 1 July 2003; received in revised form 31 October 2003; accepted 14 November 2003
Abstract It is shown that the Mn 2p spectra of Mn atoms in Pd–Mn bimetallic surface systems formed on Pd(1 0 0) exhibit a satellite, separated 1 eV from the main line, in addition to the 5 eV satellite previously observed for Mn-containing compounds and surface alloys. The main line and the low-energy satellite are both assigned to 2p5 3d6 final states whereas the 5 eV satellite is associated with 2p5 3d5 final states. The identification of the two low-binding energy peaks is based on an energy comparison with the Mn 2p → 3d X-ray absorption spectrum. The relative intensities of the two low-binding energy peaks furthermore vary depending on the Mn atomic environment, reflecting the changes in the properties of the Mn 3d band rather than the geometric position of the Mn atoms. For Mn coverages ≤1 monolayer, the highest degree of Mn 3d localization was found for structures in which the Mn atoms are spread uniformly over the surface. The observed changes in the Mn 2p spectra are discussed in terms of varying Mn–Pd interaction, exchange splitting and magnetic ordering. © 2003 Elsevier B.V. All rights reserved. Keywords: Surface alloy; Core-level photoelectron spectroscopy; Metal surface; Electron correlation; X-ray absorption spectroscopy
1. Introduction Transition metal atoms arranged in structures of reduced dimensionality often show electronic properties that are quite different from the stable bulk phases. For example, the d-electrons exhibit a much stronger correlation, which in turn can lead to strongly enhanced magnetic moments. This has been found to be the case for Mn in a monolayer (ML) on Ag(1 0 0), which shows a magnetic moment as large as 4µB , close to the Hund’s rule atomic 6 S5/2 ground state value, as opposed to the itinerant low-spin ground state formed in Mn metal [1,2]. A most relevant question is therefore how the chemical surrounding influences the properties of the d-levels and a particularly interesting class of systems in this respect is the surface alloys, since the added atoms are situated in a matrix of host atoms and the surface alloy as such is low dimensional. Core-level photoelectron spectroscopy (also denoted X-ray photoelectron spectroscopy (XPS)) offers atomic ∗ Corresponding author. Tel.: +46-18-471-35-48; fax: +46-18-471-35-24. E-mail address: [email protected] (A. Sandell).
0368-2048/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2003.11.005
selectivity due to the strong localization of the core hole. The core-level photoemission spectrum is furthermore very sensitive to changes in the valence electronic structure and these properties make the technique particularly useful for studies of alloys and compounds. The core-level photoemission spectrum, however, often consists of a multitude of different components. One effect that gives rise to additional lines is the strong correlation between the core hole and electrons in an open valence shell, which results in an atomic multiplet splitting. In addition, relaxation effects must be taken into account. Extra features can appear in the core-level spectrum as a consequence of the strong perturbation caused by the core ionization. In a simple picture, the relaxation process leads to valence excitations in the core-ionized state, i.e. they represent more highly excited core-ionized states. It can furthermore be shown that the presence of these so-called satellites (or shake-ups) is caused by the difference between the lowest energy configuration of the core-ionized final state (FS) and the ground state [3]. Consequently, the main line and the various satellites represent different electron configurations, and, in principle, each final state electron configuration can display multiplet splitting.
