Surface ferromagnetism and chemisorption: Polarized electron diffraction calculations on Fe(110)p(2×2)-S

Surface Science 139 (1984) L191-LI96 North-Holland, Amsterdam

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S U R F A C E SCIENCE LETTERS SURFACE F E R R O M A G N E T I S M A N D CHEMISORPTION: POLARIZED ELECTRON DIFFRACTION CALCULATIONS O N Fe(110)!~2 × 2)-S E T A M U R A * and R. F E D E R Theoretlsche Festkorperphystk, FB 10, Umversltat Dutsburg GH, D- 4100 Duisburg, Fed Rep of Germany

Received 2 November 1983, accepted for pubhcatlon 1 February 1984

Intensity and exchange-reducedscattering asymmetry versus energy profdes have been calculated for spm-polartzedlow-energyelectron dfffracuon(SPLEED) from ferromagnetic Fe(ll0)p(2 ×2)-S The strong sensmwty of our asymmetry results to the magnetization of the topmost substrate layer suggests SPLEED as a powerful probe of the effect of chemlsorptlon and adsorbate-mduced substrate reconstruction on surface ferromagnetism

The abd~ty of elastic Spin-Polarized Low-Energy Electron Dtffract~on (SPLEED) (cf. reviews [1,2] and references thereto) to determine, via comparison of experimental scattering asymmetry data w~th their theoretxcal counterparts, the surface magneUzat~on of 3d-ferromagnets, has recently been clearly demonstrated for clean crystalhne surfaces, for Fe(ll0), the surface magnetic moment (at room temperature) was found to be strongly enhanced (by about 30%) with respect to the bulk [3-5], wtule for NI(001) a shght enhancement ( + 5 % + 5%) (at zero temperature) was mferred [6,7], m accordance with first-prmoples self-conststent electromc ground state calculations ([8,9] and references thereto). If foreign atoms are adsorbed on a ferromagnetic surface, the magnetization is generally expected to be reduced (cf. ref [10] and references therein). Such "chermsorpuon-mduced magnetic reconstruction" should, of course, mamfest itself m SPLEED asymmetry spectra. Indeed, significant changes were recently observed m SPLEED form Fe(ll0)p(2 × 2)-S [11]. Ttus system is of parucular interest, since a LEED intensity analys~s also suggests an adsorbate-mduced geometrical reconstrucUon [12] SPLEED asymmetries should therefore be affected firstly by the change m the magnet~zaUon due to chemisorpt~on and geometrical reconstruction, and secondly by scattermg contributions from the adsorbate layer. It ~s the mm of the present study to disentangle, by means of model calculations, the different effects and to * Present address InsUtut fur Festkorperphyslk, KernforschungsanlageJuhch, Postfach 1913, D-5170 Jullch, Fed Rep of Germany 0039-6028/84/$03.00 © Elsevier Soence Pubhshers B.V (North-Holland Physics Pubhshmg Division)

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E Tamura, R Feder / Surface ferromagnettsm and chemtsorptton

explore the capability of SPLEED to determine the layer-dependent magnetization of adsorbate systems As in refs [4,7], calculations were done by means of a Schrodinger-equation-based layer-KKR LEED formalism employing spin-dependent effective scattering potentials V ° (o = + / for incident electron spin alignment parallel/antiparallel to the ferromagnetic majority spin axis) To handle the adsorbate-lnduced reconstruction, the lntra-layer part of our computer code was extended to four atoms per unit cell, as in ref [13] The intensities Io~ calculated for the diffracted beams (labelled by index n) yield the spin-averaged intensity Io~ and the exchange-induced scattering asymmetry A~x according to

I ~=(1++1:)/2,

Aex=(I+-l"_)/(I++l~)

(1)

