Straindashinduced enhanced solubility of Au in epitaxial films of Fe

surface science ELSEVIER

Surface Science 364 (19961L505-L510

Surface Science Letters

Strain-induced enhanced solubility of Au in epitaxial films of Fe C.J. P a s t o r

a, C.

L i m o n e s a j . j . H i n a r e j o s a J . M . G a r c i a a R. M i r a n d a a,., J. G 6 m e z - G o f i i b, J.E. O r t e g a c, H . D . A b r u f i a d

a Dpto. de Fisica de la Materia Condensada, Universidad Autonoma de Madrid, Cantoblanco, 28049-Madrid, Spare b Dpto. Fisica Aplicada alas Tecnologias de la Informaci6n, Universidad Politdcnica de Madrid, Crtra. de Valencia, km 7. 28031 Madrid, Spain ° Materialen Fisika Saila, Euskal Herriko Unibertsitatea, Aptdo. 1072, 20080-Donostia-San Sebastidn, Spare d Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, NY14853-1301, USA Received 8 February 1996; accepted for publication 24 April 1996

Abstract

The presence of an Au surfactant oveflayer allows the growth of high-quality Fe thin layers on Au (100) at 420 K to a depth of at least 70 .A. The surfactant layer can be removed from the surface by gentle ion sputtering. After this removal, Au is still found within the Fe film, as judged by a 0.3 eV shift to lower binding energies of the Au 4f photoemission peak. Quantification of the XPS signal indicates that the total atomic concentration of Au diluted within the Fe layer is ~ 3%, i.e. much higher than the bulk solubility at 420 K. We suggest that relaxation of the strain causes the enhanced solubility of Au in the Fe layer. Keywords: Alloys; Gold; Iron; Low energy ion scattering (LEIS); Low index single crystal surfaces; Magnetic films; Magnetic interfaces; Metal-metal magnetic thin film structures; Quantum wells; Scanning tunneling microscopy; Single crystal epitaxy; Surface segregation; X-ray photoelectron spectroscopy

The formation of surface or interface alloys is a topic that has recently attracted widespread interest. It is particularly important for thin films and short-period superlattices since it may influence (and even determine) the resulting properties. In particular, magnetic properties such as interface anisotropy, giant magnetoresistence (GMR), enhanced magnetic moments or reduced Curie temperatures depend sensitively on the extent of intermixing at interfaces [ 1 ]. Very recently it has been reported that several bulk immiscible systems do form surface-confined

* Corresponding author. Fax: + 34 1 3974758; e-mail: miranda@hobbes~fmc.vam.es.

alloys, driven by the fact that the surface is an efficient strain reliever. Examples include Au/Ni(ll0) [2], Pb/Cu(100) [3] and N a / A u ( l l l ) [41. We describe in this letter a set of experimental observations leading us to conclude that strain relief in thin epitaxial films of Fe/Au(100) enhances the solubility of Au in the Fe film by several orders of magnitude with 'respect to the bulk solubility, resulting in a new Fe-Au alloy. Fe/Au(100) is a model system for the study of supported thin ferromagnetic films. It has been claimed that, at room temperature, Fe grows in a layer-by-layer mode on Au(100), in spite of their widely different surface free energies (Fe=2.9 J m -2 and A u = l . 6 J m -2) [5]. This has been concluded from the evolution of Auger signals with

0039-6028/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0039-6028 (96) 00767-4

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C.J. Pastor et aL/Surface Science 364 (1996) LSO5-L510