8
A. Sandell, A.J. Jaworowski / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 7–14
Changes in the spectral shape can therefore sometimes be attributed to varying relative intensity of the states related to the different electron configurations. More than one final state configuration typically appear when charge transfer (CT) screening takes place and, which is of particular interest here, the Mn 2p spectra for Mn/Ag(1 0 0), Mn/Ni(1 1 0) and Mn/Cu(1 0 0) have been interpreted in this way [4–6]. For these systems, a main line was found at a binding energy (BE) of 640 eV and a satellite at 645 eV. The 5 eV satellite was in all cases assigned to a 2p5 3d5 final state configuration whereas the main line was associated with a 2p5 3d6 final state configuration. In the study of the Mn/Ag(1 0 0) system, the intensity of the 5 eV satellite relative to the main peak displayed a significant coverage dependent behavior; the relative intensity of the satellite progressively increased as the coverage was decreased [4]. The interpretation offered was in terms of different screening responses upon core ionization: At high coverages, the Mn 3d–3d interaction increases and this leads to an increased intensity of the 2p5 3d6 final states. At low coverages the Mn 3d states become more localized, due to the small interaction with the Ag 4d and Ag 5sp states, which gives rise to an increased intensity of the 2p5 3d5 final states. Concerning effects of atomic multiplet splitting it has been found that these play an important role for the shape of the Mn 2p main line in various compounds. This has been observed for the Heusler alloys, which are ternary alloys of the generic form X2 YZ or XYZ, where Y is Mn and X is, for example, Fe, Co, Ni, Cu, Pd or Rh and Z = Al, Ga, Si, Sn, Sb and In. It has been suggested that the line width of the Mn 2p main line is dependent on the local magnetic moment of the Mn atoms, that is, effects due to exchange splitting are most important [7,8]. We have previously reported detailed studies of the electronic and geometric structure of Pd–Mn bimetallic systems formed on Pd(1 0 0) [9–11]. Using X-ray absorption spectroscopy (XAS) it was demonstrated that Mn atoms in a c(2 × 2) Pd–Mn surface alloy formed on Pd(1 0 0) have a very correlated electronic structure, similar to that of the Mn/Ag and Mn/Cu systems and ionic Mn compounds. This was also the case for a Mn monolayer on Pd(1 0 0) formed at 90 K. Using photoelectron spectroscopy of the valence levels and ab initio calculations it was demonstrated that both the monolayer and antiferromagnetic (2 × 2) alloy were c and we were also able to distinguish between different types of possible c(2 × 2) surface alloy structures. In this paper, we continue the investigation of the electronic structure in well-characterized Pd–Mn systems formed on Pd(1 0 0) by discussing the Mn 2p spectrum in detail. We find that the Mn 2p main line consists of two features that are separated by 1 eV in binding energy. The relative intensities of these states vary depending on the Mn atomic environment, reflecting the changes in the properties of the Mn 3d states rather than the geometric position of the Mn atoms. For Mn coverages ≤1 ML, the highest degree of Mn 3d localization was found for structures in which the
Mn atoms are spread uniformly over the surface. This is discussed in terms of varying Mn–Pd interaction, exchange splitting and magnetic ordering.
2. Experimental The core-level photoemission data were recorded at beamline I311 at MAX II, a third generation electron storage ring at the Swedish national synchrotron facility MAXLAB. I311 is a new undulator-based beamline for studies in the VUV and with soft X-rays [12]. The end station has two separate chambers for measurements and sample preparation, respectively. A large hemispherical electron energy analyzer of Scienta type is used to record the photoelectron spectra. The Mn 2p spectra were measured at a photon energy of 750 eV (unless otherwise stated) and with a total energy resolution of 0.6 eV. The Pd 3d5/2 spectra were recorded with a photon energy of 400 eV and a total resolution of 100 meV. All spectra were obtained at normal emission and the binding energy values are relative to the Fermi edge of the sample. The XAS spectrum was recorded at beamline D1011 at MAXLAB [13] by monitoring the intensity of the Mn Auger features in the kinetic energy interval 500–512 eV using an electron energy analyzer similar to the one at BL I311. The photon energy resolution was set to 0.2 eV. The spectra were recorded at 40◦ incidence relative to the surface normal and the photon energy was calibrated using first and second-order light from the monochromator. The Pd crystal was cleaned by sputtering with 2.5 kV Ar+ ions at 900 K, followed by a 1 min anneal at 1150 K. In addition, several oxygen treatments were used to remove residual carbon. During these, the oxygen pressure was kept at 2 × 10−8 mbar while the temperature was cycled between 400 and 900 K. After oxygen treatments the crystal was flashed to 1100 K. Surface cleanliness was checked by the Pd 3d and the C 1s core-levels. Manganese was evaporated from a flake (Goodfellow, purity 99.95%) firmly placed inside a resistively heated tungsten coil. The evaporation rate was monitored by the shape and intensity of the Mn 2p spectrum, which has previously been correlated to coverages obtained by a thickness monitor (quartz microbalance). However, an amount corresponding to 0.7 ML according to the thickness monitor is close to the amount for one monolayer as inferred from the integrated Mn 2p intensities. Thus, given the well-defined monolayer situation [10], it is likely that the 0.7 ML as read by the thickness monitor in fact corresponds to a slightly higher amount.
