disorder transition at a Cu3Au(100) surface

Surface Science 159 (1985) L451-L459 North-Holland, A m s t e r d a m

L451

SURFACE SCIENCE LETTERS POLARIZED LEED INVESTIGATION OF THE ORDER/DISORDER TRANSITION AT A Cu3Au(100) SURFACE K.D. JAMISON, D.M. LIND, F.B. DUNNING and G.K. WALTER~ Department of Physics and The Rice Quantum Institute, Rice Unioersity, Houston, Texas 77251, USA Received 8 February 1985; accepted for publication 12 April 1985

The results of an exploratory polarized LEED study of the order/disorder transition at a Cu3Au(100 ) surface are reported. The data provide no evidence of a sudden change in surface composition or local order at the bulk transition temperature, and are consistent with the results of earlier LEED and low energy ion scattering studies.

In recent years there has been renewed interest in the study of order/disorder transitions at Cu3Au surfaces. Cu3Au is a classic ordering alloy that undergoes a discontinuous bulk order/disorder transition at a critical temperature Tc of 390°C. The behavior of Cu3Au surfaces in the vicinity of Tc has been investigated using a number of techniques, including low energy ion scattering (LEIS) [1], and low energy electron diffraction (LEED) [2-5]. These studies indicate a continuous surface order/disorder transition, with the possibility of a sudden component at Tc [5]. The results obtained using these techniques, however, have yet to be reconciled theoretically [6], and the nature of the transition is still not completely understood. Earlier investigations in this laboratory, and elsewhere, have shown that polarized LEED provides a sensitive probe of surface geometric structure and reconstruction [7,8]. Further, the spin-orbit effect, which gives rise to polarization effects in LEED, is strongly dependent on dtomic number Z. Thus, because copper and gold atoms have very different values of Z (29 and 79 respectively), substantial changes in composition or geometrical arrangement at a C u 3 A u surface due to an order/disorder transition might be expected to lead to marked changes in the observed LEED polarization features. Here we report the results of an exploratory polarized LEED study of the order/disorder transition at a Cu3Au(100 ) surface. The data, which provide the first polarized LEED measurements on a binary alloy, are consistent with the results of earlier investigations and provide no evidence of sudden changes in surface composition or local order at T~. The present apparatus, shown schematically in fig. 1, is similar to that used in earlier polarized LEED studies in this laboratory [8-10] except for the 0039-6028/85/$03.30 © Elsevier Science Publishers B.V.

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K.D. Jamison et al. / Order/disorder transition at Cu 3Au(l O0)

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addition of a GaAs spin-polarized electron source and a second L E E D optics assembly. Polarized LEED data are recorded using two complementary techniques. In the first, described in detail elsewhere [9,10], an unpolarized incident electron beam from a conventional L E E D electron gun is used and the polarization P(E, O) of the diffracted beam, relative to the normal h to the scattering plane, is determined by means of a Mort polarimeter. The direction of h is defined via h = k × k ' / I k × k' I where k and k ' are the wave vectors associated with the incident and scattered electrons, respectively. For these measurements the target crystal is located at the position labeled 1 in fig. 1. In the second experimental approach a spin-polarized incident electron beam is used and the spin-dependence of the intensities of the diffracted beams is measured. For such studies the crystal is moved to the position labeled 2 in fig. 1. The spin-dependence of the beam intensities is specified by means of an asymmetry parameter A ( E , 0)

A(E, 0 ) = - -1 I ( P o ) - I ( - P o ) leol I(Po)+l(-po)' where P0 is the spin-polarization of the incident beam and I(P0) and I ( - P 0 ) are the scattered intensities with the incident electron spins polarized parallel and antiparallel to h, respectively. The majority of the data reported here were obtained using the polarized electron source. The polarized electron source is patterned closely after that described by Pierce et al. [11]. Circularly polarized radiation from a GaA1As laser is directed normally onto a room-temperature negative electron affinity GaAs(100) surface, creating longitudinally polarized photoelectrons. These electrons are accelerated and passed through a 90 ° electrostatic deflector. The emergent beam,