We recall (cf refs [1,2]) that the magnetic surface information Is coded in A~x, which is experimentally accessible in special geometrical set-ups (magnetization perpendicular or parallel to scattering plane) For the clean Fe(ll0) surface, the structural and non-structural (especially potentials and their spin dependence) model assumptions are chosen as in ref [4] For the p(2 × 2)-S adsorbate, two geometrical models are used, which were put forward in a LEED intensity analysis [12] with an S - F e interlayer distance of 1 43 .A,, one model assumes the topmost Fe layer as bulk-like, and the other, found preferable in ref [12], involves a lateral (2 × 2) reconstruction of the topmost Fe layer such as to enlarge the four-fold hollows at which S atoms are adsorbed The S ion-core potential was constructed from overlapping charge densities of an S atom and its four nearest Fe neighbours, using an energy-dependent exchange approximation [14] The Debye temperature and the inner potential of the adsorbate layer are assumed to be the same as for the substrate The surface barrier is taken as refracting but non-reflecting For the magnetization of the topmost Fe layer, the bulk value as well as enhanced and reduced values are considered The other Fe layers are given the bulk value In the following we present typical results, obtained for normal Incidence of a polarized primary beam For the (½½) beam (fig 1), the spin-averaged intensity spectrum from the adsorbate system is seen to be appreciably affected by the reconstruction of the topmost substrate layer We note the good agreement w~th the calculated and measured spectra of ref [12] Comparison with an intensity spectrum calculated for the clean reconstructed substrate (fig 1) reveals the dominant role of the overlayer In accordance with earlier results from clean surfaces (cf refs [1,2,4] and references thereto), I is not affected by changes of the surface magnetization The exchange asymmetry Aex spectra obtained for M I = 1 0 (i e magnetization of the topmost Fe layer equal to the bulk magnetization) show that substrate reconstrucUon and adsorption by themselves produce comparable peak magnitudes with some slmdarities (e g around 60 eV) and marked differences (e g around 93 eV) in the hne shape Since the S overlayer has been assumed as non-magnetic, the sizeable A~x

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values obtained for the adsorbate system without substrate reconstruction must be due to multiple scattering between overlayer and substrate. The combmatlon of overlayer and substrate reconstruction ts seen (fig. 1) to produce Aex peaks about twice as big, and strong changes (e.g. around 93 eV) in the line shape. Aex of the (½½) beam is thus quite sensitive to the geometrical model assumptions. As for its sensiUwty to the magnetization M 1 of the topmost substrate layer, calculations for M 1 ranging from 0 to 1 5 reveal (fig. 1) firstly that the overall shape of the Aex profile is preserved, and secondly that most peaks (e.g. at 32, near 45, at 74 and 96 eV) grow substantially with increasing M1, wtule the peak at 59 eV is almost unaffected. Relative height ratios are thus statable to determine MI via comparison with experiment. We note that for the (½ ½) beam, the scattering plane is a (170) mirror plane, i e. parallel to the easy magnetization direction (001). r

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Fig 2 As fig 1, except (10) beam, only M I = 1 (l e as bulk), plus spectra for clean unreconstructed F e ( l l 0 ) ( - -) Fig 3 A ~ of the (10) b e a m for n o r m a l incidence on Fe(110)p(2 x 2)-S w n h reconstructed top Fe layer with m a g n e t i z a t i o n M 1 (relatwe to bulk) as indicated

For the (10) beam, which already exists for the clean unreconstructed substrate, our spin-averaged intensity results (fig 2) are again in good agreement with experiment and earher calculations [12,15] Reconstruction is seen to have a fairly small effect on I, the same holds for Aex, except for the m i n u s / p l u s feature around 45 eV In contrast, variation of M] strongly affects Aex (cf fig 3) with increasing M 1, A~. at 38 eV goes from - 12% to +24%, the m i n u s / p l u s feature around 44 eV gets weaker, whale the one around 82 eV drastically increases Ale° thus provides a very sensltwe measure for the surface magnetization [16] For the specular beam, we also fred energy regions of strong s e n s m w t y Unfortunately, no experimental A~x data are yet avadable to c o m p a r e with our calculated normal-incidence results In the hope of making contact with the only avadable A~. data, a recent specular beam spectrum measured from a segregation-prepared F e ( l l 0 ) p ( 2 x 2)-S surface [11], we also performed calculauons at a polar angle of 45 ° Fig 4 shows that the A °° spectrum is, except between 65 and 75 eV, not only lnsensltwe to the reconstruction of the topmost

