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INITIAL STAGES OF GROWTH OF Co/Pt (100) AND Co/Cr (110)
C. Boeglin, B. Carriere, J. Deville, O. Heckmann, P. Panissod, F. Scheurer
To cite this version:
C. Boeglin, B. Carriere, J. Deville, O. Heckmann, P. Panissod, et al.. INITIAL STAGES OF
GROWTH OF Co/Pt (100) AND Co/Cr (110). Journal de Physique Colloques, 1990, 51 (C1),
pp.C1-775-C1-780. �10.1051/jphyscol:19901121�. �jpa-00230030�
I N I T I A L STAGES OF GROWTH OF C o / P t (100) AND C o / C r (110)
C. BOEGLIN' , B. C A R R I E R E * , J. P. DEVILLE" , 0. H E C K M A N N * , P. P A N I S S O D * * and F. SCHEURER*
I.P.C.M.S., UM CNRS-ULP-EHICS n0380046, Groupe "surfaces-Interfacesn*
et Groupe d'Etude des Materiaux ~ ~ t a l l i ~ u e s * , 4 Rue Blaise Pascal, F-67070 Strasbourg Cedex, France
RCsumC
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L'ttude des stades initiaux de la croissance de cobalt respectivement sur la face (100) du platine et la face (110) du chrome, A temperature ambiante, a t t t entreprise en vue d'Ctablir les conditions d'obtention de multicouches mCtalliques magnCtiques pouvant avoir des proprittCs magnttiques inttressantes. Yutilisation des mCthodes d'analyse des surfaces a permis d'ttablir les modes de croissance (Franck-van der Menve), de constater qu'il n'y avait pas tpitaxie entre les diffCrentes couches m&me si la croissance ttait hornogkne et, dans le cas du syst&me CoICr, de dCcrire le dCsordre existant 5 ['interface. Pour Co/Pt, il existe un ttat Clectronique d'interface caracteristique des atomes de platine voisins de ceux du cobalt.Abstract
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The investigation of the initial stages of growth of cobalt at room temperature on respectively Pt(100) and Cr(ll0) surfaces has been undertaken to test the possibilities of obtaining modulated metallic magnetic multilayers having new interesting magnetic properties.Using surface physics analytical methods enabled to establish the growth mode (Franck-van der Merwe), t o find that no epitaxy occurs between the layers even if the growth is homogeneous. In the case of ColCr, the disorder existing a t the interface has been described. For Pt/Co, an interfacial electronic state characteristic of the Pt atoms being in the vicinity of cobalt has been demonstrated.
1
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INTRODUCTIONMetallic multilayers, artificial structures in which two metals or alloys are deposited alternatingly, with individual layer thicknesses of a few atomic layers, have been studied since the late 70-ies and, in the recent years, several interesting properties have been discovered in metallic magnetic multilayers (MMM). For example, large perpendicular anisotropies were observed first in sputtered CO-Pd by Carcia et aL [l] and, later, even larger effects were found in evaporated CO-Pd, CO-Pt and CO-Au multilayers [2,3]. Magnetic superstructures and, particularly, anti-ferromagnetic coupling, were observed in Fe-Cr sandwiches [4]. As far as transport properties are concerned, giant magneto-resistance effects have been observed with the Fe-Cr system [S]. In Fe-Ru [6], the otherwise unstable hcp iron phase has been demonstrated in which a stable magnetic moment exists on some sites. Up to now all these new properties are not well explained and may strongly depend on the choice of metals, individual layer thickness, smoothness and structure of interface,
...
Once obtained only by sputtering and evaporation, MMMs now benefit from the progress of MBE (molecular beam epitaxy) growth, used with success for tailoring new 111-V and 11-V1 semiconductors.
With this growth technique, metallic superlattices have been indeed obtained [5,6].
Among others, two systems (Co/Pt, Co/Cr) are good potential candidates for building MMMs having strong perpendicular anisotropies and/or showing interesting antiferromagnetic coupling.
Moreover, the Co/Pt system can illustrate the fundamental relations between structural and magnetic order whereas it is tempting to hope the stabilization of the CO fcc phase unstable at room temperature through an epitaxy on bcc chromium. However, since there is a large lattice mismatch between CO and Pt
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901121
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and a phase mismatch between CO and Cr, one cannot anticipate an easy epitaxial growth. I n order t o understand the stability of such multilayers, surface physics techniques [Auger and Electron Energy Loss Spectroscopies (AES, EELS), low-energy electron diffraction (LEED), photoemission (XPS) and Surface Extended X-Ray Absorption Spectroscopy (SEXAFS)] have been applied to study the initial stages of growth at room temperature of Co/Pt(100) and Co/Cr(l10).
