Characterization of the growth processes and magnetic properties of thin ferromagnetic cobalt films on Cu(100)

732

Surface

Science 211/212 (1989) 732-739 North-Holland. Amsterdam

CHARACTERIZATION OF THE GROWTH PROCESSES AND MAGNETIC PROPERTIES OF THIN FERROMAGNETIC COBALT FILMS ON Cu(100) J.J. DE MIGUEL, R. MIRANDA

A. CEBOLLADA,

Departamenro de Fisica de la Materra Cantoblanco, 28049 Madrid, Spain

C.M. SCHNEIDER, and J. KIRSCHNER

Condensada,

P. BRESSLER, *

Instrtut ftir GrenzJltichenforschung

J.M. GALLEGO, C-III,

J. GARBE,

und Vakuumph_vsik,

S. FERRER,

Universrdad

Authnoma

de Madnd,

K. BETHKE

KFA Jiilich, Postfach

1913, D-51 70 /ii&h,

Fed. Rep. of Germay Received

23 June 1988; accepted

for publication

23 September

1988

The growth of thin films of fee cobalt on Cu(100) has been characterized with medium energy electron diffraction (MEED) and thermal energy atom scattering (TEAS) in order to prepare films of reproducible structural perfection, interdiffusion profile and atomic concentration. The magnetic properties of thin films are shown to be strongly affected by these - usually difficult to control - parameters providing a clue to the possible origin of contradictory reports in this field. In particular, for carefully grown films, a clear thickness dependence of the Curie temperature, not previously reported for this system, has been found.

1. Introduction Magnetic properties of thin films offer a fascinating field of research with important technological implications. The capability of depositing the layers in ultra high vacuum (UHV) and controlling the amount deposited in fractions of a monolayer (ML) has given a boost to these studies [1,2]. In recent years, an increasing amount of information on magnetic properties of thin films has been accumulated using a variety of experimental techniques [3]. The resulting image is, however, far from being clear. Contradictions abound concerning the thickness dependence of the magnetic moments, Curie temperature or spin-resolved band structure [4-71. * Permanent address: lnstitut D-1000 Berlin 33. Germany.

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We believe that conflicting reports concerning the magnetic properties of one and the same system, such as those existing, for instance, for the Co/Cu system [4,5], are mainly due to differences in the growth of the adlayers, i.e. coverage calibration, geometric perfection, growth mode, interdiffusion, etc., and accordingly, we set out to investigate in detail the growth conditions and their influence in the magnetism of overlayers. The most commonly used techniques for the characterization of metallic films deposited on single-crystal metal substrates are Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The growth mode of the film (layer-by-layer, layer plus island, or island growth) can be determined by AES while its structural perfection is commonly assessed with LEED [2,5-71. However, it is known that only sophisticated LEED experiments reveal quantitatively the density of defects in the growing overlayer [8]. Recently, it has been suggested that reflection high energy electron diffraction (RHEED) is a technique that gives simultaneously both the number of deposited layers and their perfection [9]. An important drawback of this method is the multiple scattering nature of the interaction between high-energy electrons and solid surfaces which greatly complicates the interpretation of the observed oscillations in the intensity of the diffracted beams [lo]. This oscillatory behaviour of the diffracted beams during epitaxial growth has also been found when using another experimental technique, namely thermal energy atom scattering (TEAS) [ll]. Contrary to RHEED, it has been demonstrated that TEAS data admit a kinematical interpretation which yields a quantitative evaluation of the surface density of steps during epitaxial growth [12]. Furthermore, in this case only the evolution of the intensity of the beam during deposition - and not that of its angular profile, as in RHEED [13] needs to be considered.

2. Experimental The experiments have been carried out in two different chambers. In one of them medium energy electron diffraction (MEED) was performed using a rear-view LEED optics and the electron gun of the Auger system, the electron beam energy being 3 keV. This allowed the simultaneous acquisition of Auger electron spectra (AES) with a cylindrical mirror analyzer (CMA). In the other chamber, thermal energy atom scattering (TEAS) was performed during growth in order to characterize the structural perfection of the film. LEED and AES facilities were also available. The electronic and magnetic properties were determined either by spin-polarized photoemission using synchrotron radiation or by surface magneto-optic Kerr effect (SMOKE). The samples were Cu single crystals with (100) orientation whose perfection was assessed by TEAS prior to Co deposition. On the average, the Cu(100)

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crystals had flat terraces larger than 200 A [12]. They could be heated by electron bombardment up to the melting temperature and cooled down to 150 K. The temperature was measured by a calibrated thermocouple attached to the sample. Cycles of ArC sputtering (600 eV, 2 PA/cm’) and annealing (1100 K) were used to clean and order the surface. Evaporation of Co was achieved from an oven heated by electron bombardment. The pressure was in the high lo-” Torr range during deposition of Co overlayers.