3. Results 3.1. Geometric structure of Pd–Mn bimetallic systems on Pd(1 0 0) We will start by going through the different Pd–Mn systems that can be formed on Pd(1 0 0), focusing on the
A. Sandell, A.J. Jaworowski / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 7–14
Pd 3d5/2
Pd(100)-c(2x2)-Mn
Tdep = 90 K
I
hν = 400 eV
Intensity (arb. units)
Intensity (arb. units)
Mn 2p B
S
+1 ML Mn
+0.3 ML Mn
2p1/2
660
Clean Pd(100)
337
9
336
335
334
2p3/2
655 650 645 640 Binding Energy(eV)
635
Fig. 2. Extended Mn 2p photoelectron spectrum for Pd(1 0 0)–c(2×2)–Mn obtained after deposition of 0.7–1.0 ML Mn at room temperature followed by annealing to 570 K for 3 min.
Binding Energy(eV)
Fig. 1. Pd 3d5/2 spectra for clean Pd(1 0 0) and after deposition of increasing amounts of Mn at a substrate temperature of 90 K. The three peaks are due to pure Pd(1 0 0) surface (S), bulk Pd (B) and interface Pd atoms (I), respectively.
geometric properties for Mn coverages up to one monolayer. Fig. 1 shows the coverage dependent behavior of the Pd 3d5/2 line upon deposition of Mn on Pd(1 0 0) at 90 K. The bottom spectrum shows the spectrum for clean Pd(1 0 0) and the surface (S) and bulk (B) components are clearly resolved [14]. After deposition of 0.3 ML Mn at 90 K, it can be seen that the component due to clean Pd surface (S) has vanished and is now replaced by an interface peak (I). The increased BE of the interface peak is an effect of the bonding to Mn [10]. At the monolayer point, the interface peak has shifted more towards higher BE due to the increased Pd–Mn coordination and, at this temperature, the Mn monolayer covers the surface without intermixing with the Pd atoms [10]. A most interesting result is furthermore that the clean surface component has vanished already at 0.3 ML, meaning that at this point all atoms in the outermost Pd layer are directly coordinated to Mn. From this follows that the Mn atoms must be distributed uniformly over the surface, occupying highly coordinated sites (most probably four-fold hollow sites). The behavior at higher temperatures has been discussed extensively in previous work [9–11,15,16]. To summarize, incorporation of Mn atoms occurs at 300 K, forming a c(2 × 2) alloy, and a dramatically improved ordering of the alloy is obtained upon annealing to 570 K. At a Mn coverage of 0.7–1.0 ML, the c(2 × 2) reconstruction covers the whole surface [9–11]. Based on a comparison of the experimental and calculated electronic structures, it was proposed that the annealed alloy is two-layer thick, each of the layers consisting of Pd and Mn atoms arranged in a checkerboard pattern [10]. At submonolayer coverage (0.3 ML), it was
demonstrated that the surface consists of c(2 × 2) Pd–Mn alloy islands separated by (1 × 1) regions consisting of pure Pd [11].Thus, from the above findings it stands clear that by changing the Mn coverage and the temperature, it is possible to vary the distribution of the Mn atoms in the outermost layers in the Pd(1 0 0) host. 3.2. The Mn 2p X-ray photoelectron spectrum: observation of new fine structure The Mn 2p XPS spectrum for the c(2 × 2) alloy formed on Pd(1 0 0) is shown in Fig. 2. The alloy was prepared from an estimated amount of 0.7 ML Mn, which gives a surface fully covered by a c(2 × 2) Pd–Mn layer and, in addition, a sub-surface Pd–Mn alloy layer [10]. The spectrum covers both the 2p3/2 and 2p1/2 components and upon close inspection we see that there are basically two regions with features associated with each spin–orbit level. Focussing on the states associated with 2p3/2 ionization, there are structures between 639–641 eV and a broad feature at 645 eV. The general appearance is very similar to the spectra found for the related Mn/Ag(1 0 0), Mn/Ni(1 1 0) and Cu(1 0 0)–Mn–c(2×2) systems [4–6]. The Mn 2p spectra for these systems typically display two peaks, interpreted as a main line at a BE of about 640 eV and a feature at about 645 eV, assigned to a correlation satellite. Interestingly, the Mn 2p core-level spectrum shown in Fig. 2 also reveals fine structure in that there are two peaks in the main peak BE region around 640 eV. Such a doublet structure has not been observed for the Mn/Cu, Mn/Ni and Mn/Ag systems. One reason is simply that the earlier spectra are of such a quality that a splitting of this magnitude is difficult to discern. There is also the possibility that there are differences in the geometrical structure. For example, the fully developed Pd–Mn c(2 × 2) structure contains
10
A. Sandell, A.J. Jaworowski / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 7–14
Intensity (arb. units)
0.3 ML
0.7 ML
570 K
570 K
300 K
300 K 90 K
646
90 K
644
642
640
638
Binding Energy (eV)
646
644
642
640
638
Binding Energy (eV)
Fig. 3. Mn 2p3/2 spectra for two different Mn coverages prepared at three different temperatures.
