Initial growth and structure of Mn on Ag(100): Formation of a superficial alloy

Solid State Communications, Vol. 97, No. 9, pp. 757-761, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 003%1098/96 $12.00+.00

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0038-1098(95)00756-3 INITIAL GROWTH AND STRUCTURE OF Mn ON Ag( 100): FORMATION OF A SUPERFICIAL ALLOY P.Schieffer, C.Krembel, M.C.Hanf, D.Bolmont, and G.Gewinner Laboratoire de Physique et de Spectroscopic Electronique, URA CNRS 1435, 4 rue des F&es Lumiere, 68093 Mulhouse CCdex, France

(Received 6 September 1995; accepted 3 1 October 1995 by G. Bastard)

We have investigated the crystallographic structure of ultra-thin Mn films deposited at room temperature on a clean Ag(lOO) single crystal, using angle-resolved ultra-violet photoemission, X-ray photoelectron diffraction (XPD) and low-energy electron diffraction. The evolution of the Ag 4d valence band shows deviations from simple layer by layer growth. These results, combined with the XPD data, indicate that by - l-l .5 monolayers of Mn a two layers thick Mn-rich epitaxial MnAg alloy is formed that continues the face centered cubic lattice of the Ag substrate. Keywords: A. metals, A. thin films, A. surfaces and interfaces, E. photoelectron spectroscopies

Properties of thin film and metastable phases of magnetic 3d transition metals have generated considerable theoretical and experimental interest. The restricted geometry in thin films may lead to significant modifications

concluded that a well ordered (1x1) monolayer of Mn grows on silver at 44OK, while at RT, their results were consistent with multilayer formation [7]. In the present study, we present photoemission and XPD data which demonstrate for the first time the presence at RT of a diffuse Mn/Ag( 100) interface. Approximately 1.5 ML is found to intermix with the Ag and forms a two layer thick interfacial film where both Mn and Ag occupy face-centered cubic positions of the Ag lattice.

in the electronic properties as compared to the relevant bulk material. Typical examples are face-centered cubic (fee) Fe [l] and Co [2] which can be formed at room temperature on Cu(100) substrates. Among the transitions metals, Mn is a very interesting candidate since it presents a rich variety of crystallographic phases: the room temperature (RT) phase

All experiments

(a) is complex body-centered cubic @cc) with 29 atoms per

were carried out in an UHV

apparatus (base pressure <2.10-‘@I’)

primitive cell, the cubic p phase with 20 atoms per unit cell

equipped

with

standard low-energy electron diffraction (LEED), angle resolved ultra-violet photoemission (ARUPS) and X-ray photoelectron diffraction (XPD). For UPS, typical resolutions are -3’ in angle and 15OmeV in energy. The Ag(lOO) single crystal (miscut ~0.3” as verified by Laue diffraction) was mechanically and chemically polished, then cleaned in situ by Ar-ion sputtering and annealing at -600°C until the surface exhibited a sharp (1x1) LEED pattern with low background intensity and no impurities were detectable by XPS. Mn was deposited onto clean ordered Ag( 100) from a home-made molecular-beam epitaxy (MBE) metal vapor source operating at a typical rate of -0.3ML/min

is stable between 8OO’Cand 1lOO’C,the y-(fcc) and &(bcc) modifications are stable respectively above 1085°C and 114O’C [3]. There are only limited investigations of the structure and morphology of thin manganese films on Ag(lOO). In earlier work, G.A.Prinz and co-workers observed a body-centered tetragonal structure with a c/a ratio of - 1.13 for 14 ML Mn in Mn/Ag( 100) supperlattices [4,5]. Egelhoff et al, using X-ray photoelectron diffraction, found a similar crystal structure of the Mn films grown at 80K with contraction in layer spacing for the lowest coverages [6]. More recently, J.E. Ortega and F.J. Himpsel 757

758

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FORMATION OF A SUPERFICIAL ALLOY

pressure of -5.10-*cT. The amount of deposited Mn was

this BE window. Let us concentrate first on the changes in the overall shape of the Ag4d band brought about by Mn

monitored by means of a carefully calibrated quartz balance and by XPS. The data presented here are for a substrate

deposition. Of particular relevance here is a small but measurable shift to higher binding energy of the peak

held at room temperature (RT).

observed at 4.85 eV in the Ag4d band. This reflects major modifications in the Ag substrate electronic structure and

Figure 1 presents a series of ultra-violet photoemission spectroscopy spectra taken at a polar angle

points towards a sizable narrowing of the Ag4d band. Note also the faster attenuation of the 4.85eV peak as compared

of 20’ in the Ag(Ol0) plane with ha=2 1.22 eV photons for

to the other 4d features. This suggests that the Mn growth mode is not simply layer by layer and that no Mn islands or

various Mn coverages on Ag( 100) surface held at RT. Prominent structures in the 3.8-8eV binding energy (BE)

multilayers are formed. In this respect, in previous work on the Cr/Ag(lOO) system [8], we found that upon Cr

range reflect emission from Ag4d valence band. As can be seen from emission for clean Ag( loo), there is a wide BE

deposition the Ag4d bulk band is attenuated but without any shift in binding energy and this interface was shown to be

window of -35eV below EF, where the spectrum shows just a very low featureless background due to incoherent emission from the wide AgSs band. This is a favorable situation, since the Mn3d features are expected to appear in

sharp. For example, we present in Fig.1 the spectrum of 1ML Cr deposited on Ag(lOO) held at 440K (dashed

(1ML equivalent to the surface density of Ag( 100)) and at a

curve).