E Tamura, R Feder / Surface ferromagnetism and chemtsorpnon

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Fig 4 (00) beam m SPLEED from Fe(ll0)p(2×2)-S for incidence at a polar angle of 45 ° m a (110) azamuthal plane intensity I and asymmetry Aex calculated w~th ( ) and wtthout ( - - - ) reconstruction of the topmost Fe layer I does not depend on the magnetlzatlon M 1 of tlus layer, Aex Is shown for M 1 = 1 and M l = 0

substrate layer, but responds also very weakly to a change of the magnetization of this layer from the bulk value to zero. In contrast to the normal inodence Ale° spectrum (cf. fig. 3), the 45 ° A ~ spectrum is therefore a priori unsmtable for deterrmnlng the surface magnetization Irrespective of how well it might agree with experiment [17]. We recall that some magnetlzatlon-msensmve and therefore unsmtable Ae~ curves were found earlier also for clean surfaces [4,7], and that it is an essenual prerequisite for surface magnetism analysis by SPLEED to find Aex features, which respond strongly to changes in the surface magnetization. In conclusion, our calculations of SPLEED from Fe(l10)p(2 × 2)-S reveal the existence of several sizeable scattering asymmetry maxima, wbach are highly sensmve to changes in the magnettzatlon of the topmost substrate layer. Comparison w~th corresponding experimental data wdl therefore pernut the deternunatlon of the layer-dependent magnetization of ttus absorbate system, and extensive experiments m this area seem highly desirable. This work has been finanoally supported by the Deutsche Forschungsgemeinschaft and furthered by the cooperauve hospltahty of the Insutut fur Festkorperforschung of the KFA Juhch. Also, we would like to thank J. Karschner for &scusslons and information on experimental data.

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

[17]

R Feder, J Phys C14 (1981) 2049 R Feder, Phys Scnpta T4 (1983) 47 G Waller and U G r a d m a n n , Phys Rev B26 (1982) 6330 E Tamura and R Feder, Sohd State C o m m u n 44 (1982) 1101 U Gradmann, G Waller, R Feder and E Tamura, J Magnetism Magnetic Mater 31-34 (1983) 883 S F Alvarado, R Feder, H Hopster, F Clccaccl and H Pleyer, Z Physlk B49 (1982) 129 R Feder, S F Alvarado, E Tamura and E Klsker, Surface Scl 127 (1983) 83 O Jepsen, J Madsen and O K Andersen, Phys Rev B26 (1982) 2790 A J Freeman, J M a g n e u s m Magneuc Mater 35 (1983) 31 W Gopel, Surface Scl 85 (1979) 400 J Karschner, Surface Scl 138 (1984) 191 H D Shah, F Jona, D W Jepsen and P M Marcus, Phys Rev Letters 46 (1981) 731 R Feder, W Monch and P P Auer, J Phys C12 (1979) L179 J C Slater, T M Wdson and J W Wood, Phys Rev 179 (1969) 28 H D Shah, F Jona, U Bardl and P M Marcus, J Phys C13 (1980) 3801 For the (10) beam from the adsorbate system, the scattering plane is neither a marror plane nor parallel or perpendtcular to the (001) magnetization axas, I e we do not have one of the special geometries (cf refs [1,2] and references thereto) To obtain an experimental A~°, we propose to add, for each spin &rectlon, the (10) and (01) intensities or the (10 and (07) intensities and thus numlc the parallel or perpendicular geometry, respectively As can be seen by comparing our fig 4 with fig 4 or ref [11], the agreement is actually unsatisfactory not only for Aex, but also for the spin-averaged intensity spectrum Since our calculated intensity spectra consistently reproduce the whole set of theoretical and experimental spectra underlying the geometncal structure analysis of ref [12], the d~screpancles w~th the 45 ° intensity spectrum (and consequently Acx ) of ref [11] are likely to be of experimental o n g m As &scussed in ref [11], the surface structure prepared by sulfur segregation at 700°C [11] rmght be different from the one obtained m ref [12] by means of adsorption at room temperature Since surface structure analysis by LEED reqmres definitely more than one spectrum, the segregation-reduced geometry can, however, at present not be deternnned To resolve the issue, more experimental intensity data are necessary, preferably for the same beams and angles of incidence as already used m ref [12]