3. Results and discussion It has long been known that cobalt grows layer-by-layer on Cu(lOO), a fee phase being stable for these thin films even at 300 K [2] while for bulk Co it exists only above 750 K. To further characterize the growing layers, MEED and TEAS were used. For both techniques, the intensities of the specular beams display an oscillatory behaviour characteristic of a layer-by-layer mode of growth [9,11] as shown in fig. 1. TEAS oscillations are purely kinematic. they have a period of one deposited monolayer and yield a quantitative estimation of the concentration of defects in the film at any stage of the growth [12,14]. However, this technique is not usually available in standard molecular beam epitaxy systems. Although. in general, dynamical effects are

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Fig. 1. (a) Medium energy electron diffraction (MEED) and (b) thermal energy atom scattering data taken during deposition of Co on a Cu(100) surface. In both cases the intensity of the specular beam is measured at different substrate temperatures during growth. The angle of incidence of the electron beam in MEED has been chosen to mimic the purely kinematic behaviour of the TEAS specular beam. In these conditions the period of the oscillations corresponds to one deposited monolayer.

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important in MEED, it is feasible to find experimental conditions (e.g. with an angle of incidence at a maximum in the rocking curve [15]) where these multiple scattering effects are minimized and the shape of the MEED oscillations becomes equivalent to that in TEAS, as illustrated in fig. 1. Thus, MEED oscillations can be used as a simple and precise way of evaluating the deposited coverage. On passing, we note that simultaneous MEED and AES measurements indicate that the breaks in the Auger intensity versus evaporation time curves coincide, in these conditions, with the maxima in the intensity of the MEED specular beam [15]. For coverages above 1 ML, however, the MEED method is much more accurate than the standard Auger technique [15] in providing a measure of the thickness of the evaporated overlayer. We have used this approach to carefully control and characterize fee Co films grown on Cu(100) where magnetic and electronic properties have been measured by spin-resolved photoemission using synchrotron radiation from BESSY (to be reported elsewhere), and by the surface magneto-optic Kerr effect (SMOKE). This technique is a form of ellipsometry detecting the rotation of linearly polarized light upon reflection, due to magneto-optical interaction [16]. In order to illustrate how delicate the characterization of the deposited film should be if meaningful magnetic information is to be obtained, we show in fig. 2 the Kerr intensity at 220 K as a function of the external magnetic field for two films of Co of exactly the same coverage (2 ML) and deposited at the same temperature (450 K) but annealed, respectively, to 525 (a) and 575 K (b) prior to the measurements. The external magnetic field was in the plane of the surface. The chosen geometry for the SMOKE measurements was such that the Kerr intensity was proportional to the in-plane magnetization of the Co film. The width of the loops provides a measure of the coercivity, H,. The ratio of the peak-to-peak intensities of the Auger transitions of Co (716 eV) and Cu (920 eV) is identical within the experimental error (about 5%) in both cases. Visual inspection of the LEED pattern indicates that it is still 1 X 1 with low background. In other words, the standard techniques of characterization can hardly distinguish both overlayers. The magnetic properties, however, are completely different as shown in fig. 2. In particular, the second film shows no magnetization at room temperature. A thorough study of the interdiffusion behaviour in the Co/Cu(lOO) system [15] indicates that at 575 K Co just begins to diffuse into the Cu crystal. After the annealing period (60 s in this particular case) Co is partly dissolved into the first few layers of the Cu substrate and partly on top of the surface. Co atoms inside a Cu matrix do not possess a magnetic moment [17]. The Co atoms remaining on the surface represent a coverage whose Curie temperature T, is below 220 K (see below), and, therefore, show no magnetization. Finally it should be mentioned in this respect that an additional deposit of 0.7 ML of Co on the “interdiffused” 2 ML thick film of fig. 2b results in

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J.J. de Miguel et al. / Thin Jerromugner~c cobalt films on Cu(lOO)

Fig. 2. Surface magneto-optic Kerr effect (SMOKE) intensity as a function of the externally applied magnetic field for (a) 2 ML of Co on Cu(100). deposited at 450 K, annealed at 525 K and measured at 220 K. (b) same as (a) but annealed at 575 K and (c) after additional deposition of 0.7 ML of Co onto (b) sample. The magnetic field was applied parallel to the film plane and the Kerr intensity was proportional to the in-plane magnetization. Broken lines are guides to the eye.