K.D. Jarnison et al. / Order~disorder transition at Cu 3Au(lO0)

L453

now transversely polarized, is directed through a series of electron trans~¢ort lenses and is focussed on the target surface. The incident current at the target is - 0 . 3 - 0 . 5 /.tA. The polarization P0 of the beam, determined directly by moving the target crystal so that the beam can enter the Mott polarimeter, is - 27%. The polarization can be simply reversed, P0 ~ - P 0 , without influencing the beam current by changing the sense of circular polarization of the radiation incident on the GaAs photoemitter. The sense of circular polarization is modulated at 500 Hz using a Pockels cell. This makes possible simultaneous measurement of the spin-asymmetry parameters and spin-averaged intensities of the diffracted beams. The beam intensities are determined using a movable Faraday cup. The AC component in the current to the Faraday cup, which is measured using phase-sensitive detection techniques, gives directly the spin dependence of the scattered intensity. The average D C current to ~the cup, measured using an electrometer, gives the spin-averaged intensity. The Cu3Au crystal is mounted on the end of a thin molybdenum can that contains an electrically isolated tungsten filament. The filament is used to heat the can, and hence crystal, either radiatively or by electron bombardment. The crystal temperature is measured by a thermocouple attached to the end of the molybdenum can close to the crystal. Prior to each data taking run, the surface was cleaned by sputtering with 750 eV argon ions for - 20 min followed by annealing for 20 rain at - 500°C. Auger analysis showed that no contaminants remained on the surface following this treatment. The surface was then further annealed for - 45 min just below T~. This resulted in a well-ordered surface which, at room temperature, produced a sharp L E E D pattern with well defined half-order superlattice beams. Annealing below T~ for longer periods, even up to - 4 0 h, did not improve the LEED pattern or lead to changes in the observed intensity and polarization features. Polarized L E E D and intensity data were recorded for both the 00 and 10 beams at several crystal temperatures ranging from room temperature to 50°C above T~. The half-order superlattice beams were not investigated because their intensity decreases very rapidly as T~ is approached, making it difficult to study spin dependences close to and above Tc. The electron energies quoted here have been corrected for the work function differences between the GaAs photoemitter and Cu3Au surface and give the mean kinetic energy of the incident electrons in vacuum. Fig. 2 shows representative intensity-energy ( I - V ) and asymmetry parameter-energy (A-V) profiles for the 00 beam from a room temperature Cu3Au(100 ) surface at several angles of incidence 0 and an azimuthal angle q~ = 0 °. The vertical extent of each data point in these, and other, A - V profiles indicates the standard deviation about the mean of the measured asymmetry parameters at each energy. Sizeable spin dependences in the scattered intensities are observed that depend strongly on electron energy and angle of incidence. As in earlier polarized L E E D investigations, the largest spin-depen-

L454

K.D. Jarnison et al. / O r d e r / d i s o r d e r transition at Cu 3 Au( l O0)

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d e n t effects are typically o b s e r v e d in the vicinity of intensity m i n i m a . A direct c o m p a r i s o n b e t w e e n p o l a r i z e d L E E D d a t a o b t a i n e d using the M o t t p o l a r i m e t e r a n d the p o l a r i z e d electron source is p r e s e n t e d in fig. 3. The A - V profile for the 00 b e a m at 0 = 12 ° is shown b y the o p e n circles. T h e p o l a r i z a tion P(E, O) of the 00 b e a m , at the same angle of incidence, resulting from the scattering of u n p o l a r i z e d incident electrons is i n d i c a t e d b y the solid d a t a points. T h e m e a s u r e d values of A(E, O) a n d P(E, O) are in g o o d agreement, a n d a n y small differences a p p a r e n t in fig. 3 can be r e a d i l y a t t r i b u t e d to the s y s t e m a t i c error ( - + ~l o ) inherent in the m e a s u r e m e n t of 0. The g o o d a g r e e m e n t b e t w e e n A(E, O) a n d P(E, O) is e x p e c t e d b e c a u s e s y m m e t r y c o n s i d e r a t i o n s [12,13] require that these be equal when, as in the p r e s e n t work, the scattering p l a n e is a m i r r o r s y m m e t r y p l a n e in the crystal. T h e t e m p e r a t u r e d e p e n d e n c e of the A - V profiles for the 00 b e a m at 0 = 13 ° a n d the 10 b e a m at n o r m a l incidence are shown in figs. 4 a n d 5. H e a t i n g the crystal leads to a decrease in the m a g n i t u d e s of the various