and bulk components of clean Pd(1 0 0) [14]. It is therefore unlikely that the existence of a doublet structure is due to two chemically shifted components, such as a sub-surface and a surface peak. How general is this effect? We have also prepared other Mn-containing systems. In Fig. 5, we compare the disperse and complete annealed c(2×2) surface alloys (b and c) with a thick Mn film grown at 300 K (a) and a thin oxide film formed by oxidation and subsequent annealing of a surface completely covered by the c(2 × 2) Pd–Mn alloy (d). Fine structure in the main line region is found for both the thick film and the thin oxide. In the case of the thick film, it is possible that the split is due to a surface core-level shift Mn 2p3/2 0.7 ML Full c(2x2) surface
Intensity (arb. units)
sub-surface as well as surface Mn atoms [10], whereas the Mn atoms in the c(2 × 2) Mn–Cu alloy on Cu(1 0 0) are all situated within the surface layer [17,18]. The doublet structure found for the Pd–Mn c(2×2) system could therefore be the result of having both sub-surface and surface Mn atoms, i.e. atoms that are chemically inequivalent. In order to study the effect of different geometrical positions of the Mn atoms we utilize our ability to change the amount and distribution of Mn atoms within the topmost layers. In Fig. 3, we have collected Mn 2p3/2 spectra for two different Mn coverages, 0.3 and 0.7 ML, measured at three different temperatures. It is very hard to discern a coverage dependent effect when comparing 0.3 and 0.7 ML deposited at 90 K, and, most importantly, the 90 K situations both has doublet structure even though Mn atoms situated on the Pd(1 0 0) surface is the only type of species present. The Mn 2p spectra for the annealed situations are however clearly different, which can be due to the fact that these systems contain chemically inequivalent atoms, i.e. sub-surface and surface atoms. The simultaneous presence of surface and sub-surface guest atoms can in favorable cases be revealed by recording spectra using different photon energies. This changes the kinetic energy of the photoelectrons and hence the surface sensitivity. In addition, kinetic energy dependent diffraction effects may occur which can lead to intensity variations between peaks due to atoms in different geometrical positions. Consequently, changing the photon energy is a very sensitive method for the identification of chemically shifted peaks. Fig. 4 shows Mn 2p spectra recorded at three photon energies for 0.1 and 0.7 ML of Mn. Both systems are the ones obtained after annealing. For each coverage, the changes in spectral shape are very small and using a simple two-component curve fitting, we find that the intensity ratio between the two peaks vary by less than 10%. This variation is much smaller than that found for, e.g. the surface
hν = 770 eV hν = 750 eV hν = 730 eV 0.1 ML c(2x2) islands hν = 770 eV hν = 750 eV hν = 730 eV
641
640 639 Binding Energy (eV)
638
Fig. 4. Mn 2p3/2 spectra, recorded at three different photon energies, for two different Mn coverages prepared by deposition at room temperature followed by annealing to 570 K.
A. Sandell, A.J. Jaworowski / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 7–14
11
Pd(100)-c(2x2)-Mn
Mn 2p3/2
hν = 750 eV
Intensity (arb. units)
Intensity (arb. units)
(d)
(c)
(b)
XAS
XPS
(a)
638
640
642
644
Energy (eV)
644
642 640 638 Binding Energy (eV)
Fig. 5. Mn 2p3/2 spectra for four different systems: (a) a thick Mn film deposited on Pd(1 0 0) at room temperature; (b) 0.3 ML Mn deposited at room temperature and annealed to 570 K; (c) 0.7 ML Mn deposited at room temperature and annealed to 570 K; (d) a MnOx film on Pd(1 0 0), obtained by oxidation of situation (c).
