Cu 2p X-ray absorption spectroscopy of thin copper films grown on Fe(001)

Solid State Communications, Vol. 94, No. 7, pp. 569- 572, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 003%1098/95 $9.50 + .OO

0038-1098(94)00893-0

Cu 2p X-RAY ABSORPTION

SPECTROSCOPY

OF THIN COPPER FILMS GROWN ON Fe(O0 1)

H. Tolentino LNLS/CNPq,

Caixa Postal 6192, 13081-970 Campinas (SP), Brazil and

S. Pizzini, G. Panaccione, J. Vogel and M. Sacchi LURE, Bat, 209D, Centre Universitaire Paris-Sud, 91405 Orsay, France (Received 22 July 1994 by J. Joffrin)

We report on Cu 2p absorption spectra for l-20 monolayers (ML) of copper deposited on Fe(0 0 1). A correlation is established between the structural phase of the thin Cu layers and the lineshape of the Cu2p absorption edge. The continuous changes of the absorption lineshape as a function of copper thickness can be related to the structural change of the thin layers from b.c.c. to f.c.c. phase. We have proved that the variations in the spectra are not a surface effect and are not determined by the change of the chemical nature of the neighbours. The evolution in the 4-8 ML range suggests the occurrence of a continuous process rather than the coexistence of two distinct phases. Keywords: A. magnetic films and multilayers, E. X-ray spectroscopy.

1. INTRODUCTION IN THE LAST few years one has observed an increased activity in the field of the growth and characterisation of metastable structures, either in the form of thin films or in multilayers. Theoretical predictions and experimental findings sometimes agree and sometimes contradict each other, with the final positive result of keeping this field extremely active and dynamic. Exceptional magnetic properties of metallic multilayers, such as enhanced magnetisation, perpendicular anisotropy and giant magnetoresistance [l] have been both predicted and observed, increasing the practical interest of these studies. One system which has been extensively investigated is Fe/&, both as f.c.c. Fe grown on Cu and as b.c.c. Cu grown on Fe. The b.c.c. Cu, in particular, remains the most controversial phase regarding the experimental results as well as theoretical calculations. Total energy calculations by Moruzzi et al. [2] indicated the existence of the metastable b.c.c. Cu phase with a parameter a = 2.87A and an energy of only a few mRy per atom higher than the stable f.c.c. phase. Another calculation of the same type was recently published by Morrison et al. [3], showing

the existence of another non-cubic metastable phase, corresponding to a body-centered tetragonal (b.c.t.) structure with a = 2.76A and c = 3.09A. This result has found a certain amount of criticism from both theoreticians [4] and experimentalists [5]. Experimental evidence for a b.c.c. Cu phase in different conligurations has been provided by several groups: Cu/ Fe(0 0 1) up to several layers [6], Cu/Ag(OO 1) up to about 4 ML [7] and Cu/Fe/Ag(O 0 1) up to 10 ML of Cu [8]. The existence of a b.c.t. Cu phase has been observed on Pd(0 0 1) up to 5 ML [5]. All these studies have been performed by quantitative LEED analysis (except [8] by RHEED). Experimental data appear to be more coherent than theoretical predictions. In a recent study of grazing-incidence X-ray diffraction, Payne et al. [9] showed in a quite direct way that Cu grows on Fe(O0 1) always in the b.c.t. phase, with a c/a ratio which varies monotonically between 1.17 for 2ML and 1.36 for 1OML (c/a values for b.c.c. and f.c.c. structures are 1 and J2, respectively). Unfortunately, this experiment was carried out using sputtered-deposited Fe and Cu, making a comparison with other experiments a little hazardous. To our knowledge, less interest has been devoted

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THIN COPPER FILMS GROWN ON Fe(O0 1)

