Si multilayers

Journal of Magnetism and Magnetic Materials 93 (1991) 545-551 North-Holland

545

Structural and magnetic profiles in Fe/Si multilayers C. Du f o u r , A.Bruson, G. Marchal, B. G e o r g e and Ph. Mangin Laboratoire de Physique du Solide, UniL'ersit~ Nancy I, B.P. 239, F54506 Vandoeuvre Cedex, France

Iron-silicon multilayers were prepared by vacuum evaporation and characterized by cross-sectional electron microscopy, electron diffraction, in situ electrical m e a s u r e m e n t s and low-angle X-ray scattering. M6ssbauer spectroscopy m e a s u r e m e n t s carried out on samples with different silicon and iron thicknesses showed a distribution of the magnetic m o m e n t s of the iron atoms. T h e M6ssbauer spectra are analysed with reference to those of F e - S i amorphous and crystalline alloys. They show iron atoms in a pure bcc crystalline phase, in a bcc crystalline phase with a silicon atom as a first neighbour, in an amorphous magnetic phase with a large hyperfine field distribution and in an amorphous non-magnetic phase. A Jaccarino-Walker type hypothesis allows us to propose a model for structural and magnetic profile of the layers which is consistent with neutron scattering and M6ssbauer spectroscopy data.

1. Introduction Films layered on an ultrathin scale provide opportunities of exploring the magnetism of interfaces and, reciprocally, the study of the magnetism of multilayers can provide information on the structure of the interface and the possible occurrence of an interracial alloy. To propose a reliable interface model, the results obtained using different techniques have to be compared. We have studied F e / S i multilayers by low angle X-ray scattering, polarized neutron scattering, magnetic measurements and M6ssbauer spectroscopy. First data on M6ssbauer spectroscopy and polarized neutron scattering have been published previously [1, 2]. The aim of this paper is to present new M6ssbauer spectroscopy data, showing the magnetism of iron atoms, and to present an improved model of the interface, which is consistent with polarized neutron scattering results. M6ssbauer spectroscopy has been used by several groups [3-6] and has proved to be a powerful technique for studying the interfaces in multilayers and determining their extension. As clear relations have been shown between the M6ssbauer parameters and the chemical neigh-

bourhood of iron atoms in crystalline and amorphous iron silicon alloys [7-10], this technique is expected to be particularly fruitful in the case of F e / S i multilayers.

2. Preparation and characterization of the samples Multilayers were obtained by vacuum condensation on substrates kept at liquid nitrogen temperature. The vacuum in the evaporation chamber was better than 10-STorr during the deposition of the materials which were evaporated from electron gun crucibles. The evaporation rates (between 0.1 and 0.5,~/s) were controlled and regulated by two quartz monitoring systems [11]. The periodic structure has been confirmed by cross sectional electron microscopy by using the microcleavage technique [12] and by low angle X-ray scattering [13, 14]. The picture of fig. 1 is an electron-micrograph of a transverse section of a Fe(10,~)/Si(35 A) sample. The spots shown at the bottom are the diffraction peaks from the multilayer (Bragg peaks from a 45 modulation). Low angle X-ray scattering exhibits

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

546

67.

Dufour et al. / Profiles in Fe / Si rnultilayers

Fig. 1. Cross-sectional T E M image of a F e / S i multilayer, eft. = 10A, esi = 35,~,. The bright areas are the Si layers and the d a r k areas the Fe layers. The e l e c t r o n diffraction p a t t e r n of the cross-section is p i c t u r e d in the insert.

ness o-= 9,~, where cr represents the half width of the Gaussian distribution of the height of the interface between iron and silicon. Alternatively, the spectra could be fitted by introducing a 20A thick interface alloy between the pure components. The crystallinity of the multilayers has been determined by electron diffraction. This

the same Bragg peaks from the multilayer. The comparison between measurements and optical simulations of multilayers confirmed the periodicity of the layers but showed that it was necessary to introduce a damping which increases with the wave vector. The data could be reasonably well fitted by introducing at each interface a rough• (",I