On the other hand,

striking

valence

band

modifications, in particular, peak shifts were observed for Cr/Au(lOO) and were explained in terms of the formation of a substitutional Auloo_,& fee alloy (x<50) [9]. So our ARUPS data suggest a diffuse interface. Note that some intermixing is in line with the bulk phase diagram indicating that Mn and Ag are miscible. Actually similar photoemission Ag4d band spectra have been reported for AgMn bulk alloys. It was clearly observed that Ag-Mn alloying results in an increase of the intensity in the flat s-p band region accompanied by a narrowing of the Ag4d [ 10,1 11. Hence limited interdiffision probably takes place at the Mn/Ag interface formed at RT. Support of this statement comes from a closer examination of the flat sp band region in O-3.8eV BE range. Figure 2 displays a series of ultra-violet spectroscopy spectra for 1ML deposited at RT and for various polar angles. The emission plane is Ag(Ol0) and the photon

I

I

E,

1

1

I

I

I

I

4 5 6 2 3 BINDING ENERGY (ev,

I

I

7

8

Figure1 : Valence band photoemission spectra taken with unpdarized He1 radiation at a polar angle 8=20” for various Mn covemges on Ag(100) held at RT The light incidence is 45O and the collection plane Ag(O10). The dashed line corresponds to the spectrum of 1ML Cr on Ag(100) held at 440K shown for comparison.

energy of 16.8eV rather than 21.2eV has been selected here in order to avoid interference with Ag4d emission from the satellites at 23.09eV and 23.75eV that accompanies the He1 radiation. We show here an enlarged view of the 0-3eV spectral region where possible Mn3d induced features are expected at these energies. Some peaks due to direct transitions from AgSs bulk band are indicated by arrows. It can be seen that a 1eV wide shoulder which has no counterpart in the clean Ag spectrum appears near 2.8eV This feature is visible at all polar angles of emission with little dispersion and has essentially the same width and position as observed in previous photoemission studies of MnAg alloys [ 11,121. Hence the simplest way to account for our observations is to interpret the Mn induced feature at -2.8eV as emission from Mn3d states of weakly interacting Mn atoms in a superficial alloy. The 2.8eV feature is clearly visible up to a coverage of approximately 2ML. This seems

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FORMATION OF A SUPERFICIAL ALLOY

759

reflect emission from Mn in a bulklike environment. As Polar Angle

shown elsewhere [ 131 the persistence of Ag feature indicates Ag segregation at the surface of the film. The evolution of the valence band curves may be understood in the following way: intermixing occurs at the interface upon Mn deposition up to -lSML, the Mn atoms involved in the film having less Mn atoms as nearest neighbors and Mn-Mn interatomic distances substantially larger than in Mn bulk. With further deposition, the coordination of the Mn atoms increases and the distance between two nearest neighbours drops to a bulk value (-2.6eV). As a consequence a Mn3d electronic structure appears that is characteristic of a bulklike environment with a broad structure near 2SeV and emission at the Fermi level. Thus Mn growth cannot be interpreted in terms of 3d islands growth where strong MnMn interactions would take place. In that case the broad feature near 2.5eV characteristic of thick Mn films thicker than 2ML would be visible as soon as -1ML is deposited. Clearly up to -l-2ML interdiffusion must occur at the Mn/Ag interface formed at RT in order to explain the valence band photoemission data.

I E,

I 1

I 2

I 3

BINDING

I 4

I 5

I 6

I 7

I 8

ENERGY (eV)

Figure 2 : Valence band spectra photoemission taken with unpolarized NeI radiation for 1ML Mn deposited on Ag(100) held at RT, as a function of polar angle of emission in the Ag(O10) plane. We also show in greater detail the O4eV region where the Mn3d induced features are expected.

also inconsistent with the presence of a sharp interface since in that case bulk-like Mn should be formed by 2ML with a several eV wide Mn3d band because of strong Mn-Mn interactions. In this respect, considering the sp region of the spectra in fig. 1, we note that up to 2ML there is no evidence of any distinct Mn structure near the Fermi level E,, and that deposition of Mn increases only the background emission. Again, this indicates that the Mn growth mode is not of the multilayer type, as 3d Mn islands would present a peak near EF in their density of states as observed for bulklike Mn. Hence a marked structure at E, would be seen. In fact, this structure becomes clearly visible in figure 1 for cover-ages higher than 2ML, where a bulk-like Mn phase is then formed. Indeed, on further increasing the Mn coverage, Mn3d induced structures appear at -2SeV and at EP suggesting a qualitative change in the growth mode. Since these additionnal features are still visible at higher coverages and characteristic of thick Mn layers, they must