deposition time [6], and the persistent observation of RHEED oscillations during evaporation [7]. In order to explain the layer-by-layer growth, it has been suggested that a gold over-layer "floats" on the surface during Fe growth, modifying the surface energy balance and keeping the Fe film flat [6,8]. The presence of this "self-surfactant" Au overlayer has been deduced from the persistence of the Auger signal from Au after deposition of 20 equivalent monolayers (ML) of Fe at 500 K [6], and from the energy position of the image state in inverse photoemission spectra (IPS) [8], which remains close to that of Au(100). The Fe film deposited under these conditions is epitaxial, (100)oriented and presents the bcc structure [9]. The epitaxial relation with the Au(100) substrate is such that its lattice is rotated by 45 ° with respect to the substrate, resulting in an excellent (0.6% mismatch) lattice matching. Fe/Au(100) films have revealed interesting new properties such as the existence of quantum-well states [8,1, which are thought to be responsible for oscillatory magnetic coupling [ 10] and giant magnetoresistence (GMR) [11,1. Furthermore, the structural perfection that can be achieved in Fe/Au(100) superlattices has allowed the observation of an oscillatory GMR with a period of oscillation that depends not only on the thickness of the non-magnetic spacers [ 12], but also on the thickness of the magnetic spacer [13-1, an effect predicted by theory but only detectable for superlattices of extreme crystalline perfection. In this paper we show that at 420 K there is indeed a floating Au oveflayer on top of the Fe film, which contains 0.6 ML of Au for a film of 70 ,~ Fe. Upon removal of the Au surface layer we detect a significant amount of Au diluted within the Fe film, well beyond the limit of bulk solubility. The experiments were performed in a UHV chamber with capabilities for X-ray photo electron spectroscopy (XPS) and low energy ion scattering spectroscopy (ISS). Photons of the Mg Ke line were employed for XPS, and a beam of He ÷ ions of 500 eV at a current density of 0.1/zA c m - : was used for ISS. The scattered He + ions were detected by an hemispherical analyzer at a fixed angle of scattering of 135°. The measuring time was kept low in order to minimize sputtering effects.

Additional experiments were carried out in a chamber containing scanning tunnelling microscopy (STM), low energy electron diffraction (LEED) and ultraviolet photoelectron spectroscopy (UPS) capabilities. The Au(100) crystal was cleaned in-situ by sputter annealing until no contamination signal was detected with XPS and the surface displayed spatially extended atomically resolved STM images of the (5 x 20) reconstruction. Fe was deposited from an electron beam evaporator, with the substrate held at 420 K. The Fe evaporator provides a constant flux (2.1 +0.1 A rain -1) over extended times of continuous operation. One monolayer (ML) of Fe is taken to be 1.4 ,~ thick, according to the interlayer spacing of bcc Fe(100). The coverage is estimated to be correct within 15%. We first characterize the formation of the Au surfactant layer and quantify the thickness of the Au overlayer segregated to the surface. In Fig. 1 we display the evolution of the Fe 2p and Au 4f integrated XPS intensities as a function of the evaporation time (bottom scale) or thickness of Fe deposited (top scale). The lines are fits to the data, assuming the canonical model of an exponential increase for Fe (or decay for At]) of the XPS signals with growing thickness of the deposited film. Fe thickness (Angstrom) 0

10

20

30

40

50

60

70

80

Fe/Au(lO0) at 420 K

..a-

§2

.~

0

0

Fe2~

t~ 13.. X Au~f 0

0

i

l

l

10

20

30

40

Evaporation time (min)

Fig. 1. Evolution of the XPS integrated signals of Au 4f and Fe 2p during growth of the Fe film at 420 K. The lines are fits to the data points using an exponential attenuation model (see text) and assuming the presence of a floating Au overlayer.

