EXCHANGE-SPLITTING OF ADSORBATE-INDUCED BANDS IN CHEMISORPTION ON FERROMAGNETIC 3d-METALS

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EXCHANGE-SPLITTING OF

ADSORBATE-INDUCED BANDS IN

CHEMISORPTION ON FERROMAGNETIC

3d-METALS

G. Schönhense, M. Getzlaff, C. Westphal, B. Heidemann, J. Bansmann

To cite this version:

G. Schönhense, M. Getzlaff, C. Westphal, B. Heidemann, J. Bansmann.

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JOURNAL DE PHYSIQUE

Colloque C8, SupplBment au no 12, Tome 49, d6cembre 1988

EXCHANGE-SPLITTING OF ADSORBATE-INDUCED BANDS IN

CHEMISORPTION ON FERROMAGNETIC 3d-METALS

G. Schonhense, M. GetzlafT, C. Westphal, B. Heidemann and J. Bansmann

UniversitBt Bielefeld, Fakultiit fiir Physik, 0-6800 Bielefeld, F.R.G.

Abstract. - The influence of the ferromagnetism of 3d-metal substrates upon the electronic states of "nonmagnetic" adsorbate overlayers like oxygen and sulphur has been investigated by means of photoelectron spinpolarization spectroscopy. The spectra reveal a substantial exchange splitting of the 0 2 p and S 3pderived bands in chemisorption on Fe and FeCo. This demonstrates the strong magnetic coupling of these species with the substrate.

I n t r o d u c t i o n

The influence of chemisorption on surface mag- netism has been subject to a number of investigations employing different experimental techniques yielding the general conclusio~ that chemisorption essentially reduces the surface magnetic moments. Especially for oxygen-covered Ni the formation of magnetically "dead" surface layers is still being discussed [I]. How- ever, recent experimental data obtained via spin- polarized Auger-electron spectroscopy of oxygen on iron [2] and spinresolved photoemission from oxygen and sulphur on nickel [3] gave-first indications of magnetic interactions - especially of nonvanishing magnetic moments - within the overlayer. The latter ex- periment showed anomalous Ni 3d-photoemission line shapes which were interpreted as being due t o spin- flip scattering of photoelectrons on adatoms carrying magnetic moments. Theoretical calculations [4] could not reproduce these line shapes and other experiments [5] showed that in such systems spin-flip scattering should be negligible. It was the purpose of the present study t o gain new information on such chemisorption systems by looking a t the adsorbate-induced electronic states rather than a t an indirect influence of chemisorption upon the substrate 3d-bands. Similar investigations of empty adsorbate-induced bands for oxygen and sulphur on Ni(ll0) have recently been made via spin-resolved inverse photoemission [6].

E x p e r i m e n t a l

Our data were obtained by means of angle- and spin-resolved photoelectron spectroscopy. Experi- mental details as well as earlier results for adsor- bates on nonmagnetic substrates have been discussed in a recent review article [7]. Briefly, unpolar- ized light

(hv

= 21.22 eV) from a helium-lamp irra- diates the ferromagnetic sample at 45' angle of in- cidence. The photoelectrons emitted into a cone of

z t 3" around the surface normal are energy analyzed

( A E

-

150 meV FWHM) and focused into a Mott detector for spin analysis. From the measured photoelec-

tron intensity

I

and spinpolarization P the majority and minority spectra of the ferromagnetic sample are obtained separately, according t o IT,L = 1 ( 1 z t P) /2. The sample was cleaned by repeated Ar ion sputter- ing and heating cycles. It was magnetized by means of the common technique of a closed magnetic circuit. As magnetic substrates we used an Fe(ll0) single crystal as well as a polycrystalline sample made from an iron- cobalt alloy (49 % Fe, 49 % Co, and 2 % V). Oxygen was dosed t o the freshly sputtered probe whereas a sulphur overlayer could be produced via segregation by heating the iron sample t o temperatures above 1 000 K

for several minutes as described recently 181. R e s u l t s a n d discussion

The experimental results for atomic oxygen on iron are shown in figure 1. The spectra have been taken at normal emission and a t a photon energy of

hv =

21.22 eV (He1 line). The oxygen coverage corresponds to a dosage of 4 Langmuirs (1 L = torrs) a t room temperature. The upper curve shows a pho-

Fig. 1. - Photoelectron intensity spectrum I (upper curve) and partial spin-up and spin-down spectra IT, IL (lower curves) of oxygen on iron. The decomposition of the 0

2p-derived band around 6 eV below the Fermi-energy into the partial spin spectra reveals an exchange splitting of 0.9 f 0.2 eV.

