Chemical structure of the PtCo(0001) interface

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Surface Science 352-354 (1996) 828-832

Chemical structure of the Pt/Co(0001)interface H. Bulou a, *, A. Barbier b, R. Belkhou c, C. Guillot c,d B. Caxd~re a, J.P. Deville a a IPCMS, UMR 046 du CNRS, Groupe Surfaces-Interfaces, 23 rue du Loess, 67037 Strasbourg Cedex, France b CEA/Grenoble, D R F M C / S P 2 M / P I , 17 rue des Martyrs, 38054 Grenoble, France c LURE, Batiment 209d, Centre Universitaire Paris-Sud, 91405 Orsay Cedex, France DRECAM-SRSIM, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France

Received 5 September 1995; accepted for publication 31 October 1995

Abstract We describe the growth of platinum deposited on Co(0001) at room temperature and the composition of the interface. Core level photoemission spectroscopy (PES) experiments and simulations of the growth are reported. We show that an island growth model fits the experimental data and we suggest the possibility of an interdiffusion during the platinum deposition. Keywords: Cobalt; Growth; Interdiffusion;Photoelectron spectroscopy;Platinum

1. Introduction In the recent years, magnetic/non-magnetic metallic systems (sandwiches, multilayers...) were increasingly studied because of their magnetic properties [1-3]. A m o n g these systems, the P t - C o system is one of the most studied in terms of electronic, magnetic and structural properties [4,5]. The main interest of this system lies in its potential applications for magneto-optic recording media [6]. It was also promising to investigate the crystallographic structure and the growth mode either of platinum overlayers on cobalt or cobalt ones on platinum since they are key parameters in the understanding of the magnetic anisotropy in multilayers [7-9].

* Corresponding author. Fax: + 33 88 10 72 48; e-mail: [email protected].

In this paper, we present a set of experiments performed by means of photoelectron spectroscopy (PES) in order to analyse the growth mode of Pt on Co(0001) and to get out some photoelectron diffraction (PED) experiments having in mind to describe unambiguously the structure of epitaxial platinum overlayers [10]. After a description of the experimental conditions and a discussion of the analysis of the experimental results, we can assess the different components of the Pt4f7/2 photoemission line and discuss the growth through a comparison of the experimental data with a simulation.

2. Experiment Clean Co(0001) single-crystal surfaces were obtained by several cycles of argon ion sputtering at 300°C followed by a final annealing at 350°C in an U H V chamber with a base pressure of 1 × 10-~0 mbar. The cleanliness of the substrate was checked

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H. Bulou et al.~/ Surface Science 352-354 (1996) 828-832

by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). Each platinum deposition was carried out at room temperature by resistively heating a platinum wire. Each evaporation was characterized by AES, LEED and PES. The platinum thicknesses are estimated through the intensity of the Auger Co MVV peak. The present study was carried out at the SuperACO synchrotron source at LURE Orsay, on the beamline SA 73b with a TGM 7 ° monochromator. To study the Pt 4f7/2 photoemission line the photon energy was set up at 155 eV using the p-polarisation, optimizing the Pt crosssection, the monochromator response and minimizing the photoelectron mean free path to increase the surface sensitivity. The analyzer was set at the normal to the sample surface. The resolution was about 350 meV.

3. Results 3.1. Method of analysis

The experimental data consist of two spectra: the energy distribution curves (EDCs) of the Pt4f7/2 core electrons and the EDCs at the Fermi-level. The binding energy of the core electrons are very sensitive to the chemical environment of the atom which takes part to the photoemission transition. Thus, binding energies of core electrons outcoming from a surface atom, a bulk one or an interface one are different [11]. This fact implies that the experimental EDC of the Pt4f7/2 core electrons is composed of different individual peaks which are assumed to have a Doniach-Sunjic (DS) lineshape. Fig. 1 shows the different platinum sites which are associated with the

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0.25 0.25 0.25 0.25

0.19 0.11 0.19 0.11

70.74 71.08 71.13 71.40

above components labelled S, S *, B and B *. The DSs parameters are summarized in Table 1 [11-13]. Furthermore, for a photon energy of 155 eV, the Pt 4f7/2 core level lines are superimposed to a background of secondary electrons. This secondary electron spectrum must be removed before the decomposition of the EDC. In the case of the Pt 4f7/2 lines, the total width of the core level does not exceed a few hundred meV and a linear shape of the secondary electron background is a good approximation. After subtraction of this linear part, the experimental EDC of the Pt4f7/2 core electrons is decomposed into the above different components using a least-mean-squares fitting procedure. We have in particular to make a convolution of the theoretical lineshape with the instrumental resolution function. The overall energy resolution (monochromator plus electron analyzer) is determined from the measurements of the Fermi-edge and the resolution function is taken as a Gaussian one. It is clear that several mathematical solutions may exist for the fit when more than two lines are introduced. So, it is necessary to identify the different possible physical situations which may be present to constrain the DS parameters and the number of components. Fig. 2 shows the Pt 4f7/2 spectra corresponding to 0.60 ML up t o 8.5 ML of platinum deposited on Co(0001). In all cases, to fit these" photoemission lines, we need to introduce three components. 3.2. How can we describe the components?