but this is not very likely, given the fact that the SCLS is expected to be smaller than 0.3 eV for Mn as estimated by the well-known method [19]. In the case of the oxide, we cannot exclude that the shoulder at 639 eV binding energy is due to the presence of Mn atoms that are not oxidized, although the oxidation of surface and near-surface atoms with a high tendency for oxidation is expected to be very efficient, as exemplified by the MnOx formation on the Cu(1 0 0) surface, starting from the c(2 × 2) Cu–Mn structure [20], and the growth of a thin Al2 O3 film on the Ni–Al(1 1 0) surface [21]. Consequently, at the present stage we can merely point out that the fine structure found for the Pd–Mn systems can also be present in pure Mn and thin MnOx films formed on metallic substrates. In addition, we have also previously presented Mn 2p spectra for a 1 ML Mn/1 ML Pd/W(1 1 0) sandwich and a two-layer thick c(2 × 2) Pd–Mn alloy on W(1 1 0) formed upon annealing of the sandwich [22]. The Mn 2p spectra for these two systems were nearly identical, exhibiting two leading structures, and the spectral shape is very similar to the spectrum for the c(2 × 2) alloy on Pd(1 0 0). To summarize, we conjecture that the position of the Mn atoms within the Pd matrix has very little effect on the spectral shape for a Mn coverage of 0.7–1 ML. However, forming an alloy by annealing of a Mn amount smaller than 0.7 ML results in an increase of the relative intensity of the sharp low BE component. Likewise, the Mn 2p spectrum for a thick Mn film formed on the Pd(1 0 0) surface shows an increased relative intensity of the low BE peak.
Fig. 6. Mn 2p3/2 photoelectron (XPS) and X-ray absorption (XAS) spectra for 0.7 ML Mn deposited at room temperature and annealed to 570 K. The common energy scale corresponds to the binding energy relative to the Fermi level (XPS) and the absolute photon energy (XAS).
3.3. Final state energetics It is important to be able to identify the final state electronic configurations and to deduce how they are related in energy. When discussing the energetics of core-ionized and core-excited final states, a comparison between the XAS and XPS spectrum has in many cases proven to be most useful [23]. Fig. 6 shows the Mn 2p3/2 XPS spectrum and the Mn 2p3/2 → 3d XAS spectrum on a common energy scale. The energy of the Mn 2p3/2 XPS peaks corresponds to the energy relative to the Fermi level, whereas the energy of the XAS resonance corresponds to the absolute photon energy needed for the Mn 2p3/2 → 3d core excitation. The valence electronic structure of the Mn atoms in the alloy shows intensity reaching down to the Fermi level [10]. It is therefore justified to assume that the XPS peak with lowest (binding) energy corresponds to a final state screened by an electron at the lowest energy state, i.e. at the Fermi level. The consequence of this is that the energy of the lowest-BE XPS peak pinpoints the position of the Fermi level in the XAS spectrum [23]. The XAS process is governed by the dipole selection rule and the XAS spectrum can furthermore to a very good approximation be viewed as valence states projected on the core-excited atom. The XAS spectrum for the c(2 × 2) Pd–Mn alloy show a strong resemblance to the spectrum for Mn2+ in a high spin 3d5 ground state, exhibiting the characteristic atomic multiplet structure [10]. One can therefore assume that the final state created by the 2p → 3d X-ray absorption process is 2p5 3d6 . It is seen in Fig. 6 that the energy regime of the XPS peaks of interest agrees with a 2p5 3d6 FS configuration. In fact, the position of the high BE
12
A. Sandell, A.J. Jaworowski / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 7–14
component agrees with the position of the XAS resonance, whereas the position of the low BE component falls on the XAS onset. Hence, the main line regime is associated with 2p5 3d6 final states, in agreement with the previous studies of Mn compounds and surface alloys [4–6,24,25]. To verify that the state at a binding energy of 645 eV is associated with 2p5 3d5 final states, as previously suggested, one can make a comparison with a recent XPS study of atomic Mn [26]. The final state observed in the Mn 2p spectrum is in this case definitely due to a 2p5 3d5 configuration since charge transfer screening is not possible. The binding energy of the feature at lowest BE given in this work is 650.0 eV versus the vacuum level. In order to compare with our spectra, which are referenced to the Fermi level, one must subtract the appropriate work function. For the c(2 × 2) alloy, the work function has been determined to be about 5 eV, a result obtained from the DFT calculations presented earlier [10,27]. From this follows that the 2p5 3d5 final state configuration is expected to occur at about 645 eV binding energy relative to the Fermi level, which agrees very well with the broad satellite observed in the Mn 2p spectrum for the c(2 × 2) alloy.
