Oscillations of the galvanomagnetic properties of evaporated nickel films due to the adsorption of carbon monoxide

S~rfaee Science 58 (~9'76) 597-600 © North-Holland Publi,,t.hingCompany

OSCILLATIONS OF THE GALVANOMAGNET1C PROPERTIES OF EVAPORATED NICKEL F [LMS DUE TO THE ADSORPTION OF CARBON MONOXIDE Received 14 April 1976

In the recent years the adsorption of CO on Ni films evaporated under uhv conditions has been investigated intensively. A survey of the measurements of the adsorption isotherm, 1:he heat of adsorption, the desorption spectra, the work function, and the change in resistivity is given in ref. [1]. The measurements of the change in resistivity and the change in thermopower are sumlaadzed in ref. [2]. The ferromagnetic metals Fe, Co, Ni are very good catalysts for the hydrogenation of CO. Therefore it seemed desirable to investigate in addition the influence of the adsorption of CO on the galvanomagnetic and on the magnetic 5roperties of the Ni films. Ti~c Ni films were evaporated under pressures lower titan 8 X 10 -10 Totr, condensed on glass substrates at room temperature, and annealed at the s~me temperature. The experimental procedure was similar to that described in ref. [3]. The investigated prop~:ties were the resistivity p, the perpendicular magnetoresistivity APmag (preferably the decrease in resistivity above the saturation magnetization). the extraordinary Hall coefficient RHE , the ordinary Hall coefficie:at RHO, and the saturation magnetization B S determined from the point, of inters"ction of the two linear branches of the curve Hall voltage versus magnetic field [4]. The magnetic field strength could be varied continously between 0 and 13 kG. Tlae properties of the clean Fdms, especiaUy the thickness dependence of the effects mentioned above are published elsewhere [5]. The CO was a0mitted in small amounts by means of ampoules. The relative coverage 0 (0 = 1 corresponds to a monolayer, see ref. [1])was determined from the adsorpzion isotherms and from the calibrated curves Ap versus 0. The results found at full coverage (0 = 1) agree very well with those published by other authors: (a) The resistivity of the Ni f'dm increases considerably. This has been explained in former papers [1,6--8] mainly as the consequence of an additional scattering of the conduction electrons by the adsorbed species. (b) The extraordinary Hall coefficient and the ordinary Hall coefficient increase [9]. This can be ascribed to an anisotropic scattering of the charge carriers at the adsorbed molecules as has been shown for the system Cu/CO [3]. (c) The saturation magnetization decreases. The same has be,m reported by Sel597

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woo,t [ ! 01 for supported Ni catalysts. This author attributes the effect to the filling of the d-band (d) The decrease in r¢.~istivity in a perpendicular magnetic field gets smaller, both the total naagnetoresistwity [9] (between 0 and 13 kG) and the ordinary part of the magnetoresistivity (between the field at the saturation magneti::ation and 13 kG). After Sommerfeld and Bethe [11] the decrease in resistivity ~,f a ferromagnetic ~,.etai in a magnetic field is due to the additional symmetry of the oriented spins. This orientation might partially be destroyed by the adsorbed CO molecules thus di nini.~fing the decrease in resistivity. ~ . ; remits cbtained at low coverages have, to our knowledge, not yet been observe¢. Fig. 1 sbows the coverage dependence of the or6inary Hall coefficient which has ~-e.n studied most intensively. In order to compare the res:.lts obtained with four films of different thickness the ordinate is normalized to tl,.e value of the Hall coeffident at (/= 0.225. The most striking observation is the o.,c~!lating behaviour of the ,:~rdinars' Hall coefficient with increasing coverage. The ~mplitude ,of the oscillati..:,ns is much greater than the experimental erroz (as indicated in fig. 1)and is grea~er than "he overall increase h~ the ordinary Hall coefficient between O 0 and O = 1. At coverages 0 > 0.6 the reproducibility for different f'thns is not so good as at coverages 0 < 0.6. As ;hewn in fig. 2 an oscillating behaviour is also found for the magnetoresistivit.,,' APm~,o above the saturation magne:ization (fig 2d) and for the saturation magnetization B s (fig. 2c) with increasing coverage. The oscillations of the magnetoresistivity are opposite to those c,f RHO , in the case oi" the saturation magnetization regions of oscillations parallel or opposite to those of RHO are observed. The chJnge ir~. the extraordi~aa~, Hall coefficient RHE shows only one maximum for 0 -< 0.6. Although the reproducibility of the measurements of &PmagO, BS and RHE for different ill,ms is worse than that of RHO , the amplitude of the oscillations is considerably greater than the experimenzal error (as indicated in the figures). ,';in,:c file transport properties - resistivity p (fig. 2a) and thermopower [2] - do =

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Fig. 2. Schematit: representation o£ the changes of (a) the resistivity p, (b) the extraordinary Hall coefficient EHE , (c) the saturation magnetization BS, (d) the ordinary p~LrtOf the magnetoresistivity AprnagO, (e) the ordinary Hal| coefficient RHO in arbitrary units as a function of the cuverage 0.