to the characterisation of the electronic properties of Cu thin films. Lee and coworkers have reported photoemission spectra for a b.c.c. Cu layer on Ag [7] and for b.c.t. Cu layers on Pd [5]. The aim of the present study is not to answer any of the open questions mentioned above, but to add some information about the properties of thin Cu layers deposited on Fe(O0 1). We believe that for the moment it is important to supply inputs from other directions than those extensively exploited so far. Our study is based on Cu2p X-ray absorption spectroscopy (XAS) measurements of Cu thin films grown on Fe(0 0 l), as a function of the Cu layer thickness. 2. EXPERIMENTAL Copper thin layers were deposited on an Fe(0 0 1) crystal substrate cleaned in ultra-high vacuum (UHV) by repeated cycles of Ar-sputtering and annealing (T z 600°C). To limit the presence of contaminants diffusing from the bulk to the top layers, the final surface was prepared by evaporating a thick (= 20 A) layer of Fe and heating at T M 250°C. Auger analysis showed no sign of nitrogen and sulfur, which were detectable after prolonged annealing at T = 600°C. Oxygen and carbon were also below the detectable limit. The Fe substrate showed very sharp LEED spots on dark background. Cu was evaporated at room temperature from high purity droplets premelted in vacm on tungsten wires. An evaporation rate of about 1 A min-’ was obtained in a vacuum of 4-5 x 10-lOmbar (base pressure of 1 x lo-i’mbar). A thickness-dependent study was performed from 1 ML to 20 ML by subsequent deposition of Cu starting from the same clean Fe substrate. Cu2p XAS measurements as well as LEED and Auger analysis were performed for each thickness. XAS 2p absorption measurements were carried out on the SA22 beamline of the SuperACO storage ring at LURE (France), using a double Beryl crystal monochromator (E/AE x 3000at 1 keV). The total electron yield from the sample was collected by a biased (+lOOV) channeltron. The XAS spectra were measured at normal incidence, with the polarisation vector of the X-ray beam parallel to the sample surface. For a limited number of samples the spectra were also measured in grazing incidence, with the polarisation vector of the X-ray beam about 15” off the sample normal. The thickness of the Cu layers was estimated in three different ways: (i) using an oscillating quartz; (ii) comparing the Auger intensities of Fe and Cu (both LVV and MVV); (iii) comparing the absolute intensities of the Cu2p spectra, referred to the clean Fe

Vol. 94, No. 7

substrate. The first method gives an absolute but not very accurate value for the thickness. The second gives a good relative calibration, but to determine the absolute values one needs the escape depths of the Fe Auger electrons, and the final accuracy is not necessarily better than in the first method. With the third method only relative values can be obtained. The three methods turned out to be extremely coherent both for absolute (i and ii) and relative (ii and iii) calibrations. Nevertheless, we think that an error bar of f30% should be maintained. When referring to 1 ML, we mean a density of Cu atoms equal to that of one layer of Fe (% 1.2 x lOI at cm-* % 1.46A). The Fe substance was not heated during the evaporation, but some intermixing of Fe and Cu at room temperature cannot be excluded. Two layers were evaporated and measured at 100 K and did not show any variation with respect to the equivalent samples prepared at room temperature. 3. RESULTS A summary of the LEED and Auger data measured for the Cu layers is given in Fig. 1. The amount of monolayers is estimated by comparison of the Auger intensities, and using the escape depth of the Fe Auger electrons. The square LEED pattern of the Fe(O0 1) substrate was clear and sharp after the deposition of 1

100 -

h

60 -

?

5

c

GO-

E

a

E

40-

t 9 a

20-

o-

I

I

I

I

I

0

2

4

6

I

Evaporation time (min) Fig. 1. Summary of the LEED and Auger data measured for Cu thin layers deposited on Fe(0 0 1), as a function of the evaporation time. The saturation, in terms of relative intensities Auger lines, occurs for about 4min of evaporation, which corresponds roughly to 6ML. The LEED was very clear up to about 2 ML and became faint in the region from 2 ML to 4 ML.

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THIN COPPER FILMS GROWN ON Fe(0 0 1)

0 930

940

950

960

Energy (eV)

Fig. 2. Cu 2p X-ray absorption spectra measured for 1 to 20 Cu monolayers deposited on Fe(0 0 1). and 2 monolayers of copper and still visible on a higher background up to 4 ML. For larger coverages no clear pattern was visible. It should be noted, however, that our front-view LEED analyser only allows visual inspection of the pattern, making it difficult to point out weak spots on high backgrounds. These qualitative results suggest that up to 4MLCu grows with the metastable b.c.c. structure imposed by the Fe substrate. Figure 2 collects the Cu 2p XAS spectra for several thicknesses of Cu grown on Fe(0 0 1). The raw data for the thinnest layers (< 6 ML) were normalised to the spectrum taken in the same conditions (geometry, energy region and counting rate) for the clean Fe substrate. For the thickest layers only a linear background was subtracted. A clear evolution in the spectra can be observed, especially concerning the structures after the main absorption peak of the L3 edge (peak A). The thickest (20ML) layer strictly corresponds to bulk f.c.c. Cu; the 16ML and 12ML ones are not too different. On the contrary, the spectra for thicknesses between 1 ML and 4 ML are quite similar to each other but differ from those of bulk copper. The main difference between the two series of spectra is the presence of a double structure (peaks B and C) after the main absorption peak for the thick layers, whilst only one structure is present for the thin layers. The 6 and 8 ML can be seen as an intermediate situation between the two. The same changes can be seen, less clearly, at the L2 edge. Without making any assumption about the origin of these changes, one can at least associate them with