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asymmetry features. However, no pronounced change in the form of the A - V profiles is observed as the crystal is heated through Tc. Heating also results in a sizeable decrease in the magnitudes of the intensity features, but again no changes in the form of the I - V profiles are apparent in the vicinity of T~. The reduced beam intensities at the higher temperatures lead to somewhat increased statistical uncertainties in the measured asymmetry parameters. In order to obtain a more detailed picture of the temperature dependence of the asymmetries, measurements were made at selected electron energies as a function of temperature. Expansion of the lattice on heating will result in small changes in the positions of the various asymmetry features [14,15], and thus may lead to a temperature dependence in the asymmetries. To minimize this effect measurements were confined to regions where the asymmetry parameter is not strongly energy dependent. Data for the 00 beam at 63 eV and the 10 beam at 49 eV are shown in fig. 6. For each data set the magnitude of the asymmetry parameter decreases steadily with increasing temperature. The rate of this decrease, however, increases markedly in the vicinity of To. Fig. 6 also shows the corresponding intensities which, as noted in earlier LEED studies [4,5], display a marked departure from simple exponential behavior as T~ is approached. The decrease in the magnitudes of the asymmetry features observed as the crystal is heated is attributable to diffuse scattering. Polarized LEED measurements are sensitive to diffuse scattering because the most pronounced asymmetry features generally occur in the vicinity of intensity minima, where the diffracted beam current is small, and because diffuse scattering is spin independent [7]. This spin-independence was confirmed by positioning the Faraday cup so that only electrons produced by diffuse scattering entered. The asymmetry parameters measured under such conditions were equal to zero, within

L456

K.D. Jamison et al. / Order~disorder transition at Cu ~Au(lO0)

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experimental error, at all energies. Diffuse scattering thus provides a spin-independent contribution to the total Faraday cup current. As the sample is heated, the diffracted beam intensities decrease whereas diffuse scattering increases. Diffuse scattering therefore accounts for an increasing fraction of the total Faraday cup current, resulting in a decrease in the magnitudes of the observed asymmetry features. This decrease is typically less pronounced for the 00 beam than for the 10 beam because the 00 beam intensity is larger. For both beams, however, the rate of this decrease with temperature increases significantly in the vicinity of Tc. This is expected because the beam intensities begin to decrease more rapidly in this region. However, the de.creased intensities also suggest an increase in diffuse scattering, which would further decrease the magnitudes of the asymmetry features. Measurements of the diffuse back-

K.D. Jamison et al. / Order~disorder transition at Cu 3Au(l O0)

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Fig. 5. Temperature dependence of the asymmetry parameter-energy profiles for the 10 beam at normal incidence. ground showed that diffuse scattering accounted for much, if not all, of the observed decrease in the magnitudes of the asymmetry features with increasing temperature. No significant changes in the form of the A - V profiles, or new asymmetry features, are apparent at temperatures in the vicinity of, or above, T~. Since the spin-orbit effect, which gives rise to the observed spin-dependent effects, depends strongly on Z, this implies that no sudden changes in surface composition occur in the vicinity of Tc. This is consistent with the results of LEIS studies which show that the gold concentration in the first and second surface layers remains approximately constant at 0.5 and 0, respectively, below Tc and changes only very slowly with temperature above To. This indicates that any disordering that occurs near Tc must result from repositioning of atoms