2 - EXPERIMENTAL
The LEED, AES and EELS experiments have been performed in a conventional chamber fitted with a cylindricai mirror analyzer (CMA) using the derivative mode. The clean surfaces, evidenced by well contrasted LEED patterns [5 X 1 for Pt(100) and l X 1 for Cr(llO)] and no contamination detected by AES, were obtained by classical surface cleaning methods. The cobalt vapor deposition, controled with a quartz microbalance, was obtained by resistively heating a cobalt wire (0 0.25 mm). More details on the flux calibration can be found in (71. Photoemission studies were carried out either in a conventional XPS apparatus or at the LURE radiation synchrotron facility where SEXAFS experiments were also conducted.
3
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GROWTH OF COBALT ON PLATINUMIn this part, we shall focus our attention on the low-energy Auger transitions. Fig. 1 shows the AST curve describing the growth profile obtained for successive cobalt deposits a t room temperature on Pt(100). It is well known that the three major growth modes can be adequatelly determined with AES by the observation of curves (called AST curves) giving the intensity of a substrate (resp. adsorbate) Auger peak as a function of the number of deposited atoms [8]. Here, for sake of clarity, the AST plot is given only for the Pt N 6 W Auger transition. We can see three breaks (possibty a fourth o n e , although being more difficult to ascertain) which indicates the completion of successive monolayers (ML) of cobalt atoms. It is thus clear that during 3 or 4 layers, cobalt is growing on platinum with the Franck-van der M e w e growth mode (layer by layer). 'Afterwards, the formation of cobalt islands (Stranski-Krastanov growth mode) probably occurs. Since the 5 X 1 LEED pattern, characteristic of clean Pt(100), disappears before the completion of the first cobalt monolayer, it means that, a t room temperature, cobalt atoms are not epitaxially deposited on platinum.
Fig. 1 : Auger signal vs. time kinetics of cobalt growth on Pt(100). The segments represent a calculated best fit for a layer by layer growth. The time scale is expressed in close-packed cobalt monolayers (i.e. 1.83 x 1015 atoms cm-2)
3.5, 4.5 and 5 ML. T h e last curve (G) is obtained with a thick cobalt deposit (more than 10 ML). O n e can clearly see the increase of the cobalt M2 3VV Auger feature which keeps the same structure all over t h e deposition process. O n the contrary, the'platinum N6VV, N7VV doublet shows a gradual increase of the low-energy feature. It has been establish in [7] that this phenomenon is not due t o a n artifact (modification of the secondary electron cascade, overlap with the cobalt Auger peak,...). It is not either due t o a n interdiffusion of cobalt in platinum, o r a n alloying process a t the interface. This can be demonstrated by XPS measurements which d o not show any shift of the CO 3 p and Pt 4f core level lines.
This increase of the 64.5 e V feature with respect to the 68 e V one is in fact only due to the modification of the N7 density of states. Characteristic electron energy spectroscopy shows indeed the same evolution of t h e N7 and N6 edges; photoemission experiments give the same results. I t seems possible t o relate this modification of the platinum 4f density of states with the appearance of a n interface state of platinum atoms in the vicinity of cobalt ones. Such a modification of Auger spectra a t a n interface has been shown by Pavlovska and Bauer [9] with the Au/Mo system. A more detailed description of the CoPt(100) interface has been obtained with photoemission experiments run a t SuperACO.
Fig. 2 : Evolution of the low-energy Auger spectra as a function of time of cobalt deposition on platinum (100).
A to G correspond to various coverages indicated in the text.
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Photoemission spectra have been recorded for various photon energies ranging from 40 t o 125 eV.
It is well known that the photoionization cross section varies quite rapidly a s a function o f energy. F o r example, for platinum the cross section for 5d electrons has a maximum for about 40 eV. At 120 e V the cross section is decreased by two orders of magnitude. For CO 3d electrons the evolution is different and the ratio between the cross sections at 40 and 120 e V is about 0.5. At 70 eV, the cross sections for both Pt 5d and CO 3d electrons a r e about the same. Fig. 3 shows the valence band photoemission spectra recorded for cobalt coverages o n platinum ranging from 0 t o infinity. Features a , b and c evidenced o n the clean Pt(100) a r e thus characteristic of platinum. The intensities of these features decrease a s a function of CO coverage (b more rapidly than the others) and, in the same time, the cobalt 3d contribution t o the valence band increases and becomes predominent for the thick 10 ML cobalt coverage. Since feature b disappears a t about 1 ML cobalt coverage, it is attributed t o a surface state of platinum. But a t 1 ML a new feature (c') appears and seems t o reach its maximum intensity a t 1.5 ML. If we look (Fig. 4) a t the photoemission spectra recorded for this coverage respectively a t 40 and 70 e V photon energy, o n e can see that this c' state is enhanced a t 40 e V where the platinum cross section is higher : feature c' is thus clearly evidenced a s being due t o platinum. If we keep in mind that the electron mean free path for these energies is about 3 ML the evolution of the intensity of this c' feature could be explained by a n interface state, characteristic of the platinum atoms surrounded partly by cobalt atoms. How this interface state is responsible of the enhancement of the 4f7I2 photoemission line (or N 7 V V Auger transition) with respect t o the 4fsn ( N 6 W ) is not understood a t the present time.