571

the presence/absence of the LEED pattern. The transition between the two types of spectra corresponds with the disappearance of a clear b.c.c. LEED pattern. Various calculations exist for the f.c.c. Cu2p absorption spectra [lo], usually performed in the APW scheme. However, the exact shape of the spectrum in terms of relative positions and intensities has never been accurately reproduced. The main transition (peak A) is usually related to a small contribution of 3d states in the bands just above the Fermi level, giving a sharp 2p -+ 3d transition superimposed on the smooth 2p -+ ns transitions. The peaks at 939eV (B) and at 943 eV (C) have been attributed to transitions towards 4p states of neighbouring Cu atoms that present, when projected on the absorbing site, the correct symmetry for dipole transitions. This picture might suggest that the modification of B and C as the Cu layer thickness increases could be related to the change in the chemical nature of the neighbours, either Cu or Fe, whose 4d density of states are different. As the layer thickness increases the absorbing Cu atoms “see” more and more Cu neighbours. However, this argument would not explain why the spectra do not change in the l4ML range, where the average nature of the neighbours changes the most. To disentangle this question, we measured the 1 ML sample at normal and grazing incidence, in such a way to probe the density of empty states in the plane and along [00 l] direction. Within the limits imposed by the noise (< 5%) no difference could be observed between the two spectra. Moreover, the spectrum measured for 1 ML Cu after a heat treatment up to 300°C which caused interdiffusion of Cu in Fe, was also found to be identical to that measured for the as-deposited sample. All these measurements rule out the possibility that the difference between the spectra measured for thick and thin Cu layers might be due to the difference in the chemical nature of the neighbours of the absorbing Cu atoms. To dissipate any doubts about a possible surface origin of the spectral shape for the thin layers, we show in Fig. 3 the comparison between the f.c.c. Cu, the 1 ML Cu/Fe(O 0 1) and the 1 ML Cu/Ni( 1 10) [ 111. The comparison between the Cu spectra for thin layers grown on a b.c.c. (Fe) and an f.c.c. (Ni) substrate leaves little doubt about the association of the two different spectra with the b.c.c. and f.c.c. structures of copper. The evolution of the Cu/Fe spectra in the 4-8 ML range suggests the occurrence of a continuous transition from a “b.c.c.” to an f.c.c. phase rather than the coexistence of two distinct phases. This supports the model of a continuous b.c.t. phase transition suggested by Payne et al. [9].

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THIN COPPER FILMS GROWN ON Fe(O0 1)

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shown that the presence of a single satellite structure, instead of two as in f.c.c. Cu, is not a surface effect only (1 ML Cu/Ni has the same structure as bulk Cu) nor is determined by the chemical nature of the neighbours. It appears that the iineshape is related to the b.c.c. structure of the thin layers where Cu has 8 neighbours instead of 12 as in the f.c.c. phase. We have tentatively attributed the changes versus thickness to the different behaviour of the empty Cu4d orbitais in the two phases.

-J: :u/Ni(llO)

REFERENCES 1. 0

935

945

940 Energy

950

(eV)

Fig. 3. Cu2p X-ray absorption spectra measured for 1 MLCu/Fe(OO 1) and 1 MLCu/Ni(l 10) are compared with spectra measured for bulk metallic Cu and for Cu in its atomic phase. Figure 3 also shows the spectrum measured on the same ~amIine for copper in its atomic phase 1121.In atomic copper, the 3d states are full and we do not observe any sharp transition around 934 eV. A sharp, structured peak is observed at 940eV and it has been attributed to 2p --f 4d transitions. Its energy location is not too far from that of the B and C peaks of f.c.c. Cu. and of the second peak in Cu/Fe. This suggests that the difference between the spectra for b.c.c. and f.c.c. structure might be due to the different splitting in the empty 4d states in the symmetries associated with the two structures. 4. CONCLUSIONS In this study we have found a correlation between the structural phase of thin Cu layers on Fe(0 0 1) and the lineshape of the 2pCu absorption edge. We have

6. 7. 8.

9. 10. 11. 12.

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