4. Discussion From the above results it can be concluded that the presence of the doublet structure at 639–640 eV BE is not due to two chemically inequivalent species. Instead, the structures must be interpreted in terms of main line and satellite states. That the satellite at 1 eV higher BE is due to a 2p5 3d5 final state, in contrast to the 2p5 3d6 final state of the main line, is furthermore contradicted by the energy comparison with the Mn 2p spectrum for the free atom. In the following we will therefore assume that the structures at 639–640 eV binding energy are due to 2p5 3d6 final states and focus the discussion on effects due to non-local versus local screening, variations in the band structure, multiplet splitting and magnetic ordering. The presence of a low-energy satellite peak can be due to two different responses to the core ionization. This model has been employed in order to explain the Ni 2p spectrum of NiO, where a 1 eV split doublet is seen [28]. It was proposed that both peaks stem from final states of the same 3d-count. The low BE component (main line) was attributed to screening from O nearest neighbors and the process was written as 2p6 3dn → 2p6 3dn+1 L where L denotes a hole in ¯ was interpreted the oxygen valence band. The 1¯eV satellite 6 n 6 n+1 as due to a 2p 3d → 2p 3d L process where the extra ¯ d-electron is provided by neighboring NiO6 units. Thus, the term non-local screening was coined to describe the satellite process, as opposed to the local screening associated with the main line. From this follows that variations in the spectral shape may occur even though only one formal final state electron configuration is present.
The scheme with non-local and local screening mechanisms is very attractive, since it gives a very direct picture of the coverage dependent effects seen for the annealed situations (Fig. 3). Within this model, the low BE peak can be associated with a 2p5 3d6 L final state configuration, where L denotes a hole in the Pd valence band (“local screening”). The 1 eV satellite would then correspond to a final state in which screening is provided by neighboring MnPdx units, giving a 2p5 3d6 configuration (“non-local screening”). As previously mentioned, when decreasing the coverage from 0.7 ML down to about 0.3 ML, the situation after annealing changes from a fully reconstructed c(2 × 2) Pd–Mn surface to a surface with disperse c(2 × 2) Pd–Mn islands separated by (1 × 1) Pd parts. Consequently, the average number of neighboring MnPdx units is reduced and hence also the relative intensity of the 1 eV satellite, as seen in Fig. 3. This behavior is thus directly comparable to that found when comparing the Ni 2p spectra for bulk NiO with that of a NiO monolayer/MgO(1 0 0) and dilute Ni in MgO [28]. The processes above can also be discussed more in detail by considering the valence electronic structure, where one can envisage both pure Mn 3d states and Mn 3d states hybridized with Pd 4d states. Calculations show that the occupied valence band, which reaches down to the Fermi level, is rather broad due to mixing of Mn 3d majority spin states and Pd 4d states [10]. The unoccupied part of the DOS dominates by a sharp peak 1.5 eV above the Fermi level, which has a pronounced Mn 3d minority spin character [10]. Since the screening involves population of an initially unoccupied state at the core-ionized site, one must consider the unoccupied density-of-states. The energy cost for the promotion of an electron from the Fermi level to the empty state of strong Mn 3d character agrees in energy with the shift between the main line and the satellite, as confirmed by the direct comparison of the XPS and XAS spectra shown in Fig. 6. In this context, it is important to note that the effect of the Mn 2p hole on the position of the empty Mn 3d states is very small [10]. Consequently, in an attempt to merge the non-local/local screening picture with the band structure picture, one could, very qualitatively, say that local screening produces a FS typical of a metal, that is, the screening electron resides at the Fermi level, where Mn–Pd mixed states dominate the DOS, whereas non-local screening produces an atomic-like 2p5 3d6 FS, that is, the screening electron resides in a pure Mn level, a state which is situated at 1 eV higher binding energy. As previously noted, the presence of atomic multiplet splitting is also expected to affect the spectral shape. In a study of Co2 –Mn–Sn and Pd2 –Mn–Sn Heusler alloys, different shapes of the Mn 2p main line was found, and it was stated that this was partly due to different local magnetic moments, which gives rise to differences in the exchange splitting, and partly due to varying intensity of the sub-levels due to dipole matrix elements effects [7]. It was argued that the latter effect comes into play since the peaks
A. Sandell, A.J. Jaworowski / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 7–14
