CHARACTERIZATION OF THE Yb/Pd (111) INTERFACE BY LIII-SXAS AND 4f PHOTOEMISSION

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CHARACTERIZATION OF THE Yb/Pd (111)

INTERFACE BY LIII-SXAS AND 4f

PHOTOEMISSION

E. Beaurepaire, B. Carrière, D. Chandesris, C. Brouder, G. Krill, P. Légaré, J.

Lecante

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Supplkment au no 12, Tome 49, dkcembre 1988

CHARACTERIZATION OF THE Yb/Pd (111) INTERFACE

BY

PHOTOEMISSION

E. Beaurepaire (I), B. Carrihre (I), D. Chandesris (2), C. Brouder (3), G. Krill (3), P. L6gar6 (*) and J. Lecante (2)

(I) I.P.C.M.S. (UM CNRS 380046), Universite' Louis Pasteur, 4 rue Blaise Pascal, 67070 Strasbourg Cedeq

France

(2) LURE ( L P 08 CNRS), Bdt. 209 D, 91405 Orsay, France

(3) L ~ ~ o T u ~ o ~ T ~ de Physique des Solides (UA 155 CNRS), Universite' de Nancy, BP 239, 54506 Vandeuvre-les-

Nancy, France

(4) Laboratoire de Catalyse et Chimie des Surfaces (UA 423 CNRS), 4 m e Blaise Pascal, 67070 Strasbourg,

France

Abstract. - Surface LIII absorption and UV photoemission experiments are reported for the characterization of the interface between deposited Yb (0.4-10 nm) and a Pd (111) surface. A schematic description of interface formation is proposed combining the two types of experiment and taking advantage of Yb valence sensitivity to the local environment.

Characterization of the interface between a Rare- Earth (RE) and a Transition Metal is essential to the understanding of the electronic properties of artificially modulated compounds (multilayers and/or superlat- tices), which will be of substantial technological inter- est in the near future. In order t o study such proper- ties, it is essential, in the first place, t o obtain accurate information on model systems whose physical proper- ties are well documented. In this work we use the high sensitivity of the Yb valence to any modification of its local surroundings: e.g. in the case of Yb,Pdl-, al- loys, previous work [l] has shown that Yb is divalent in Yb rich alloys and trivalent in P d rich alloys, with a continuous valence change when the alloy composition varies and with a critical composition slightly greater than x = 0.5. This information on the RE valence state can be obtained, with good surface sensitivity if one is interested in the early stage of interface formation, by LIII surface X-ray absorption spectroscopy (SXAS) and UV photoemission (UPS) [2].

Typical spectra for thin Yb films (0.4-10 nm) evap- orated on a P d (111) surface a t T = 300 K in an ul- tra high vacuum environment are reported in figures 1

and 2. The data presented here have been obtained a t LURE (Orsay), at the SEXAFS and PS1 stations. The quantity of Yb evaporated on the Pd (111) sur- face was measured using a quartz microbalance. The equivalent film thickness is hereafter reported in nm, assuming that all evaporated Yb is uniformly spread over the Pd and retains the fcc structure of the (di- valent) metal. Experimental details will be reported elsewhere [3].

The analysis of Yb LIII SXAS data is easy since two absorption structures, characteristic of divalent Yb atoms (4f14 configuration in the ground state) and trivalent Yb (4f13) are resolved around 8935 and 8942 eV, respectively, allowing the determination of the average Yb valence.

Valence band UV photoemission spectra (recorded at a photon energy hv = 160 eV) are dominated by

8910 8920 8930 8940 8950

P h o t o n e n e r g y ( e V )

Fig. 1. - Yb LIII edge spectra recorded at room tempera- ture for different Yb coverages on Pd (111).

Yb 4f lines [4]. These have been separated in figure 2

into yb3+ atomic-like multiplets (peaks labeled A) and two shifted doublets (peaks Bs and Bb) arising from yb2+. The fits presented in the figure are obtained by convolution of the theoretical multipIets line shape [5] with the appropriate Doniach-Sunjic and spectrometer functions [3, 41. From the photon energy dependence of the relative intensity of these lines, the peaks B, are assigned t o the divalent Yb atoms a t the top of the surface 161.

Examination of figure 3 suggests that the interface formed at 300 K is not abrupt. If it were so, the valence

C8

-

U P S hv = 160 e V

B i n d i n g e n e r g y ( e V )

Fig. 2. - Valence band UPS spectra (hv = 160 eV) recorded at room temperature and for various Yb cover- ages on Pd (111). The observed structures are assigned to Yb atoms in different ground states: yb3+ (A), surface- like yb2+ (B,) and bulk-like yb2+ (Bb)

.

3 would be possible only for those Yb atoms directly in contact with Pd, producing a very rapid variation of the mean Yb valence with coverage, particularly sen- sitive in UPS since this technique gives information on the first few atomic layers.

We shall distinguish two steps in the growing pro- cess:

for nominal coverage in the range 0.4-1.6 nm, UPS spectra remain quite similar, with an average Yb va- lence v = 2.5. We believe that this behaviour is the sig- nature of the formation of ~ b : +

-

z below the critical concentration), the yb2+ contri- bution resulting from the atoms at the surface as men- tioned above. The difference between the value of v measured by UPS and LIII SXAS can thus be quali- tatively understood by the different surface sensitivity of these experiments (around 0.7 nm and larger than 10 nm, respectively). The surface character of divalent Yb in this coverage range is confirmed by the large size of B, peaks, as shown in figure 2.

For Yb nominal coverage larger than 1.6 nm, the Yb concentration of the layers just below the surface reaches the critical concentration where the valence transition takes place, provoking the appearance of the

Y b C o v e r a g e ( A )

Fig. 3. - Variation with nominal coverage of the average Yb valence deduced from LIxI (circles) and UPS (squares) data. Open symbols refer to data recorded at 300 K and

full symbols to data recorded at 77 K. The arrows indicate coverage above which bulk-like yb2+ lines (Bb peaks in

Fig. 2) are observed in UPS spectra. Lines are a visual guide.

so-called bulk yb2+ (Bb) and a rapid attenuation of the yb3+ signature.

Therefore, this mechanism is essentially a diffusion process that takes place over a very short time scale. The occurrence of this diffusion process is also strongly suggested by the persistence of the Pd MVV Auger line even a t very high Yb coverage (> 10 nm), indicating the presence of such atoms at the surface.

A qualitative test to evidence the diffusion be- haviour would of course be to perform experiments at lower temperature. The analysis of UPS spectra recorded a t liquid nitrogen temperature is also re- ported in figure 3. It appears that if the general trends are the same as at 300 K, significant differences should be emphasized. In particular, the plateau occurs for lower values of the Yb valence (v = 2.3 instead of 2.5 at 300 K). This shows that the Yb quantity that has diffused in Pd is reduced at 77 K.

[I] Malterre, D., Siari, A., Delcroix, P., Durand, J., Krill, G. and Marchal, G., J. MAgn. Magn. Muter. 63 & 64 (1987) 521.

[2] Rossi, G., Chandesris, D., Roubin, P. and Lecante, J., Phys.

Rev.

desris, D., Krill, G., Lecante, J. and LBgarB, P., to be published.

141 Domke, M., Laubschat, C., Sampathkumaran, E. V., Prietsch, M., Mandel, T., Kaindl, G. and Middelmann, H. U., Phys.

Rev.

[5] Gerken, F., J. Phys. F. 11 (1982) 121.