Fig. 3 : Photoemission results of the valence band of the Fig. 4 : Comparison of the photoemission curves CoPt(lO0) system at various cobalt coverages (in ML). obtained for a 1.5 M L cobalt coverage on Pt(100) Incident photon energy : 70 eV at respectively 40 and 70 eV incident photon energy
exponential decrease (increase) of the Cr (CO) peaks, characteristic of an homogeneous growth process. As in the case of Co/Pt, it has been possible to show some breaks in the initial part of the AST curves due to the completion of the successive cobalt monolayers; their number depends critically on the chromium surface cleanliness. Thus, generally, the growth mode is the Stranki- Krastanov one (&e. one o r two monolayers of adsorbate followed by the growth of islands). The Auger fine structure and the photoemission lines are not at all affected by the deposition, their energy width and position being the same as in the bulk. This indicates that no interdiffusion occurs during the RT deposition. But there is no epitaxy between CO and Cr as it is deduced from LEED experiments which show only a weaker and weaker (1x1) pattern when increasing the cobalt depos.ition from 0.2 to 1 ML.
SEXAFS is thus one of the few methods available to get further data about the local order at the interface. The S E W S experiments were performed at LURE on the DC1 storage ring. The variation of the X-ray absorption coefficient of the sample is measured above the K edge of cobalt (7709 eV) in the total yield mode. The Fourier transforms for the room temperature deposited layers are given on figure 5 (full lines). Up to 4 ML they have the typical shape which has been found several times for an amorphous structure : shorter calculated distances, smaller amplitudes for the first shell, very weak higher order shells with respect to bulk crystalline cobalt [Magnan, Chandesris, Rossi, Jezequel, Hricovini and Lecante, to be published]. When reaching thicknesses larger than 2 ML the Fourier transform indicates a gradual change towards the bulk values which are finally obtained for 12 ML coverages.
Gentle annealings of the small coverage layers at various temperatures have been performed in order to induce the recrystallisation of the layer. SEXAFS results (dotted lines) show indeed a partial crystallisation, evidenced by the narrowing and intensity increase of the first shell peak.
However, a t 600K, the faint LEED pattern seems characteristic of the (1x1) Cr(ll0) surface. It is thus believed that cobalt forms small islands. In this case, AES data which show an increase of the chromium Auger transitions confirm this hypothesis.
Fig. 5 : Radial distribution functions for cobalt deposited on Cr(ll0) at various coverages full lines : room temperature adsorption; dotted lines : after annealing at 600K.
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5 - CONCLUSION
As a prerequisite before processing CO-Pt and CO-Cr mctallic magnetic multilayers, we havc investigated the growth, at room temperature, of cobalt ultrathin layers on Pt(100) and Cr(ll0) surfaces.
In both cases, we have found, as it could be expected, that the growth mode could be strongly affected by the cleanliness of the substrate surface. In the case of clean enough surfaces, a layer - by
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layer (Franck - van der Merwe) growth mode could be observed on Pt(100). For Cr(ll0) after the completion of two cobalt monolayers, one could observe the appearance of cobalt islands (Stranski - Krastanov growth mode). It should be emphasized that chromium surfaces are very sensitive to external contamination and to impurity surface segregation; a better cleaning process could thus favors a satisfactory layer - by-
layer growth.For the two systems, we have shown that the interface is sharp on an atomic level and that no interdiffusion occurs between the substrate and the adsorbate. But in neither system epitaxy did occur : the LEED patterns became fuzzy before the completion of the first cobalt monolayer. This indicates that there are very small cobalt microdomains or even amorphous cobalt layers. In the case of Co/Cr(llO), the disorder has been characterized by SEXAFS and it was found that a better crystallinity of the cobalt overlayer could be obtained by gently annealing the surface. However, this thermal treatment induces a coalescence of cobalt atoms and the growth of islands. It seems thus that the interfaces, grown at room temperature, between cobalt and, respectively, platinum (100) and chromium (110) surfaces are of a good quality in terms of homogeneity and lack of interdiffusion. Work is now in progress to find thermal treatments able to favor the cobalt epitaxy on both metals.
REFERENCES
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The authors would like to thank D. Chandesris, C. Guillot and J. Lecante for their help a t LURE synchrotron radiation facility and fruitful discussions about SEXAFS and angular-resolved photoemission.
