Pd(111) interface: spectroscopic and structural studies

Surfacc Sc cncc 269/270 (19921 731-736 North-Holland

"surface science

The Eu/Pd(lll) interface: spectroscopic and structural studies F. B e r t r a n ", T. G o u r i e u x ", G. Krill ", M. A l n o t b, j . j . E h r h a r d t b a n d W. Felsch ¢ " Laboratotre de Ph~,~que du Sohde URA 155. Unn erszt~ de Nancy I, B P 239, 54506 Vandoeuvre-lds-Nancy, France t, Laboratowe Maurtce Letort, UPR CNRS 6851, Route de Vandoeut re, 54600 l,Tllers-I&-Nancy, France c Erste~ Phystkah~ches lnstttut, Utm ersttat Gottmgen, Bunsenstrasse 9, D-3400 Gbttmgen, Germany Recewed 2 August 1991, accepted for pubhcatmn 11 September 1991

Growth of Eu ultra thin films on a Pd(111) single crystal, kept at room temperature, has been stu&ed by spectroscopic (XPS, UPS, AES) and structural ( R H E E D ) methods Eu atoms of the first two deposited monolayers are dwalent and form a p(2 × 2) arrangement on the Pd(! 11) surface Durmg the completion of the third Eu monolayer, th~s ordered structure smears out m connection with the appearance of trwalent Eu atoms at the interface. The fine structure mo&ficatmns of Pd and valence band XPS spectra indicate the 4d band filhng, wa a charge transfer mechanism, of the Pd atoms revolved m the interface For thinker Eu coverages a diffusion process leads to the formation of &sordered dwalent E u - P d alloys After anneahng the sample at 820 K, R H E E D diffraction patterns reappear (p(2 x 2)) and, s,multaneously, an increase m the Eu mean valence is observed. XPS results show that segregation of divalent Eu occurs at the surface of the new ordered bulk-trwalent E u -P d compound which forms at the interface This behavmur occurs whatever the thickness of the Eu deposit is

1. Introduction In the past few years an increased number of rare earth ( R E ) / t r a n s m o n metal (TM) interfaces have been studied by evaporating ultra thin films of RE on a TM single crystal. One reason for this interest is related to the magnetic and electronic properties one can observe on R E / T M multilayers or superlattices (e.g. T b / F e [1]). As pointed out before [2], when the two metals involved are known to form intermetallic compounds, the formation of a compound or an alloy is expected to take place at the interface, depending on the substrate temperature and the thickness of the RE overlayer (e.g. Yb/N1 [2-4], Y b / P d [5], S m / C u [6], N d / C u [7,81. . . . ) t he E u - P d system is a smtabie case for studymg such effects because of ~ts rich phase diagram [9] and because of thc existence of amorphous alloys over a wide range of concentration [10]. Due to the small energy difference between the 4f" and the 4f "+~ configurations, Eu can be rather divalent or trivalent depending on its chemmal environment: it is found to be divalent

m the bulK for Eu metal, EuPd and EuPd 2, and trivalent in the bulk for EuPd 3 and EuPd 5. In the case of amorphous alloys, there is a mixture of divalent and trivalent Eu s~tes leading to an heterogeneous mixed valence, the mean value of which depends on thc Eu concentration A ~harp transition of this Eu mean valence from 3 to 2 is found around a 27% Eu concentration (i.e. just above the concentration of the crystalline EuPd s compound). In the present work the behaviour of the E u / P d ( l l l ) interface is studied as a function of the substrate temperature and the thickness of deposited Eu. This work is based on spectroscopic (XPS, UPS and AES) and structural ( R H E E D ) experiments. This paper contains two main parts. I ne first one ~ oeu0tcu tu the iuuJn temperature stud}': m th~s part, the roam results obtained for very low Eu coverages, which have been already published [11], will be summarized. Then, the case of thicker Eu coverages w£: be discussed. The second part concerns the results obtained after annealing the Eu films which were deposited at room temperature.

0039-6028/92/$05 00 ,(, 1992 - Elsevier Smencc Pubhshers B V All rights reserved

732

F Bertran et al / The Eu / P d ( l l l ) interface

The experimental set-up for the evaporated films has been described in detail m ref. [11]. The XPS (AI K a ) results were obtained with a resolution of 1.0 eV and the binding energies (BE) refer to the Fermi edge of the clean P d ( l l l ) surface. The thickness of evaporated Eu is given assuming a Eu density of 5.25. In order to have reference spectra, EuPd v polycrystalline compounds (y = 1, 2, 3, 5), whose structure was checked by X-ray diffraction, were prepared by arc melting in reduced Ar atmosphere.

