Cr interfaces

150

Journal of Magnetism and Magnetic Materials 93 (1991) 15(1-154 North-Holland

Growth and Electronic Structure as studied by Photoemission of F e / C r and C o / C r interfaces F. S c h e u r e r , E. B e a u r e p a i r e ~, V. S c h o r s c h L C. B o e g l i n , B. C a r r i b r e , O. H e c k m a n n a n d J.P. D e v i l l e I.P.C.M.S., Groupe Surfaces-Interfaces and '~Groupe d'Etudes des Mau;riatcc M~talliques, UM 380046, 4 rue Blaise Pascal, 67070 Strasbourg Cedex, Frunce

The valence bands of ultrathin Fe and Co layers deposited on a Cr(100) surface at room temperature are studied by ARPES and UPS. Transitions from Fe minority and majority spin bands are still observed for coverages of about 1 monolayer, showing that Fe remains ferromagnetic. These studies agree with theoretical calculations which predict an enhanced magnetic moment for ultrathin Fe layers on Cr(100). Measurements on Co layers let us suppose that a Stranski-Krastanov growth mode occurs, with a bcc Co phase for the very first layers.

1. Introduction The electronic structure of the Cr(100) surface and its related magnetic properties are quite well understood by means of A R P E S (Angle Resolved Photoemission) experiments which agree with theoretical band calculations [1-4]. It is d e m o n s t r a t e d that the Cr(100) surface is ferromagnetic with a strong e n h a n c e d magnetic moment of about 3p~B. In the case of Fe(100), A R P E S [5] and spin resolved photoemission have been performed on the valence band [6, 7] and transitions from majority and minority spin bands have been identified. Theoretical calculations [8] show that Fe is antiferromagnetically coupled to the Cr(100) surface layer, with a 2.4# B magnetic m o m e n t in comparison to the 2.2/~B bulk value. Studies by S P L E E D [9], S M O K E and Brillouin scattering [10] of F c - C r - F e sandwiches, as well as magnetization m e a s u r e m e n t s on F e / C r superlattices [11], seem to confirm these calculations. However, the ferromagnetic nature of the first two Fe layers is not clearly established. In that framework, we explored by photoemission the valence band of evaporated ultrathin Fe layers (from 1 to 10 monolayers) on a Cr(100) single-crystal. We followed the evolution of the

valence band features attributed to magnetic band transitions. We also studied the 3s core level multiplet splitting, which is supposed to be correlated with the magnetic m o m e n t [12]. In the same way we studied the C o / C r ( 1 0 0 ) system, for which multilayers grown in the last years [13] showed a bcc Co phase. Theoretical calculations exist for C o / C r superlattices [14-16], but only few experimental work on the electronic structure and magnetic properties of bcc Co has been done at the present time [17]. These photoemission studies should be considered as a preliminar investigation.

2. Experimental A clean Cr(100) single-crystal surface was typically obtained by Ar-sputtering and annealing cycles in a U H V c h a m b e r (the pressure lay in the 10 m T o r r range). The surface cleanliness was characterized either by A E S and L E E D or by XPS. Fe and Co were in both cases evaporated from a filament by joule heating. The Cr substrate was held at room temperature. Fe is known to grow layer by layer on Cr(100) [9] with an abrupt interface at room temperature. In our case we checked that the L E E D pattern was very

0304-8853/91/$03.50 ~ 1991-Elsevier Science Publishers B.V. (North-Holland)

F. Scheurer et al. / Growth and electronic structure of (Fe, Co) / Cr interfaces

well defined even at a thickness of 10 M L showing thus a good epitaxy. The layer thicknesses were estimated by the attenuation of the Cr low energy Auger peak and the 2p Cr XPS peaks. Mean free paths of 5.~ (resp. 10A) for the Cr low energy Auger peak (resp. for the 2p Cr XPS peaks), in agreement with previous experiments [10, 21], were used in the derivation of the thickness• Photoemission experiments were performed on the Super-Aco storage ring at L U R E ( F e / C r system only). The spectra were recorded in p polarization at 100 and 160eV at normal emission and for a 30 ° electron emission angle with respect to the surface normal. We also used for both systems a He discharge lamp (21.2eV) and nonmonochromatized A1 radiation (1486.6 eV).

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3. Results and discussion

Fe / Cr At normal emission and 100eV (see fig. 1) the bulk Fe spectrum shows a strong feature at 0.65eV below the Fermi level, referred to as feature 1; another weak feature (referred to as 2) is seen at about 2.8 eV. For 1.5 monolayers (ML), we notice the apparition of a third feature very close to the Fermi level, and a little shift (about 0.15 eV) of features 1 and 2 toward higher binding energies. The spectrum for 0.8 ML Fe coverage shows very little difference with the clean Cr one. At 100eV and normal emission a region of the Brillouin zone near the F point (where the bulk Fe bands are quite flat) is probed. If we consider the spin-polarized photoemission measurements of Brookes et al. [7], feature 1 in the bulk Fe spectrum (fig. 1) is actually a two component peak arising from a minority F2[ spin state and from a majority FI~2 spin state. This is in good agreement with band calculations, in a free electron final state model• In the work of Kisker et al. [6], the F~ state cannot appear very strongly because of the use of s polarized light. Feature 2 in the bulk Fe spectrum (fig. 1) is probably a mixing of the F~.~ majority spin state [6] and of a minority surface resonance [7], both having ap-