can be described as direct transitions from Mn 2p states into final state d bands, i.e. it is a way to describe the screening process. Changes in spectral shape due to screening effects have been discussed above, but the exchange splitting has so far not been considered. This effect provides a possible origin to the varying widths of the 1 eV satellite as shown in Fig. 5. It can be observed that the width of the 1 eV satellite in the Mn 2p spectrum for the annealed 0.3 ML situation (Fig. 5b) is narrower than in the spectrum for the annealed 0.7 ML situation (Fig. 5c). This would suggest a smaller exchange splitting for the satellite final state in the case where Mn atoms are situated within disperse c(2 × 2) islands than for the satellite final state when Mn atoms form the fully developed c(2 × 2) alloy. A plausible reason for this is that a decreased hybridization with the surrounding Pd atoms increases the local magnetic moment. If this interpretation is correct, it furthermore implies that the local magnetic moment has increased even further for the oxide film, since this spectrum is considerably broader (Fig. 5d). To summarize the discussion so far, we can state that the Mn–Pd interaction for the disperse and fully developed c(2 × 2) situations are expected to be different and this in turn affects the correlation of the Mn 3d states. An increased correlation gives rise to an increased relative intensity and width of the 1 eV satellite, associated with an increased weight of atomic-like 2p5 3d6 FS and a concomitant increased exchange splitting. The relative intensity of the low BE peak thus shows a minimum at a Mn coverage of around 0.7–1.0 ML, indicating a maximized correlation of the Mn 3d states in this coverage regime. This finding is at variance with the Mn/Cu and Mn/Ag systems, for which a progressively increased localized character was noted with decreasing coverage. The reason for this discrepancy is likely to be caused by different Mn–host interaction strengths; due to their larger overlap in energy, the Mn 3d states are expected to interact more strongly with the Pd 4d states than with the Ag 4d states, prohibiting the localization effect found for the MnAg system. However, another important effect concerns magnetic ordering of the Mn atoms, which can have a strong effect on the localization of the Mn 3d states. It is well known that the magnetic ordering depend on the number of atoms having a magnetic moment and how they are distributed. For instance, if one has small, well-separated islands, the correlation between the islands is reduced as compared to a homogeneous surface and this leads to a lower degree of magnetic ordering. It is therefore not unlikely that the separate c(2 × 2) islands formed at 0.3 ML has a lower degree of magnetic ordering than the uniform alloy formed at 0.7 ML. Likewise, the overlayer formed by deposition of 0.3 ML Mn at 90 K is more uniform than the disperse c(2 × 2) islands since the atoms reside in the same layer and are uniformly distributed over the whole surface. A higher magnetic ordering increases the 3d localization and this is consistent with the finding that the uniform situations show a more localized behavior.
13
Turning finally to the temperature effects of the 0.3 ML situation, it is clear that the 90 K case has Mn situated on top of the Pd surface whereas the annealed situation features separate c(2 × 2) islands, with Mn at the surface as well as in the bulk. Clearly, the Mn–Pd interaction is expected to be stronger upon substitution and alloy formation. Moreover, we have previously stated that the presence of a doublet is not due to having chemically inequivalent Mn atoms. However, it is important to be aware of the fact that the relative intensities of the doublet peaks can vary depending on whether the Mn atom resides in the surface or bulk layer. In fact, calculations indicate that the Mn 3d DOS differ between the bulk and surface atoms; the 3d states of the bulk atoms are more strongly hydridized with the Pd 4d states [10]. Thus, it is feasible that the Mn 3d states of the bulk atoms are more delocalized than the 3d states of the surface atoms. This leads to a low BE component which is more pronounced for the bulk atoms than for the surface atoms, which may account for the 10% variation in relative intensity of the two peaks seen at different photoelectron kinetic energies.
5. Summary We have shown that the Mn 2p spectra of Mn atoms in Pd–Mn bimetallic surface systems formed on Pd(1 0 0) exhibit a satellite, separated 1 eV from the main line, in addition to the previously observed 5 eV satellite. The main line and the low-energy satellite are both assigned to 2p5 3d6 final states, whereas the 5 eV satellite is associated with 2p5 3d5 final states. The identification of the two low-binding energy peaks is based on an energy comparison with the 2p → 3d X-ray absorption spectrum. The relative intensities of the two low-binding energy peaks furthermore vary depending on the Mn atomic environment, reflecting the changes in the properties of the Mn 3d band rather than the geometric position of the Mn atoms. For Mn coverages ≤1 ML, the highest degree of Mn 3d localization was found for structures in which the Mn atoms are spread uniformly over the surface. The observed changes in the Mn 2p spectra can be attributed to varying Mn–Pd interaction, exchange splitting and magnetic ordering.
Acknowledgements The helpfulness of MAXLAB staff is gratefully acknowledged. This work has been financially supported by the Swedish Natural Science Research Council.
References [1] P. Schieffer, C. Krembel, M.C. Hanf, G. Gewinner, Phys. Rev. B 57 (1998) 1141.