2. Room temperature deposition

2.1. E l l / P d ( l l l ) mterface: low Eu corerages When Eu coverage was in the submonolayer range, reflection high energy electron diffraction ( R H E E D ) showed that a p(2 × 2) overlayer structure develops on the P d U l l ) surface (see fig. 1

and ref. [11]). The Eu 3d and 4d X-ray photoelectron spectra obtained for these low coverages revealed a dwalent Eu electronic state, in agreement with Johansson's calculations [12] which indicate that the Eu 3+ configuration is not stable at the surface. In fact, adsorption of Eu atoms on a Pd surface is a rather peculiar situation because the formation of a surface alloy (i.e. strong 4 d - 4 f interactions) was expected instead of adsorption. In order to complete this information He II photoemission ( h u = 40.8 eV) was performed. Because of the very small escape d e p t h ( ~ 4 ,~) of the emitted photoelectrons, most of the signal obtained with this photon energy is a measure of the density of states near the surface. The introduction of any adsorbate modifies these surface states and thus the He II spectrum. According to Louie [13], because these changes are due to the removal of surface states and resonances, they must not be very sensitive to the nature of the

Fig I RHEED diffraction patterns obtained m the [112] real space direction at different Eu coverages and substrate temperatures. The same symmetry was ,,bsem,ed m the [CH]] real space d~rectlon (a) Pd substrate, (b) 3 A Eu, (c) 24 A Eu and anneahng at 620 K. (d) 24 A Eu and anneahng at 820 K

F Bertran et al / The E u / P d ( l l l ) mte;fa¢e

733

E u - P d disordered interface was formed. Thls was also observed for a polycrystalhne Pd subs t r a t e [ 14].

2.2. Eu / Pd(l l l) mterface: "thick" Eu cot'erages "7

b/ 0

c/

0

~ . 1 .

-10

L

-8

l

.l.~-J---~

-6 -4 -2 Bmdmo energy (eV)

0

2

Fig 2. Difference between the P d ( l l l ) H e l l spectrum and the He II spectrum taken after del~osmon of. (a) H on Pd(111 ) Lome's calculatmn, {b) H on P d ( l l l ) experiment, (c) Eu on P d ( l i l ) wtth a coverage 0 = 0 2 F,gs 2a and 2b are taken from ref [13] Fhe zero binding energy refers to the P d ( l l l ) Fermi edge

adsorbate. The condition is of course that the adsorbatc itself doe,, not contribute m a s~gmficant way to the s~gnal (i e. the coverage is very low). A confirmation o f Louie's calculations is p r e s e n t e d in fig. 2: the H e II photoemisalon diff e r e n c e spectra obtained for a Eu coverage 0 = 0.2 (0 = 1 when a p(2 x 2) E u layer is c o m p l e t e d on the P d ( l l l ) surface) is in accordance with the case of H on Pd(111). For further Eu coverages the streaky R H E E D diffraction patterns and A u g e r intensities measu r e m e n t s led to an epitaxial layer by layer growth model up to 0 = 2 H o w e v e r this p(2 × 2) str, mture became instab!e during the third mono!ayer growth. At this stage, R H E E D diffraction patterns disappeared and the appearance of trlvalent Eu at the Interface was observed on the E u 3 d end 4d XPS spectra. A p r o n o u n c e d m a x i m u m of the Eu mean valence at a value of 2.5 was obtained for 0 = 3. For higher coverages (0 > 3) the Eu m e a n valence progressively went down to 2 and a mixed

Thick Eu deposits lead to a rather complicated interface. H e r e are summarised the main results obtained for this range of thickness; a detailed analysis will be given in a forthcoming article [15]. Four different thicknesses of Eu (10, 21, 64 and 122 ~,) were investigated. Considering the previously defined p a r a m e t e r 0, 10 A roughly c o r r e s p o n d to 0 = 5. In this range of thicknesses, R H E E D patterns disappeared and the Eu m e a n valence decreased. E u 3 d and 4d photoemisston results, gave values close to 2.1 for the 10 A deposit, and close to 2 for the thicker coverages. As suggested by Selgts and R a a e n [14] the continuous decrease in the Eu m e a n valence can be interpreted as the formation of more and m o r e divalent sites as the Eu deposit increases. This suggests a continuous re-organization of the interface as a function of coverage. The normalized XPS valence band spectra obtained after deposttion at room temperature are presented m fig. 3, A continuous shift ot the band toward higher bmdlng energies is observed w'~th increasing Eu coverage, whmea~ ~ts w~dth decreased. T h e structure which appears at - 1.3 eV for the thicker coverages can be attributed to E u 4 f states. Comparisons of these spectra with those obtained on the E u P d y polycrystailine compounds have been p e r f o r m e d [15]. The 10 coverage spectrum compares well with the EuPd~ one, w h e r e a s the 64 A spectrum looks like the EuPd one. On the other hand, the behaviour of the intensity ratios (IEu/Ied) obtained from the Eu 3d and P d 3 d spectra (see fig. 4) confirms the Pd co_ncentration reduction at the mixed E u - P d interface when the Eu coverage increases Ho~,ever, it ~s difficult to say wether a saturatton occurred or not m the curve of the hg. 4 at very high Eu coverages. The case of non-saturatton would d e n o t e the poss~billty to get pure Eu on a Pd substrate at room t e m p e r a t u r e . These qualitative observations strongly support &e interpretation m terms of E u , P d t , alloy'