C.J. Pastoret al./Surface Science364 (1996) L505-L510 The signal of Fe has been fitted to lve(t)=I~[1-exp( Rt/2~45)] where R is the evaporation rate (2.1 A min-1), ,~w~545is the inelastic mean free path ( I M F P ) for electrons of 545 eV of kinetic energy through the Fe film, I ~ is the signal of a very thick Fe film, and t is the evaporation time. The signal of Au has been fitted to co 1168 IAu(t)=IAu[exp(--Rt/2v~ ) ] + I 0 , where ~v~1168 is the I M F P of Au 4f photoelectrons in the Fe film, IA~ is the signal of the clean Au(100) substrate and Io is the XPS signal corresponding to the floating Au overlayer (i.e. the value obtained for ~ 7 0 A Fe). The mean free paths obtained from the fits ~545 _- - 10.1 A and ~1168 _- - 18 A are in excelZ~Fe Z~Fe lent agreement with experimental values previously determined under the same experimental conditions [14]. It must be noted that the signal of Au is still detected for 70 ~t thick deposits. The morphology of the growing film was visualized by means of STM. In the early stages of the growth, there is a place exchange between Fe and Au, whereby the Fe atoms are located underneath the surface. This process is facilitated by the excess of Au atoms present at the reconstructed layer with respect to bulk-like Au(100). The growth of the islands locally removes the (5 x 20) reconstruction, which is characteristic of clean Au(100). Deposition of ~0.2 M L Fe completely destroys the surface reconstruction [15]. Fig. 2 presents a large-scale (1400 A x 1720 A) topograph of a 8/~ thick Fe film annealed to 420 K, illustrating the microstructure of the film. The surface is composed of islands with a density of ~ 8 x 1022 cm -2 and a characteristic lateral size of ,-~80 A. Two monoatomic steps cross the image horizontally while a third one runs vertically. In contrast to other metal-on-metal systems [16], there appears to be no preferential nucleation of islands at step edges. Larger magnification images [15] (not shown) demonstrate that the islands are atomically flat. For the purpose of this work, the most relevant observation is that on each terrace ( ~ 800 A wide on average) only three atomic heights are simultaneously present. The resulting roughness is only 1 A. Based on these observations, we conclude that the film is growing in a layer-by-layer mode under close to ideal conditions. The presence of Au signal in the XPS, even when

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Fig. 2. STM topograph of 8 A of Fe/Au(100) annealed at 420 K and recorded at room temperature. The size of the image is 1400 A x 1720A. The tunnel junction was stabilized at V~=0.55 V and •-0.3 nA. the Fe signal is saturated (see Fig. 1), indicates that there is Au within the escape depth of XPS. This can arise from the proposed floating Au overlayer or from a uniform alloy. In order to ellucidate this, ISS spectra, strictly sensitive to the surface composition, were recorded during the deposition of Fe at 420 K. Some selected spectra are presented in Fig. 3. The evaporation of Fe produces a minor decrease in the Au signal and only a very slow increase of the Fe signal. Even after deposition of 70 A Fe, a clear Au peak is detected. This conclusively demonstrates that Au is in fact present at the external surface o f the film. Fig. 4 presents the time evolution of the intensities of Fe and Au ISS peaks during evaporation. The Au signal decreases to about 54% of its original intensity after deposition of 70 A Fe. Since the roughness of the film does not increase substantially, as demonstrated by the STM study described above, one can estimate that ,~ 0.54 __0.06 M L Au is present at the external surface under these conditions. The surface composition estimated from the decrease of the Au signal is consistent with that obtained from the absolute values of Au and Fe signals and the corresponding scattering cross-sections [17]. This

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C J. Pastor et al./Surface Science 364 (1996) L505-L510

FelAu(100) at 420 K

Au

~.

He*, E~ = 500 eV

~

=c

0, = 135 °

-g _=

.

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ii

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Fe Clean Au(lO0)

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removed by a brief and gentle ion sputtering. This was accomplished by Ar + (1/tA cm 2, 500eV, 7 min), eroding --~2 M L of material from the surface. As shown in the lower part of Fig. 3, the Au signal essentially disappears from the ISS spectrum upon sputtering (e.g. its intensity falls to only 4.7% of the initial value, while the Fe signal increases to a value which is 52% larger shall the initial value). This, in turn, confirms that 0.5-0.6 M L Au is present at the surface after deposition of 70 it Fe at 420 K. We now turn to the characterization of the film underneath the floating Au layer. XPS analysis of the sputtered sample after removal of the Au overlayer unexpectedly shows peaks corresponding to Au, as shown in Figl 5. The Au 4f peaks are half as intense as those of the Au overlayer, and are shifted to lower binding energies by 0.33 eV with respect to those of Au(100) and the floating overlayer. Since the Fe film is --~70 A thick, i.e. much thicker than the escape depth of Au photoelectrons ( ~ 18 A), the signal cannot be due to the Au(100) substrate. If the remaining XPS signal were due to Au at the surface, it would correspond to 0.3 ML, a surface coverage well above that detected by ISS (0.05 ML). We thus conclude that