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proximatively the same binding energy. For 1.5 ML coverage we still observe feature 2 (fig. 1) but slightly shifted• This can be explained either by a shift toward higher binding energies of the t+ • F25-11ke state, or by a relative enhancement of the minority surface resonance, due to the lower Fe coverage• Measurements at 30 ° emission angle lead us to believe that both effects are present. Feature 3 in the 1.5 ML spectrum (fig. 1) is interpreted as outcoming from the actual F2~ state: a slight shift (about 0.1 eV) toward the Fermi level of that state with respect to its bulk position, accompanied by a 0.15eV shift of the F~-like

F. Seheurer et al. / Growth and electronic structure of (Fe, Co) / Cr interfaces

152

state (feature 1 in the 1.5ML spectrum) toward higher binding energies, allows now the resolution of the two states, which was not possible for bulk Fe. However, the position of feature 3 is very close to that of the strong feature near the Fermi level seen in the 0.8ML and clean Cr spectra (fig. 1) so that one could in fact consider feature 3 as a Cr contribution. We reject this hypothesis for several reasons: the Cr peak at (}.3 eV binding energy is attributed by Klebanoff et al. [2] to two surface states (not resolved here). These states persist in the 0.8ML situation for which the Cr(100) surface is probably not entirely covered, allowing surface states to be observed. For the 1.5 ML coverage we should have a complete Fe layer on the Cr surface. Spectra recorded at 160eV and normal emission (not displayed here) confirm this: for 0.8 ML the surface states are still present but for 1.5 ML they have completely disappeared. So we conclude that feature 3 cannot be a Cr contribution. The bulk Fe spectrum recorded in our study with Hel light (fig. 2a) is not exactly comparable

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to the polycrystalline Fe valence band spectrum shown by Hiifner [18]; this is probably due to the limited angle resolution of the spectrometer in the case of our epitaxial layers, so that we only sample k vectors in the [100] direction near the H point of the bulk Fe Brillouin zone. We see in the bulk Fe spectrum an important structure marked by a tick, whose center lies at 0.9eV. (A weaker and broader feature lying at about 3.4 eV which will not be considered here, corresponds to the s - p band.) When reducing the coverage, this strong feature progressively moves toward higher binding energies and is finally replaced by a Cr feature lying at 1.7eV. In fig. 3 we represent an attempt of subtracting a clean Cr valence band contribution for Hel spectra. The subtraction should allow to separate the Fe contribution from the Cr one, but we cannot exclude a possible interface contribution. The difference spectra of fig. 3a present the same behaviour as the spectra in fig. 2: we notice that for the 1 ML situation, the strongest peak is shifted by 0.25 eV toward higher binding energies with respect to the Fe bulk

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153

F. Scheurer et al. / Growth and electronic structure of (Fe, Co) / Cr interfaces

ment, is enhanced with respect to bulk Fe. For about 1 M L the enhancement would be in the order of 10%. Measurements of the multiplet splitting of the 3s Fe core level support this value. These results seem to be in contradiction with the S P L E E D measurements of Carbone et al. [9], who do not observe an electron polarization for the first two Fe layers. This can easily be explained in terms of antiferromagnetically coupled Cr terraces as proposed by Blfigel et al. [19].

situation. This shift becomes smaller when increasing the Fe coverage. We can suppose that for bulk Fe the strong peak is essentially a majority spin structure with a great part of H2~- states. For 1 ML Fe coverage (fig. 3a) we have a bidimensional layer, so we should explore a region near the F point of the surface Brillouin zone and our strong peak should partly correspond to feature 1 in the 1.5 ML spectrum of fig. 1, which is assumed to outcome from a F~ state. To sum up, we found that the Fe spectra for bidimensional layers are very similar to those of bulk Fe, that the peaks corresponding to the F~2 t+ and the F25 majority spin states are shifted toward higher binding energies, and that the peak corresponding to the F2~ majority spin state is shifted toward the Fermi level. If our interpretation is correct, this would mean that for ultrathin Fe layers epitaxially grown on Cr(100) the exchange splitting between majority and minority spin bands, and consequently the magnetic mo-

Co/Cr

The major difference between F e / C r and C o / C r for H e I spectra, is that for the latter system a marked peak due to Co appears at the Fermi level, and lies at this position up to a coverage of 5 ML (fig. 2b). We find another broad and strong feature that moves from 1.7eV binding energy for clean Cr, to 1.2 eV for a 1.5 ML Co coverage, and moves back to 1.5eV for 3ML. When subtracting a Cr contribution (fig. 3b), we

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154

F. Scheurer et a L / Growth and electronic structure ~[ (Fe, Co) / (Jr interfaces

notice strong modifications in the 1.5 M L situation, as c o m p a r e d to bulk Co: two well s e p a r a t e d peaks are observed, the first lying at 0.1 eV below

show that even for low coverages ( < 2 ML), Fe may have an i m p o r t a n t m a g n e t i c m o m e n t .

the Fermi level, and the second one at 1.0 eV. A

In a second part we noticed that for 1.5 M L Co on Cr(100) the HeI p h o t o e m i s s i o n spectrum is

third but much w e a k e r structure is localized at about 4.3 eV. This third structure is only clearly

more compatible with a bcc D O S rather than a hcp one. We expect a bcc Co phase in the first

identified for the 1.5 ML situation and has completely d i s a p p e a r e d above 3 ML. T h e peak at the

stage of growth followed by an island growth as in the case of C o / C r ( 1 101 [21].