K, DI Jamison et al. / Order~disorder transition at Cu 3 A u ( l O0)

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within individual surface layers and not from the transfer of atoms between layers. Evidence of such disordering is provided by the increased temperature dependence of both the intensities and asymmetry parameters in the vicinity of Tc [4,5,16]. N o abrupt changes in these temperature dependences are evident at To, suggesting that the associated phase transition is continuous, although the experimental uncertainties are such that the presence of small discontinuous component cannot be ruled out. The failure to observe any changes in the form of the asymmetry profiles, however, suggests that residual order even in the top layer persists above T~. These conclusions are consistent with those derived from earlier LEED studies. In summary, the polarized LEED data provide no evidence of sudden changes in surface composition or order at Tc; rather the data suggest that the surface disorders continuously and retains local order above T~. Preliminary polarized LEED studies of a C u 3 A u ( l l l ) surface were also undertaken. The data displayed the same general characteristics as those for the (100) surface,

K.D. Jamison et aL / Order/disorder transition at Cu 3Au( l O0)

and again provide no evidence of a sudden surface order/disorder

L459 t r a n s i t i o n at

L. It is a p l e a s u r e to a c k n o w l e d g e v a l u a b l e d i s c u s s i o n s w i t h A. I g n a t i e v a n d H . L . D a v i s a n d t h e a s s i s t a n c e o f W. P r i b b l e in d a t a a c q u i s i t i o n . T h i s r e s e a r c h is s u p p o r t e d b y the D i v i s i o n o f M a t e r i a l s Sciences, O f f i c e of Basic E n e r g y Sciences, U S D e p a r t m e n t o f E n e r g y , a n d the R o b e r t A. W e l c h F o u n d a t i o n .

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

T.M. Buck, G.H. Wheatley and L. Marchut, Phys. Rev. Letters 51 (1983) 43. V.S. Sundaram, R.S. Alben and W.D. Robertson, Surface Sci. 46 (1974) 653. H.C. Potter and J.M. Blakely, J. Vacuum Sci. Technol. 12 (1975) 635. V.S. Sundaram and W.D. Robertson, Surface Sci. 55 (1976) 324. E.G. McRae and R.A. Malic, Surface Sci. 148 (1984) 551. V. Kumar and K.H. Bennemann, Phys. Rev. Letters 53 (1984) 278. For a comprehensive review of polarized LEED, see R. Feder, J. Phys. C14 (1981) 2049. A.H. Mahan, T.W. Riddle, F.B. Dunning and G.K. Walters, Surface Sci. 93 (1980) 550, plus references therein. M. Kalisvaart, M.R. O'Neill, T.W. Riddle, F.B. Dunning and G.K. Walters, Phys. Rev. B17 (1978) 1570. J.K. Lang, K.D. Jamison, F.B. Dunning, G.K. Waiters, M.A. Passler, A. Ignatiev, E. Tamura and R. Feder, Surface Sci. 123 (1982) 247. D.T. Pierce, R.J. Celona, G.-C. Wang, W.N. Unertl, A. Galejs, C.E. Kuyatt and S.R. Mielczarek, Rev. Sci. Instr. 51 (1980) 478. G.-C. Wang, B.I. Dunlap, R.J. Celotta and D.T. Pierce, Phys. Rev. Letters 42 (1979) 1349. R. Feder, Phys. Letters 78A (1980) 103. T.W. Riddle, A.H. Mahan, F.B. Dunning and G.K. Waiters, J. Vacuum Sci. Technol. 15 (1978) 1686. J. Kirschner and R. Feder, Surface Sci. 104 (1981) 448. A.A. Maradudin and P.A. Flinn, Phys. Rev. 129 (1963) 2529.