not show any oscillations with increasing coverage in the absence of a magnetic field the oscillations, superimposed on the basic effects which are discussed above in connection with full coverage, should lze attributed to enezgetic quantum levels due to the magaetic field. Such quantum states might be energy" levels of spin waves (magnons) or volume or surface Landau levels. An influence ofmagnons on adsorption and desorption properties of paramagnetic metals [12] and on catalytic properties of ferromagnetic metals 113] ha:~ been discussed in the literature. On the other han,:l it is known that the chemisorption of CO leads to the agpear~LII~,~ UI ~ , I I I ~ , I I I t I ~ O I [ . . J I . I O I - I t~,~l~ W l | l t ~ l l . I , ) I I U..... L U ~ I ~,' ~., L. I.U.I .I . can be uu~,~ ....... v~uJ in the _L spectrum [14,15]. The metal electrons that occupy these levels of the adsorption complex must be withdrawn in some way or other from the highest occupied Landau level. Thus with increasing CO coverage the highest Landau levels should be continuously emptied, one after the other. Such a process leads to oscillauons of both the magnetization and the galvanomagnetic properties as is knov,n from the De Haas-Van Alphen effect and the Shubnikov-De ttaas effect [16]. In these effects, however, the change in occupation number is achieved by changing the position of the Landau leveis relative to the Fe,:mi level by increasing the magnetic field Ul3tlll~..L

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H. Schn¢ck. G. Wcdler / Gah'anoma~encticproperties of evaporated Ni films

strength [17]. In our case the strength of the effective magnetic field above the saturation magnetization ~emains nearly cons*ant. NormaUy quantum effects like the De Haas Van Alphen effect can only be ,~bserved at low temperalures since the energetk. ~eparation of the Landau levels must be greater than /~7". However, there are some indications that this condition is fulfilled in the case of Ni even at room temperature" The existence of levels with a separation greater than kT can be sug~sted from the cyclotron radius estimated from the thickness dependence of tt'-c ordinary Hall coefficient (Sondheimer oscillations) of the clean films [5]. Addition~.dty such levels are postulated by Krinchik et al. [18] to explain oseillatiot:s of the surface reaction rate of Ni and CO to Ni(CO) 1 at room temperature in ma~,.netic fie~ds of even smaller strenth. H. SCHNECK and G. WEDLER

b~s:qmt fi~r Phy~ikalische und 77~eoretische Chemie der Universitiit Erlangen-Niim.. berg, l-,'gerlandstrasse 3, D-8520 t,'rlangen, Germanv

References [ 1! [2] [3] [41 !5l [6] [71 I ~!

G. Wedler, H. Papp and C,. Schroll, Surface Sei. 44 (1974) 463. G. Wedler. H. Reichenbcrger and H. Wenzei, Z. Naturforsch. 26a (1971) 1452. G. Wedler and W. Wieb=uer, l'hin Solid Films 28 (1975) 65. J. I.e Bas, Thin Solid Films 10 (1972) 429. (; ~edler and ti. Schneck, to be published. G %edler and P. Wismnann, Surface Sci. 26 ~197 t) 389. P. 'Vissmann, Z. Physik. Chem. (Frankfurt,;M.) 71 (1970) 294. P. ,kissmann, The Ele :trical Resistivity of Pure and Gas Covered Metal Films, :in Springer F'~,:t~ in Modern Ph: ics:~-rgebnisse der Fxakten Naturwissenschaften, Vol. 77, St,~face Phy,qcs (Springer, Berlin, 1975). [91 W. kirstein, Thesis, Hamburg (1974). [10] P.W. Sehvood. Cherr,;~'~rption and Magnetization (Academic Press, New York, 1975: p. i '.~7 [I 1] A. :,cmmerfeld and cl. Bethe, Elet~tronentheorie der Metalle (Springer, Berlin, 1967; p. 285. [ 12] H. Suid, J.H. Smith .rod P. Kumar, Phys. Rev. Letters 25 (1970) 1442. [131 E. llisca, Phys. Rev. Letters 24 (1970) 797. [ t4] D.E. 1 astman and J.K. Cashion, Phys. Rev. Letters 27 (1971) 1520. I 1";] K ('hr,stmann, O. S.:hober and G. Ertl, J. Chem. Phys. 60 (1974) 4719. [ 16] G. Ki:tel, Qunntentheorie der Festk6rper (O!denbourg, Mfinchen, 1970) p. 233. I~. 7]. C. 3.! . . ; ~,,.-,~_,"r ... . .""11. !!fleet in Metals and Alloys (Plenum, ",,,., ..... v^.t,, ,,,,-, i o-r-~ .., ,-, p. ,,.'~' [18] C.S.b.rin,'bit, R. a . S~artsman and Ya. Kipnis, JFTP Letters 19 (1974) 231. c I

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