F Bertran et al / The Eu / P d ( l l l ) interface

734

,fi

2

-8 -4 0 4 Binding energy (eV)

8

Fig 3 XPS valence bands measured after deposltmn of Eu on the P d ( l l l ) surface kept at room temperature The zero binding energy refers to the P d ( l l l ) Fermi edge. The maximum height of the spectra has been normahsed to umty Full hnes are a grade for the eyes

formation and of Pd4d band filhng which has been prevtously developed on the basis of the fine structure modtficatlons of Pd Auger spectra [11]. A careful analysis of the P d 3 d photoemission spectra was consistent with this point of view: they show an increasing chemical shift and a continuous decrease of the asv',ametry parameter as a function of coverage [15i. This indicates a weakening density of states of the interface at the Fermi edge and strong E u - P d interactions. If we assume an homogeneous interface, then we conclude that an amorphous alloy Eu~Pd t _~ forms. The concentration x is an increasing function of Eu coverage. However, because the electron mean free path is between ,.., 7 and ~ 2 0 / k for XPS experiments, there may exist a concentration gradient across the interface which cannot be detected from these results alone.

3. T h e a n n e a l e d

E u / P d ( l 11) i n t e r f a c e

3.1. R H E E D results

I

I

'

'

'

3,

/

300K

o

e-

_=

820K 1

/

0 0

i

i

i

20

40

I

I

I

60 80 100 Eu thickness (,~)

I

120

140

Fig 4 Pd3d and Eu3ds/2 measured mtenslttes ratios as a funchon of Eu coverage and substrate temperature The total area of the Pd3d hne has been measured including the 3d~/2 and the 3d5/e contributions and the high binding energy satelhtes Onl~ the area o f E u 3 d % 2 hne and ~tssatelhtc has been measured because of the overlap between the Eu 3d3/2 hne and the Pd MVV Auger hne For the two cases, a ShMey's type background has been removed The full circles correspond to the room temperature case, the full squares to the anneahng at 820 K Open circles ant. ~quares correspond to the data not corre,.ted for the P d ( l l l ) suh,,trate as discussed msectton 3

An ~mportant structural rc-orgamzatton of the interface occurred after annealing: R H E E D patterns re-appeared and led to a p(4 x 4) structure at 620 K, then a p(2 × 2) structure at 820 K (see fig. I). Thus, an ordered E u - P d c o m p o u n d forms at the interface. The p(2 × 2) structure is observed whatever the thickness of the evaporated Eu film is (even for the low coverages). In opposition, the p(a × 4) structure is observed up to 21 Eu deposits, but not for very thick Eu coverages. Thus, the p(4 × 4) structure seems to be a thickness d e p e n d e n t phase. Here, we must notice that evaporation of Pd on the 820 K annealed films gives rise to a 1 ;,< 1 R H E E D pattern (i.e. PdClll) can be grown again) [15]. opening the possibihty of making a new type of superlattices: T M / T M RE compounds. 3 2. Photoenusston results and discusston As the p(2 × 2) structure obtained at 820 K is the most stable phase, photoemlss~on results arc discussed for this case only