magnetization of the adlayer as shown in fig. 2c. These selected examples show that a careful study of the interdiffusion processes in this magnetic films is a crucial prerequisite to any reliable study of their magnetic properties. SMOKE data taken on well characterized Co films show that the magnetization vector lies in the plane of the surface for coverages above 2 ML of cobalt. Below this coverage an orientation of the magnetic moments in the film plane could not be found at 300 K. A possible orientation of the magnetization vector perpendicular to the plane of the film at these low coverages can be ruled out on the basis of polar SMOKE measurements. This effect is due to the lack of long range ferromagnetic order at this temperature. Actually, the Curie temperature, T,, of fee Co films of various thicknesses can be easily determined from SMOKE hysteresis loops recorded at increasing temperatures. A representative set of measurements is displayed in fig. 3 for 2 ML of Co on Cu(100). Upon increasing the temperature, the hysteresis loop shrinks

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Fig. 3. SMOKE intensity as a function of the in-plane applied field H for 2 ML of Co on Cu( 100) at increasing substrate temperatures. The broken lines are guides to the eye. From this type of measurements the Curie temperature can be easily obtained.

Fig. 4. Curie temperature, T,, of fee Co films grown on Cu(100) as a function of the thickness of the film as obtained from SMOKE hysteresis loops. The broken line indicates the Curie temperature of bulk cobalt. The inset shows a particular example of the determination of Tc. The method used was to plot the Kerr intensity at saturation (extrapolated to zero) as a function of 7’. The error bars ( f 20 K, + 0.1 ML) represent upper limits of both experimental uncertainties and other methods to determine r, from the data.

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down. From the data, a plot of the Kerr intensity at saturation (extrapolated to H = 0),IK(0), versus T can be constructed for each film as shown in the inset of fig. 4 for 2 ML of cobalt. With the criterium that T, is taken to be the temperature where 1k(O) goes to zero, the thickness dependence of Tc for fee Co films can be obtained. The data points are shown in fig. 4. They span the range from 1.5 to 2.5 ML. At coverages larger than 3 ML, the Curie temperature was certainly above the dissolution temperature of Co into the Cu crystal, preventing a precise determination of T, values. On the other hand, for coverages below 1.5 ML the Co film did not show any magnetization at the lowest temperature available in this particular experimental set-up, i.e. 1.50 K. It should be mentioned that photoemission spectra (not shown) taken in the 0.3 to 2 ML range display the same features as the spectra corresponding to the thicker coverages suggesting that the different magnetic behaviour is not due to a change in the electronic structure [18]. Accordingly, fee cobalt films of thicknesses below 1.5 ML may also have a ferromagnetic ground state but these fihns will not exhibit long range magnetic order above 150 K in contradiction to recent reports [5]. In conclusion, our data show a strong dependence of the Curie temperature with the overlayer thickness for the Co/Cu(lOO) system in agreement with previously published data for Co/Cu [4], Fe/Cu(OOl) [7], Ni/Re(OOOl) [19] or Ni/Mo superlattices [20] but in strong contradiction to recent data for the same system [5].

Acknowledgement

We gratefully acknowledge financial support from the DGICyT through contract PB86-0117 and from the Spanish and German Ministeriums for Research through Joint Action number 3/22 type A.

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[9] J.H. Neave, B.A. Joyce, P.J. Dobson and N. Norton, Appl. Phys. A 31 (1983) 1. [lo] P.K. Larsen, P.J. Dobson, J.H. Neave, B.A. Joyce, B. Bijlger and J. Zhang, Surface Sci. 169 (1986) 176. [ll] L. Gomez, S. Bourgeal, J. Ibaiiez and M. Salmaron, Phys. Rev. B 31 (1985) 2551. [I21 J.J. de Miguel, A. Cebollada, J.M. Gallego, J. Fe&n and S. Ferrer, J. Crystal Growth 88 (1988) 442. [13] M.C. T&ides and M.G. Lagally, Surface Sci. 195 (1988) L159. [14] J.J. de Miguel, A. Sanchez, A. Cebollada, J.M. Gallego, J. Fe&n and S. Ferrer, Surface Sci. 189/190 (1987) 1062. [15] J.J. de Miguel, CM. Schneider, R. Miranda and J. Kirschner. to be published. [16] SD. Bader, E.R. Moog and P. Grtinberg, J. Magnetism Magnetic Mater. 53 (1986) L295. [17] W. Wei and G. Bergmann, Phys. Rev. B 37 (1988) 5990. [18] CM. Schneider, J.J. de Miguel, P. Bressler, J. Garbe, S. Ferrer, R. Miranda and J. Kirschner, Proc. ICM’88, Paris, August 1988, J. Phys. (Paris), to be published. [19] R. Bergholz and U. Gradmann, J. Magnetism Magnetic Mater. 45 (1984) 389. 1201 I.K. Schuller and M. Grimsditch, J. Appl. Phys. 55 (1984) 2491.