Fig. I. The different possible sites occupied by the platinum atoms on the Co(O001) and their associated pbotoemission lines.

At this step, the question is to identify the structural situation corresponding to each individual peak, both for the thinner coverage and for the thicker

H. Bulou et a l . / Surface Science 352-354 (1996) 828-832

830

ones. Peaks (1) and (3) (respectively S and B * ) can be easily identified because of their characteristic binding energies [11-13]. But, concerning component (2), the identification is more difficult. Indeed, this component can correspond either to S *, or to B, or can be a mixing of both due to the fact that both binding energies are very close together (namely 50 meV). Then we did two series of fits, the first one with S, S * and B * and the other one with S, B and B *. It is worth noting that for each fit, the evolution of the intensity of the components versus the platinum thickness are very similar. So, we show on Fig. 3a the evolution with the platinum thickness of the difference between the binding energy of S and the binding energy of component (2).The binding energy of S should be always constant, so this representation mode allows us to suppress the error about the position of the Fermi-level. The energy difference varies from about 310 meV for the thinner deposits to about 420 meV for the thicker ones. Actually, a difference of about 320 meV is characteristic of a component S * and a difference of about 400 meV of

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Fig. 3. Difference b e t w e e n (a) the S * / B photoemission line o f the P t 4 f T / 2 core level spectra a n d the S o n e versus platinum thickness, (b) the B * p h o t o e m i s s i o n line o f the P t 4 f T / 2 core level spectra a n d the S one versus platinum thickness.

component B [12,13]. So, it is quite logical to believe that for the thinner deposit, component (2) represents S * while for the thicker deposit, it represents B.

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After having identified all the components, let us now turn our attention to the growth mode using the representation of the intensity of the different photoemission lines versus the platinum thickness. Fig. 4 shows the comparison between the experimental data and two limit cases: the simulation of an island growth mode and a layer by layer one. These simulations are done on an fcc (111) lattice. In agreement with molecular dynamics simulations, atoms arriving from the gas phase stick when they are supported by three atoms in the lower layer. Moreover, these

H. Bulou et a l . / Surface Science 352-354 (1996) 828-832

simulations are done without diffusion of the platinum into the cobalt, and the cobalt surface is considered with no roughness. For S and B, the best agreement between the experimental data and the simulation is for the island growth mode; concerning the layer by layer one, any convergent result could never be reached. So, the growth mode is very doubtless an island one, a conclusion which is in agreement with the results of AES [91. Now, an important point to approach is the interdiffusion between platinum and cobalt atoms, these two elements being able to form a wide variety of substitutional alloys and ordered compound [5]. This possible interdiffusion has not been introduced in the simulation but can be present in our experiments and an island growth mode is quite compatible with this possibility. This fact could explain the weak disagreement between the experimental data and the S* compo-

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nent of the island growth mode simulation for the thinner deposits where it is difficult to distinguish the situation with platinum atom inserted in the fh'st plane of the cobalt substrate and the starting up of the islands. Indeed, if we suppose that a part of the first platinum atoms which arrive on the surface goes deep into the substrate then the component S * would be weaker than the one of the simulated island mode, like in the experimental data. On the basis of this fact, we can also explain the behaviour of the component B* (Fig. 5). It is clear that without interdiffusion, the calculated intensity of the component B * should be systematically weaker than the experimental data. Moreover, as we can see it on the Fig. 3b, the binding energy difference between S and B * shifts from about 730 meV for the thinner deposits (characteristic of atoms of platinum of interface [12,13]) to 860 meV for the thicker ones (characteristic of atoms of platinnm in a C o 7 5 P t 2 5 alloy [12,13]). The experimental data of the Fig. 5 show clearly two regimes below and above 3 ML for the intensity evolution of B*, These facts allow us to assume the tentative following scheme for the growth: below 3 ML, the platinum grows by forming islands in a twinned fcc structure [10] with the possibility of a weak interdiffusion; above 3 ML, the islands coalesce and furthermore the interdiffusion between the platinum and the cobalt is made easier by the twinned fcc structure of the islands and leads to the formation of a mixing of platinum and

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H. Bulou et aL / Surface Science 352-354 (1996) 828-832

cobalt between the islands with the C075Pt25 stoichiometry. It is important to note that such an interdiffusion has been also observed in the [111] oriented C o / P t superlatfices [14].

4. Conclusion Core level PES has been applied to study the growth mode of platinum on Co(0001). With the help of a simulation, evidence has been found that it is 3D with the possibility of interdiffusion. The 3D mode of growth should be confirmed by STM and a characterization of the mixing of platinum and cobalt should be done, in particular by the introduction of the interdiffusion in the simulations and with the help of a study by PED. Moreover among the important parameters, it seems that the evaporation rate of the platinum and the temperature of the substrate are playing a major role with respect to the interdiffusion of the platinum in the cobalt as well as the diffusion on the surface. So another study using PES of the influence of these two parameters has to be considered.

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