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C. Dufour et al. / Profiles in Fe / Si multilayers

showed that the silicon was always amorphous. For samples obtained at a low t e m p e r a t u r e and observed at room temperature, iron was amorphous if its thickness was below 18,~ and exhibited the rings of bcc iron for larger thicknesses. In order to know when and how the iron crystallization occurs, the electrical resistivity of iron was measured during its deposition on silicon at different substrate temperatures. The resistancesquare multiplied by the square of the thickness (Re 2) is pictured versus the iron thickness in fig. 2. The slope of the curve is the dynamical resistivity. The curves can be divided in three parts. The first one (A) corresponds to a large resistivity typical of amorphous materials. The second one (B) is a drop of resistivity interpreted as a crystallisation and the third (C) one corresponds to crystalline iron. In stage C, the larger resistivity measured at low t e m p e r a t u r e can be attributed to the presence of defects which are non-present when the t e m p e r a t u r e of the substrate is high. The critical thickness of crystallisation is 23,~ at 9 0 K and 18A at room temperature. It was also observed by resistivity measurements, that samples whose iron thickness was between 18 and 2 3 A did not crystallise during the deposition at liquid nitrogen t e m p e r a t u r e but during the heating of the sample to room temperature. This is consistent with the electron diffraction results. Thus 3 kinds of samples, depending on the iron thickness, have been fabricated by deposition on a substrate kept at liquid nitrogen temperature: o

i) eve < 18,~: T h e r e is no iron crystallisation. ii) eFt > 23,~: Iron crystallises during the deposition. Silicon is deposited on crystalline iron. iii) 18A < eF¢ < 23A: Iron crystallises during the warming of the sample. During the fabrication, silicon is deposited on amorphous iron. We value duced which

did not see any significant difference in the of the interface roughness which is introto fit the low angle X-ray scattering data, was always 9A. The refiectivity of the

547

amorphous i r o n / a m o r p h o u s silicon samples was not larger than the reflectivity of the crystalline i r o n / a m o r p h o u s silicon samples. This would suggest that the roughness is controlled by the chemical nature of the layer rather than their crystallinity.

3. M6ssbauer spectroscopy To get more information on the interface, we carried out M6ssbauer spectroscopy measurements. This technique is very sensitive to the local order and is well suited for the FeSi system. In this system, correlations between the local order and the chemical neighbourhood have been extensively studied in crystalline phases as well as in amorphous structures [7-10]. We present results from two sets of samples. In samples A, the silicon thickness is kept constant (35A) and the iron thickness varies from 10 to 3 7 A (fig. 3). In samples B, the iron thickness is 2 5 A and the silicon thickness changes from 10 to 37.~ (fig. 4). The spectra of fig. 3 can be decomposed in four parts which, however, are not necessarily all present at the same time. All four components occur significantly at spectrum of fig. 3b. They are: - A sharp sextuplet with a 330kOe hyperfine field at room temperature. This sextuplet can be attributed to iron atoms involved in a bcc phase and surrounded by eight iron atoms. This sextuplet is the main component of spectrum 3a. It is not present in spectra 3d and e. - A second sharp sextuplet with a 290 kOe hyperfine field at room temperature. In particular, it gives rise to the peaks shown by arrows in fig. 3b. The hyperfine field of this component is very close to that attributed to iron atoms involved in a crystalline bcc phase and surrounded by seven iron atoms and one silicon atom [7, 8]. - A central doublet, which is the only component of fig. 3e at room temperature and whose relative contribution decreases as the iron thickness increases. This doublet corresponds to non-mag-

548

C. D u f o u r et al. / Profiles in Fc / Si multilayers

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netic iron atoms, a n d exhibits M 6 s s b a u e r p a r a m eters c h a r a c t e r i s t i c o f an a m o r p h o u s F e S i alloy rich in silicon. - A b r o a d c o m p o n e n t , o c c u r r i n g with the doublet in fig. 3d a n d which can clearly be seen in fig. 3b a n d c b e t w e e n the b a s e line a n d the m i n i m a b e t w e e n the peaks. This c o n t r i b u t i o n can be int e r p r e t e d as that of iron a t o m s involved in an a m o r p h o u s m a g n e t i c phase. T h e lowering of the t e m p e r a t u r e modifies mainly s p e c t r a 3d and e. In s p e c t r u m 3e, a signif-

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icant fraction o f the d o u b l e t is t r a n s f o r m e d into a b r o a d sextuplet, which m e a n s a fraction of iron atoms, n o n - m a g n e t i c at r o o m t e m p e r a t u r e , bec a m e magnetic. In fig. 3d, this p h e n o m e n o n goes with a b r o a d e n i n g of the m a g n e t i c a m o r p h o u s c o m p o n e n t . As shown in fig. 5, the p u r e l y a m o r p h o u s s p e c t r a can be fitted with a hyperfine field d i s t r i b u t i o n [15]. T h e hyperfine field distribution exhibits a low field c o m p o n e n t d u e to the c e n t r a l d o u b l e t and a high field c o m p o n e n t . Such a distribution can be c o m p a r e d with those o b t a i n e d

