HAL Id: jpa-00229068
https://hal.archives-ouvertes.fr/jpa-00229068
Submitted on 1 Jan 1988
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of
sci-entific research documents, whether they are
pub-lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
MÖSSBAUER EFFECT STUDY OF Fe-Si
MULTILAYERS
C. Dufour, A. Bruson, B. George, G. Marchal, Ph. Mangin
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, SupplBment au no 12, Tome 49, dhcembre 1988
MOSSBAUER
EFFECT STUDY OF Fe-Si MULTILAYERSC. Dufour, A. Bruson, B. George, G. Marchal and Ph. Mangin
Laboratoire de Physique du solide (U.A. au C.N.R.S. 155), Universite' de Nancy 1, B.P. 239, 54506 Vandceuvre Les Nancy Cedex, France
Abstract. - Fe-Si multilayers have been obtained by alternate deposition of iron and silicon on substrates kept at liquid nitrogen temperature. Low-angle X-ray diffraction patterns give evidence of a modulated structure. Mossbauer spectroscopy shows that an amorphous alloy is located between amorphous silicon layers and crystalline bcc iron layers. The crystalline iron disappears for iron thickness smaller than 20
A.
One of the more difficult problem in multilayers is the knowledge of the atomic profile at the inter- faces between individual layers. It is well known that a sharp profile of atomic composition is required to get a large X-ray reflectivity. The roughness between indi- vidual layers gives rise to a damping of the reflectivity
[I]. Experimentally, the thickness of the interlayer can be deduced from the damping of the X-ray scattering, provided an indice profile for the interlayer is assumed. Another way to get information on the interface is to use techniques very sensitive to the local order. The goal is then t o built a model of the interface consistent with the distribution of local environments and the low angle X-ray scattering pattern.
For that purpose, the study of Fe-Si multilayers by Mossbauer spectroscopy is an ideal case to con- sider. Indeed, the correlations between the parameters from Mossbauer spectra of iron atoms (isomer shift, quadrupolar splitting, hyperhe field distribution) and the chemical environments have been studied in detail for the Fe-Si alloys. A well defined correlation has been demonstrated between the hyperfine field at an iron nucleus and the neighbourhood of this atom in the crystalline bcc phase [2, 31. On the other hand, the hy- perfine field distribution and the quadrupolar splitting have been reported for a large number of magnetic and non-magnetic Fe, Sil-, amorphous alloys [4, 51.
Here, we report the results of Mossbauer spec- troscopy on Fe-Si multilayers. The samples were ob- tained by evaporation of Fe and Si in a high vacuum chamber and deposition on substrates kept at liquid nitrogen temperature during the fabrication process. As described in [6], the evaporation rates of Fe and Si were controlled by two independent quartz monitoring systems. A shutter was alternately placed above each
of the two evaporation sources. The theoretical thick- nesses of Fe and Si are those expected for pure materi- als as indicated by the quartz systems calibrated with the densities of standard materials.
The result presented here are from samples whose theoretical thickness of silicon layers is 35
A.
The num- ber of bilayers (N) varies from 70 to 105. As shown in-7 I . . - - , . . . . I . . . I . . .
0 .5 1 1.5 2 2.5
THETA
Fig. 1.
-
X-Ray diffraction pattern Log (R) = f ( 8 ) of a Fe- Si multilayer with eFe = 25A
and es; = 35A
( N = 100). Ris the reflectivity; 6 is the incidence angle and X = 1.788 A.
figure 1 for the eFe = 25 layers the modulation was checked by small angle X-ray scattering. To obtain good results with the computer simulation we need to introduce a roughness n = 6
A
(the program is based on the Fresnel equations with the roughness introduced by a exp- (4ao sino / x ) ~
factor) [7]. The modulation has also been checked by electronic microscopy using the microcleavage technique [8] (Fig. 2).The Mossbauer absorption spectra collected at room temperature from samples deposited on kapton films are pictured in figure 3. A first clear information which can be obtained from the spectra is the hyperfine field orientation. The field is lying in the plane of the layers as expected from the demagnetisation factor.
The spectra collected from the eFe = 35
A
and eFe =25
A
samples can be decomposed in three components: i) a crystalline magnetic component. It is made of two sextuplets whose hyperfine fields correspond to iron atoms surrounded by 8 iron atoms, or 7 iron atoms plus one silicon atom, respectively 13, 41. The weight of the second sextuplet is significantly larger in theeFe = 25
A
sample (19 % instead of 9 % ). These two sextuplets are the contribution of the iron atoms located in the central part of iron layers;C8 - 1782 JOURNAL DE PHYSIQUE
value of the splitting, 0.6 mm/s, is not consistent with
- *
.
.As.x -*, &. the quadrupolar parameters from the crystalline com-pounds of Fe-Si phase diagram, but it is that found in amorphous Fe-Si non-magnetic alloys [9]. This doublet is certainly the contribution of iron atoms located in an amorphous non-magnetic alloy at the interface;
iii) an amorphous magnetic contribution. Since it is spread below the sharp crystalline peaks, it is not very obvious at first glance, but it is necessary to take Fig. 2. - Cross-sectional T E M image of a Fe-Si multilayer
eFe = 25 A, eS; = 95 A. The bright areas are the Si layers and the dark areas are the Fe layers.
Fig. 3. - Mossbauer spectra of Fe-Si multilayers with
es; = 3 5 A a n d with eFe = 1 5 A ( ~ = 1 0 5 ) , ew = 2 5 A
( N = 100) and e h = 35 A ( N = 70).
ii) a non-magnetic component. It is made of a non- magnetic doublet in the inner part of the spectra. The relative weight of this doublet increases as the iron thickness is lowered (19 % for eFe = 35
A,
27 %for epe = 25
PI
). This doublet corresponds t o the quadrupolar splitting of non-magnetic atoms. Theaccount of it to fit the data.
The spectrum collected from the ep, = 15 sample can be decomposed in two components (the crystalline contribution has completly disappeared):
i) an amorphous non-magnetic component made of the same doublet seen in the other samples. But the weight of this doublet is more important (39 % );
ii) an amorphous magnetic contribution which is quite evident.
In conclusion, the Fe-Si multilayers can be described
as amorphous layers of silicon, crystalline layers of iron and amorphous interlayers of Fe-Si alloys. To a first approximation the amorphous interlayers can be di- vided in two parts: a magnetic part on the iron side and a non-magnetic part on the silicon side. When the theoretical thickness of iron is too small there is no more room for pure crystalline iron.
Work is in progress with polarised neutron diffrac- tion.
[I] Bruson, A., Piecuch, M. and Marchal, G., J. Appl.
Phys. 58 (1985) 1229.
[2] Stearns, M. B., Phys.
Rev.
129 (1963) 1136. [3] Haggstrom,L.,
Granas, L., Wappling, R. and De-vanarayanan, s., Phys. Scr. 5 (1973) 125-131. [4] Marchal, G., Mangin, Ph., Piecuch, M. and Janot,
C., J. Phys. France 37 (1976) 763.
[5] Marchal, G., Mangin, Ph. and Janot, C., Solid
State Comrnun. 18 (1976) 739.
[6] Marchal, G., Mangin, Ph. and Janot, C., Philos.
Mag. 32 (1975) 1007.
171 Underwood, J. H. and Barbee, T. W., Appl. Opt.
20 (1981) 3027.
[8r Lepetre, Y. and Rasigni, G., Opt. Lett. Q (1984) 433.
[9] Bansal, C., Campbell, S. J. a k Steward, A. M.,