~--

+7 rain sputtering 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

E/E o

Fig. 3. RepresentativeISS spectra recorded during deposition of Fe at 420 K onto Au(100). The lower spectrum reproduces the result after grazingAr + sputteringof the 70 A Ire evaporated Au(100) substrate. The Au signalis almost completelyremoved. segregated surface layer acts as a kind of selfsurfactant and induces the layer-by-layer growth of Fe. The segregation is probably driven by the large difference in surface energy between Au and Fe. The Au overlayer at the external surface can be

Fe thickness (Angstrom) 0

10

20

30

50000

40

50

60

70

80

i 0

Fe/Au(100)

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000000

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i

i

I

10

20

30

40

Evaporation time (rain)

Fig. 4. Intensity of the Au and Fe ISS signals as a function of the amount of Fe evaporated on Au(100) at 420 K.

C.Z Pastor et aL/Surface Science 364 (1996) L505-L510

can write

i Au,,

Fe/Au(100)

at 420 K

Iau IF¢

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x

'

A

f

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t

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V

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1150

1 1 5 5 1 1 6 0 1165 1170 1175 Kinetic energy (oV)

Fig. 5. Au 4fXPS spectra of(a) clean Au(100) (b) clean Au(100) plus 70 ,~ Fe evaporated at 420 K, and (c) after 7 rain of grazing Ar + sputtering (500 eV). The shift observed in this latter case is AE =0.33 eV. The small peak at 1161 eV of kinetic energy is the Fe 3s level.

Au is dissolved inside the Fe film. The different chemical environment of Au within the Fe matrix could explain the shift in the binding energy of the Au 4f peaks with respect to those of the floating layer. The total amount of Au within the Fe film can be quantified using a method developed for uniform alloys [18]. Following this approach, the intensity of the Au 4f XPS peak is given by IAu = F o N A u f l ( A u 4f)~22anoyDo(l168), 1168

(1)

where Fo is the X-ray flux, NA~ is the density of Au atoms in the alloy, # is the cross-section for photoionization, £2 is the solid angle of acceptance of the detector, Z~alloy ~t168 is the mean free path of 1168 eV electrons in the (mostly Ire) alloy, and Do(I158) is the analyzer transmission at this electron energy. For the Fe 2p peak, it follows that IFe -= FoNv, fl(Fe 2p)f22anoyDo(545). 545

(2)

Taking into account the corresponding expressions for the intensity of pure crystals, Iv°~ and In~, one

oe IAu

NAu -

Nre

x

~

Ire

]1168

NF~ x

- -

NA~u

/t,alloy x

~1168

";Au

,

(3)

where In,JIr~ and I A~/IF~ are taken from the experiment, NF~/NA~u is the ratio of bulk atomic densities, Z~alloy ;1168 18 A since the alloy is mostly Fe, and "~Au~1168----14 A [19]. With these data, we obtain NAu/NF¢ =0.03, implying that the alloy contains 3% of Au atoms dissolved into the bcc Fe matrix. This number is fully consistent with the Au concentration at the surface detected by ISS, and further demonstrates that a uniform, dilute alloy has been formcd. The almost linear decrease of the surface concentration of Au (see Fig. 4) also indicates that Au is lost at a constant rate, i.e. we can expect a certain amount of Au uniformly diluted inside the Fe film. It should be mentioned that the existence of this dilute alloy of Au in the Fc film might explain the poor agreement between theoretical calculations of LEED I-Vspectra and experimental curves previously reported [20]. Although from a quantitative LEED analysis a bcc structure was concluded for the Fc film, the value of the Pendry R factor (Rp = 0.52) was unacceptable, clearly indicating that some important structural feature was missing from the model [20]. The solubility of Au in bulk bcc Fe at the maximum temperature achieved is negligible (< 10-5%) [21], while we have detected a concentration of a few per cent in a thin film. In the low dilution limit, the solubility is basically limited by the strain induced by foreign atoms in the crystal matrix. Two foreign atoms will repel each other due to the overlap of the strain fields, and consequently the system accommodates only a limited concentration at a given temperature. Atomic-size mismatch tends to render the elements immiscible in the bulk. Therefore the presence of strainrelieving mechanisms in the bulk, such as dislocations or grain boundaries, lead to higlaer local concentrations of diluted atoms. Since it is known that the surface allows a rapid strain relief, the surface solubility is greatly enhanced with respect to the bulk, and one can find surface-confined alloying of immiscible elements of different size [22]. For thin-film pseudomorphic growth, strain