14
A. Sandell, A.J. Jaworowski / Journal of Electron Spectroscopy and Related Phenomena 135 (2004) 7–14
[2] P. Schieffer, C. Krembel, M.C. Hanf, G. Gewinner, Surf. Sci. 400 (1998) 95S. [3] N. Mårtensson, A. Nilsson, in: W. Eberhardt (Ed.), Applications of Synchrotron Radiation: High-Resolution Studies of Molecules and Molecular Adsorbates on Surfaces, Springer, Berlin, 1995. [4] P. Schieffer, C. Krembel, M.C. Hanf, G. Gewinner, J. Electron Spectrosc. Rel. Phenom. 104 (1999) 127. [5] O. Rader, T. Mizokawa, A. Fujimori, A. Kimura, Phys. Rev. B 64 (2001) 165414. [6] O. Rader, E. Vescovo, M. Wuttig, D.D. Sarma, S. Blügel, F.J. Himpsel, A. Kimura, K.S. An, T. Mizokawa, A. Fujimori, C. Carbone, Europhys. Lett. 39 (1997) 429. [7] Yu.M. Yarmoshenko, M.I. Katsnelson, E.I. Shreder, E.Z. Kurmaev, A. Slebarski, S. Plogmann, T. Schlathölter, J. Braun, M. Neumann, Eur. Phys. J. B 2 (1998) 1. [8] S. Plogmann, T. Schlathölter, J. Braun, M. Neumann, Yu.M. Yarmoshenko, M.V. Yablonskikh, E.I. Shreder, E.Z. Kurmaev, A. Wrona, A. Slebarski, Phys. Rev. B 60 (1999) 6428. [9] A.J. Jaworowski, S. Gray, M. Evans, P. Uvdal, A. Sandell, Phys. Rev. B 63 (2001) 125401. [10] A. Sandell, P.H. Andersson, E. Holmström, A.J. Jaworowski, L. Nordström, Phys. Rev. B 65 (2002) 035410. [11] A.J. Jaworowski, R. Ásmundsson, P. Uvdal, S.M. Gray, A. Sandell, Surf. Sci. 501 (2002) 83. [12] R. Nyholm, J.N. Andersen, U. Johansson, B.N. Jensen, I. Lindau, Nucl. Instrum. Meth. A 467–468 (2001) 520. [13] J.N. Andersen, O. Björneholm, A. Sandell, R. Nyholm, J. Forsell, L. Thånell, A. Nilsson, N. Mårtensson, Synchrotron Radiat. News 4 (1991) 15.
[14] R. Nyholm, M. Qvarford, J.N. Andersen, S.L. Sorensen, C. Wigren, J. Phys. Cond. Matter 4 (1992) 227. [15] D. Tian, S.C. Wu, F. Jona, P.M. Marcus, Solid State Commun. 70 (1989) 199. [16] D. Tian, R.F. Lin, F. Jona, P.M. Marcus, Solid State Commun. 74 (1990) 1017. [17] T. Flores, M. Hansen, M. Wuttig, Surf. Sci. 279 (1992) 251; M. Wuttig, et al., Surf. Sci. 292 (1993) 189; M. Wuttig, et al., Phys. Rev. B 53 (1996) 7551. [18] M. Wuttig, Y. Gauthier, S. Blügel, Phys. Rev. Lett. 70 (1993) 3619. [19] B. Johansson, N. Mårtensson, Phys. Rev. B 21 (1980) 4427. [20] P.J. Knight, R. Toomes, S.M. Driver, D.P. Woodruff, Surf. Sci. 418 (1998) 521. [21] R.M. Jaeger, H. Kuhlenbeck, H.-J. Freund, M. Wuttig, W. Hoffman, R. Franchy, H. Ibach, Surf. Sci. 259 (1991) 235. [22] A.J. Jaworowski, A. Sandell, Surf. Sci. 477 (2001) 141. [23] N. Mårtensson, A. Nilsson, J. Electron Spectrosc. Rel. Pheom. 75 (1995) 209. [24] A.J. Nelson, J.G. Reynolds, J.W. Roos, J. Vac. Sci. Technol. A 18 (2000) 1072. [25] K. Okada, A. Kotani, J. Phys. Soc. Jpn. 61 (1992) 4619. [26] Ph. Wernet, B. Sonntag, M. Martins, P. Glatzel, B. Obst, P. Zimmermann, Phys. Rev. A 63 (2001) 050702(R). [27] E. Holmström, private communication. [28] M.A. van Veenendal, G.A. Sawatzky, Phys. Rev. Lett. 70 (1993) 2459; D. Alders, F.C. Voogt, T. Hibma, G.A. Sawatzky, Phys. Rev. B 54 (1996) 7716.