At this stage let us mention our LEED observations. Upon Mn deposition at RT, the LEED pattern shows a ~(2x2) superstructure from 0.5 up to 3ML Mn with a low background with respect to clean Ag( 100). The intensity of the (l/2, l/2) spots is maximum at around 2 ML and with further deposition decreases rapidly. A structural model for this superstructure cannot yet be proposed but we merely note here that a similar ~(2x2) structure has been already observed for Mn formed on (100) surfaces of Cu [14], Ni[15], Pd [16] and was attributed to surface alloy formation. Moreover we note the following points: first, the quality of the diagram reveals the presence of a long-range crystallographic order in films with coverages of l-2ML Mn, second, the integral-order beams are located at the same angular positions as for clean Ag(100). This means that the films formed have the same in-plane lattice spacing as the Ag(100) substratei.e. they are epitaxial.

In order to get more information about the crystallographic structure of the surface alloy, we have performed XPD measurements. This technique has proven to be a powerful method in surface stucture studies [17]. The XPD curves present intensity maxima corresponding to the nearest neighbors bond directions in the sample. These maxima can be used to probe directly the local crystallographic structure of a given species. Figure 3 presents the Mn2p312 core level intensity modulations as a function of polar angle along [IO] and [ll] azimuths mesh) for various (referred to the square surface coverages. Up to -1.5ML, the XPD curves in figure 3a

Vol. 97, No. 9

FORMATION OF A SUPERFICIAL ALLOY To obtain

a better

understanding

of

the

crystallographic environment of the Mn atoms, we have compared the Mn 2p modulations to the curves obtained for fee Ag( 100) single crystal. The curves are well structured and exhibit the well characteristic forward peaks at O”, 35”, 54” along [lo], 0”and 45” along [ 1l] of a fee crystal [ 181.It is apparent that, up to 2ML, the Mn modulations closely resemble those of clean Ag with identical peak positions. These results indicate that the crystallographic environment of the Mn atoms is fee as for the Ag atoms in the Ag substrate. Thus, since the MnAg films are matched in-plane with the Ag( 100) as shown by LEED, the vertical spacing must be also the same as in the Ag lattice. Keeping in mind the ARUPS data, this indicates the formation of a fee intermixed Mn-Ag film which preserves the Ag fee structure, i.e., Mn occupies fee lattice sites. However a major point is that there is no peak at O”for coverages up to

-10

0

10

20

30

40

POLAR ANGLE

Figure 3 : Angular distributions intensity for various Mn layers taken respectively: (a) along the [1 l] azimuth. The azimuth is

50

60

70

(deg)

of the Mn2p3/2 core level deposited on Ag( 100) and [lo] azimuth; (b) along the referenced to the square

surface mesh. Also shown for comparison are the XPD profiles of the Ag(100) substrate.

1SML and this feature is still weak by 2 ML. Clearly this means that Mn is essentially located in the first and second topmost layers of the Mn/Ag film. This indicates that the Mn-Ag interfacial superficial alloy formed by 1.5ML Mn deposited at RT must be Mn rich and cannot be viewed as a dilute Ag based Mn alloy. Now on increasing Mn coverage, the forward scattering peak at 0’ appears indicating that third layers sites on top of the Mn-Ag film are filled. An essentially pure bulk like Mn phase starts to grow epitaxially on top of the intermixed film. Above two 2ML, as stated above, both ARUPS and XPD measurements reflect this change in growth mode. In particular one observes shifts towards larger angles in XPD features that can be interpreted as a reduction in vertical interlayer spacing i.e. growth of a body centered tetragonal phase. In agreement with previous work [4,6] we obtain c/a = 1.13 from observed angular shifts for this bulk-like Mn phase.

(azimuth [I 11) exhibit a well marked feature at 45’. For a 2ML Mn coverages, an additional small peak is observed at OO.With increasing Mn coverage, the peak located at 45” shifts towards larger angles indicating a different behavior. These data confirm the qualitative change in mode of growth observed in valence band photoemission. For coverages above -2ML the XPD curves resemble those measured by Egelhoff et al [6] and correspond to a bulk-like body-centered tetragonal (bet) form of Mn [4,6). These data will be discussed in detail elsewhere [13]. Let us concentrate here on the data for low coverages (
In conclusion, we have investigated the initial growth mode at RT of Mn on Ag. Valence band photoemission studies indicate that a simple layer by layer cannot account for the observations below -2ML coverage. In particular we observed a perturbation of the Ag4d bulk band and a atomic-like Mn3d induced feature that suggests alloying. These results combined with XPD data clearly indicate that intermixing occurs at the Mn/Ag( 100) interface formed at RT but is limited to two atomic layers. Since by - 1.SML the atoms must essentially reside in the two topmost layers, we are forced to conclude that an Mn rich Mnx AglOo.xfccalloy (x-0.75) alloy that adopts the Ag fee structure must be formed. This implies an Mn-Mn nearest neighbor distances of 2.88A as opposed to 2.61A in bulklike Mn formed at higher coverages with drastically reduced Mn3d overlap and probably an enhanced atomic like magnetic moment.

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