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C8 - 1644 JOURNAL DE PHYSIQUE

toelectron intensity spectrum taken with an

x

- y

recorder. The spectrum shows the well-known 0 2 9

derived band centered around 6 eV below the Fermi energy, which has been observed a t almost the same binding energy on many metals [9]. The 0 2p-feature

is superimposed to a rather large background of sec- ondary electrons caused by the Fe 3d-bands just below E F , which have been studied previously [lo]. The 0 2p-derived peak is anomalously broad on iron as

compared with other non-magnetic metal substrates [9] and furthermore seems t o have an asymmetric line profile. The majority and minority partial spectra (full and open circles, respectively), taken by means of the Mott detector, reveal the reason for this anomaly: the

0 2p-band is split by roughlj. 1 eV into a spin-up and spin-down doublet due to exchange coupling with the iron substrate. Note that "up" and "down" refers to the quantization axis of the spin detector which is an- tiparallel to the target magnetization.

The corresponding spectra for atomic sulphur on iron are shown in figure 2. The behavior is quite similar as in the case of oxygen, the splitting being a factor of 1.5 larger so that the S 3p-doublet appears already in the intensity spectrum. Obviously, the exchange coupling is stronger for sulphur than for oxygen. The peak shapes in the spin-resolved spectra indicate that there might be a further multiplet structure present in the S 3p-features (p,,, p,)

.

This issue will be studied in more detail by taking spectra a t various coverages and emission angles.

-0

E-E, lev)

Fig. 2. - The same a s figure 1 but for sulphur on iron. Note that the exchange splitting of S 3p is already visible in the total intensity spectrum (upper curve).

The observed exchange splittings of chalcogen np- bands on a 3d-ferromagnet are not an exception but seem t o exhibit a common phenomenon. We have measured an oxygen 2p-splitting of 0.7 f- 0.2 eV even on a polycrystalline FeCo-sample. By means of inverse photoemission, splittings of the empty 0 2p- and

S

3 p derived bands on Ni(ll0) have been observed [6]. The values of 80

f

20 meV for 0 and UD t o 200 meV for S on

Ni again reflect the stronger magnetic coupling of sulphur. Furthermore, these numbers demonstrate that the splitting roughly scales with the exchange splitting of the substrate d-bands. Temperature - as well as coverage - dependent measurements are in progress.

Acknowledgments

We acknowledge financial support by the Deutsche Forschungsgemeinschaft through Sonderforschungs- bereich 216.

[I] Polarized Electrons in Surface Physics, Ed. by

R.

Feder (World Scientific, Singapore) 1985, p. 583 ff. [2] Allenspach, R., Taborelli, M. and Landolt,

M.,

Phys. Rev. Lett. 55 (1985) 2599.

[3] Schmitt, W., Kamper, K.-P. and Guntherrodt, G., Phys. Rev. B 36 (1987) 3762.

[4] Rodriguez, A., Krewer, J. W. and Feder, R., Solid State Commun. 63 (1987) 341.

[5] Abraham, D. L. and Hopster, H., Phys. Rev. Lett. 59 (1987) 2333.

[6] Schonhense, G., Donath, M., Kolac, U. and Dose, V., Surf. Sci., in press;

Donath, M., PhD thesis, Universitat Wiirzburg (1988).

[7] Schonhense, G., Appl. Phys. A 41 (1986) 39. [8] Kirschner,

J.,

Surf. Sci. 138 (1984) 191.

[9] For a recent investigation of the dispersion behavior of this oxygen band on Cu(llO), see Courths, R., Cord, B., Wern, H., Saalfeld, H. and Hufner, S., Solid State Commun. 63 (1987) 619.