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CJ. Pastor et al./Surface Science 364 (1996) L505-L510

energy is accumulated in the film. In cases such as Fe/Au(100), the tensile strain in the bcc Fe film can be relaxed by including larger atoms such as Au in the structure. We suggest that this relaxation of the strain leads to the observed enhanced solubility of Au in the Fe matrix. In summary, we observe the layer-by-layer growth of Fe films on Au(100) at 420 K due to the presence of an Au overlayer on top of the system, which acts as a surfactant. We have detected the presence of Au diluted in the Fe film, at levels which are orders of magnitude above the bulk solubility at 420 K. ISS curves and the simple intensity analysis of the XPS signals reveal a homogeneous distribution of the diluted Au, which originates from the continuous loss of Au atoms in the overlayer and from a small diffusion from the substrate.

Acknowledgements This work has been financed by the DGICyT through projects PB94-1527 and PB92-0167. One of us (J.E.O) thanks the Basque Government for financial support.

References [ 1] For a general view, see for instance, J. Magn. Magn. Mater. 121 (1993).

[2] L. Pleth Neilsen et al., Phys. Rev. Lett. 71 (1993) 754. [3] C. Nagl, E. Platzgummer, O. Haller, M. Schmid and P. Varga, Surf. Sci. 331-333 (1995) 831. [4] J.V. Barth, H. Brune, R. Schuster and G. Ertl, Surf. Sci. 292 (1993) L769. [5] L.Z. Mezey and J. Giber, Jpn. J. Appl. Phys. 21 (1982) 1569. [6] S.D. Bader and E.R. Moog, J. Appl. Phys. 61 (1987) 3729. [7] Y. Suzuki, T. Katayama, S. Yoshida and K. Tanaka, Phys. Rev. Lett. 68 (1992) 3355. [8] F.J. Himpsel, Phys. Rev. B 44 (1991) 5966. [-9] E.F. Wassermann and H.P. Jablonski, Surf. Sci. 22 (1970) 69. [10] J.J. de Miguel, A. Cebollada, J.M. Gallego, R. Miranda, C.M~ Schneider, P. Schuster and J. Kirschner, J. Magn. Magn. Mater. 93 (1991) 1. [113 S.S.P. Parkin, N. More and K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [12] K. Shintaku, Y. Daitoh and T. Shinjo, Phys. Rev. B 47 (1993) 14584. [13] S.N. Okuno and K. Inomata, Phys. Rev. B 51 (1995) 6139. [-14] J. Alvarez, J.J. Hinarejos, E.G. Michel, G.R. Castro and R. Miranda, Phys. Rev. B 45 (1992) 14042. [15] J.M. Garcia et al., unpublished. [16] J. de la Figuera, M.A. Huerta-Garnica, J.E. Prieto, C. Ocal and R. Miranda, Appl. Phys. Lett. 66 (1995) 1006. [17] W. Heiland and E. Taglauer, Nucl. Instrum. Methods 132 (1976) 535. [-18] C.J. Powell and M.P. Seah, J. Vac. Sci. Technol. A 8 (1990) 735. [19] S. Evans, R.G. Pritchard and J.M. Thomas, J. Phys. C 10 (1977) 2483. [20] A.M. Begley, S.K. Kim, J. Quinn, F. Jona, H. Over and P.M. Marcus, Phys. Rev. B 48 (1993) 1779. [21] H. Okamoto, T.B. Massalski, L.J. Swartzendruber and P.A. Beck, Bull. Alloy Phase Diagrams 5 (6) (1984). [22] J. Tersoff, Phys. Rev. Lett. 74 (1995) 434.