F Bertran et at / The E u / P d ( l l l ) mterfiwe

The X-ray photocmisslon spectra recorded for the 122 ,& deposnt after anncahng at 820 K are prescntcd m fig. 5. As tdentical spectra were o b t a m e d for all the Eu thicknesses, and as the 3d intensity ratios d e d u c e d from these spectra were also thickness i n d e p e n d e n t (see fig. 4), one had to conclude that the same ordered c o m p o u n d forms whatever the initial thickness of Eu is. The apparent discrepancy observed for the 10/~ deposit can be understood as an electron mean free path effect: for low Eu coverage, the thickness of the c o m p o u n d is of the same order of magnitude as the electron mean free path. Thus, the P d ( l l l ) signal is not completely attenuated; it contributes to the total P d 3 d signal and reduces the measured 3d intensity ratm IEJlrd. The corrected data, deduced from a fit to the P d 3 d line, is plotted in fig. 4. After annealing, a trivalent feature located at about - 1 1 3 4 eV a p p e a r e d on the E u 3 d spectrum (see fig. 5). This spectrum is quite in agreement with the one obtained for the bulk-trivalent EuPda compound. For thxs compound, the struc-

Pd 3d I

I

735

ture located at - 1125 eV is attributed to divalent Eu atom,, present at thc surface [16]. Thus. onc concludes that a bulk-trivalent ordcrcd comp o u n d has formed after annealing. The comparison of the XPS E u P d 3 valence band with the annealed film confirms the trtvalent state of Eu atoms (the 4f ~' states are located at about 7 eV) as well as the energy position of the P d 3 d line. which is the same in both cases. Thus, XPS spectra of the annealed film compare well with those of E u P d 3. However, a more detailed comparison of the XPS valence bands shows a measurable difference of width. An investigation of the Pd 3d lines confirmed this difference because a more symmetrical Pd 3d line is found in the case of the annealed f~lm than in the case of the EuPd 3 one. In fact, the m d t h of the valence band and the symmetry of the P d 3 d spectrum of the annealed film compared well with the divalent EuPd 2 compound. One must conclude that the trivalent compound which forms at the interface, after anneallng at 820 K, looks lnke E u P d v but has the

Eu 3ds: z

Valence band I

I

I

I

I

1

t.-.

~2

t.

<

,4-,7--.

."

EuPd

t¢19

o

=_ t~

.q

EuPd~ , ' / / ~ ' , / / ' , /

E

I -350

-345 -340 -335 Binding energy (eV)

I -33O -15

I

I

-10 -S 0 Binding energy (eV)

5

-t145

-11"~5

-lb25

-lllS

Bmdnng energy {eV)

Fng 5 XPS ~pectra of the valence band, Pd3d and Eu3ds/2 levels of the 122 A deposit after anneahng at 820 K (dashed hnes) A cornpamon with the EuPda and the EuPd 2 ones ns made (sohd hnes) The zero binding energy refers to the P d ( l l l ) Fermi edge The rnaxlmurn height of the spectra has been normahsed to untt.~,

736

F Beman et al. / 7he Eu / P d ( I 1 D mterface

electronic parameters of EuPd 2. This result may be explained in three different ways: (1) The EuPd~ compound has formed at the interface. Thus, the observed discrepancies with the EuPd 3 polycrystalline c o m p o u n d may originate from the monocrystallinity of the annealed EuPd3/Pd(111) interface. In that case, the lattice distortion, which must occur in order to match the Pd(111) lattice, perhaps explains these differences. (2) A new trivalent EuPd 2 compound has formed at the interface. In that case, the previous explanation can be used to explain the trivalency ( E u P d : is divalent in its normal phase) of Eu: in order to match the P d ( l l 1) lattice, a contraction of the lattice parameter is necessary and may induce an Eu trivalent state. (3) A new compound EuPd2+ ~ has formed at the interface.

4. Summary and conclusion The deposition of Eu on P d ( l l l ) has been studied by XPS, UPS, AES [11] and RHEED. Based on these results, the behaviour of the interface is divided into three regions depending on the Eu coverage. At low coverage (0 < 2), dwalent Eu is found to adsorb on the P d ( l l l ) substrate and grows m a layer by layer mode wtth an epitaxial p(2 x 2) structure. When 2 _< 0 < 3, the p(2 x 2) structure vanishes and trivalent Eu appears at the interface, leading to a maximum Eu mean valence at 0 = 3. At higher Eu coverage (0>_3), the formation of amorphous E u ~ P d t _ ~ alloys is suggested, the concentration of which is coverage dependent When annealing at 820 K, a stable trlvalent compound has formed ieadmg to a pt2 × 2) R H E E D pattern for whatever thickness of the mmal Eu depostt The formula of this trwalent compound can be written as EuPd_~.,, 0 <,~ < 1 Dtffractton experiments and structural compar-

tsons would enable us to determine the actually unknown x valuc.

Acknowledgements The authors would like to acknowledge G. Schmerbcr and J.P. Kappler for the preparation of the polycrystalline samples.

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