Fermi level seems to b r o a d e n and shift very progressively to its bulk position at 0.4 eV. T h e intensity of the peak at 1.0eV is r e d u c e d when the Co

References

coverage is increased. The Co bulk s p e c t r u m is only o b t a i n e d well above 7 M L Co. The particularities of the 1.5 ML spectrum (fig. 3b) are possibly related to a bcc Co phase in the early growth

[1] L.E. Klebanoff, S.W. Robey, G. Liu and D.A. Shirley, Phys. Rev. B 31 (1985) 6379. [2] I,.E. Klebanoff, R.H. Victora, L.M. Falicov and D.A, Shirley, Phys. Rev. B 32 (19851 1997. [3] ('.L. Fu and A.J. Freeman, Phys. Rev. B 33 (19861 1755. [4] G. Allan, Surface Sci. 74 (19781 79. [5] A.M. Turner, Yu Jeng (;hang and J.L. Erskinc, Phys. Rev. Len. 48 (19821 348. [6] E. Kisker, K. Schr6der, W. Gudat and M. Campagna, Phys. Rev. B 31 (19851 329. [7] N.B. Brookes. A. Clarke, P.D. Johnson and M. Weinert, Phys. Rev. B 41 (19901 2643. [8] C.L. Fu and A.J. Freeman. J. Magn. Magn. Mat. 54 57 (1986) 778. [9] ('. Carb(mc and S.F. Alvarado, Phys. Rcv. /3 36 (19871 2433. [10] F. Saurenbach, Thesis, Diss. Univ. K6ln (1986). [1 I] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau and F. Petroff, Phys. Rev. Len. 61 (1988) 2472. [12] C.S. Fadley and D.A. Shirley, Phys. Rev. A 2 (197(I) 11119. [13] R. Walmsley, J. Thompson, D. Friedman, R.M. White and T.H. Geballe, IEEE Trans. Magn. MAG-19 (19831 IU92. [14] F, Herman, P. Lambin and O, Jepsen, Phys. Rev. B 31 ( 1985) 4394. [15] F. tlcrman, P. Lambin and O. Jepscn, J. Appl. Phys. 57 (1985) 3654. [1(~] 1t. Hasagawa and F. Herman, Phys. Rev. B 38 (19881 4863 and refs. therein. [17] G.A. Prinz, E. Kisker, K.B. Hathaway, K. Schr6der and K,[I. Walker, J. Appl. Phys. 57 (19851 3024. [18] S. Hi,ifner, in: Photoemission in Solids 11, Topics in Applied Physics, eds. L, Ley and M. Cardona (Springer, Berlin, 19791p. 179. [19] S. Bliigel. D. Pcscia and P.H. Dederichs, Phys. Rev. B 39 (19891 1392. [20] P.M. Marcus and V.L. Moruzzi, Solid State Commtm. 55 (19851 971. [21] O. Heckmann. Thesis, Strasbourg (19891. O. tleckmann, E. Beaurepaire, B. Carri~re, J.P. Deville, P. Pauissod, F. Scheurer, D. Chandesris and H. Magnan, Conf. Proc., w~l. 25, 2rid Eur. Conf. on Progress in X-Ray Synchrotron Radiation Research, eds. A. Balerna, E. Barnieri and S. Mobilio (PIF, Bologna, 19911)p, 509.

stage, rather than to the usual hcp or fcc phases. In regard of the density of states ( D O S ) for bcc and fcc Co [16, 20], it appears that the bcc D O S is more a p p r o p r i a t e d to describe the 1.5 M L situation, even if there are some discrepancies in the positions of the peaks. The position and the intensity of the peak at the Fermi level are in good a g r e e m e n t with a minority spin feature in the calculated bcc DOS. T h e second peak at 1.0eV could be a t t r i b u t e d to a strong majority spin peak, but shifted toward the F e r m i level. The third peak has no e q u i v a l e n t in the fcc calculated DOS and is also b e t t e r described by the bcc DOS. For higher Co coverages the a t t r i b u t e d bcc structures disappear: after o n e or two layers in a bcc phase, hcp Co islands probably begin to grow, and would very progressively screen the bcc features; when the coalescence of the islands is achieved, we observe a polycrystalline hcp DOS. Growth mode studies are u n d e r progress to test this hypothesis.

4. Conclusion In a first part we tried to identify Fe p h o t o e mission transitions from majority and minority spin b a n d s for u l t r a t h i n Fe layers deposited on a Cr(100) surface at room t e m p e r a t u r e . O u r results