C. Dufour et al. / Profiles in Fe / Si multilayers

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from homogeneous F e - S i amorphous alloys. In both cases, there are high field and low field components with a minimum around 80kOe. In amorphous alloys, the low field component was interpreted as an artificial contribution from the central doublet which rather exhibits a quadrupolar splitting. The shape of the high field distribution was shown to be related to the number of iron atoms surrounding an iron atom in a random statistical distribution of nearest neighbours. The clear separation between the two components was interpreted by a J a c c a r i n o - W a l k e r rule stipulating that, below a critical number of nearest iron neighbours, an iron atom was no longer

549

magnetic [16]. This number was between 6 and 7 (6 at low temperature and for the more iron-rich samples and 7 at room temperature for the less iron-rich samples). The main difference between amorphous alloys and multilayers is the width of the hyperfine distribution: it is larger for multilayers than for amorphous alloys. This would mean the iron layers are in reality an iron-silicon alloy with a gradient of composition. So, when iron is deposited on silicon, an alloy with a composition gradient is formed. This alloy is then enriched in silicon when the next silicon layer is deposited. If we refer to the results obtained for the alloys, when the nominal iron layer thickness is 10.&, the richest iron region is around FesoSis0, and when the nominal thickness reaches 15A, the region richest in iron seems to reach Fe0.6Si0, 4 or Fe0.7Si0. 3. The fit of the spectra of fig. 3a, b and c, with reference to F e - S i alloy spectra, shows that 14A of iron is not in the pure bcc phase. This corresponds to 7 A of iron (3 dense layers) involved in each interface. The spectra of the set of samples B, pictured in fig. 4, exhibit the same components. The most obvious evolution is the simultaneous decrease of the inner doublet and of the silicon thickness. The spectra do not change as long as the silicon thickness is larger than 18A, which means that there is still a thin layer of pure silicon. When the silicon thickness is smaller, the proportion of non-magnetic amorphous alloy decreases, which means there is no longer enough silicon to build the interface between pure iron and pure silicon. Thus 9,& for silicon are involved in each interface.

4. Structural and magnetic profile In a first attempt to interpret polarized neutron scattring data, we assumed a one step model of the interface with a homogeneous non-magnetic alloy between silicon and iron [1]. In the antiparallel configuration, it gives rise to a square contrast between the interface and both pure iron and pure silicon (dashed line fig. 6c). The inver-

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Fig. 5. M6ssbauer spectra and hyperfine field distribution of Fe/Si multilayers with esi=35,~. (a) eft,= 10A, T=4.2K; (b) eft, = 15,~, T= 4.2K: (c) e~,, = 15A, T= 300K. sion of the flipping ratio of the fourth p e a k in a Fe(25 A ) / S i ( 9 5 A ) s a m p l e is c o n s i s t e n t with a 16 thick interface. A s s u m i n g iron a t o m s exhibit a hyperfine m a g n e t i c field which follows t h e J a c c a r i n o - W a l k e r model, a simple linear chemical profile of the i n t e r f a c e (fig. 6a) gives a m o r e realistic view of the M 6 s s b a u e r s p e c t r o s c o p y results and is consistent with the n e u t r o n data. T o d e t e r m i n e the m a g n e t i c profile, t h e structural profile of fig. 6a was divided in 10 slabs from j = 1 to 10. E a c h slab was c o n s i d e r e d as an a m o r p h o u s alloy of c o m p o s i t i o n x ( j ) . T h e p r o b a b i l i t y

Pi(J) of finding an iron a t o m s u r r o u n d e d by i iron a t o m s a m o n g 12 n e i g h b o u r s in the slab j follows a b i n o m i a l law. T h e n a m o m e n t /x i a n d a hyp e r f i n e field H i w e r e a t t r i b u t e d to iron a t o m s s u r r o u n d e d by i iron a t o m s in each layer. W e took into a c c o u n t the fact that a m o r p h o u s alloys with x < 0.3 a r e no l o n g e r c o m p a c t a n d we ass u m e d t h a t in t h e s e alloys all the iron a t o m s w e r e n o n - m a g n e t i c . T h e m a g n e t i c profile d e d u c e d is p i c t u r e d in fig. 6b. It is m o r e a b r u p t t h a n the s t r u c t u r a l profile, gives rise to a b r o a d distribution of hyperfine fields a n d allowed us to fit the

C. Dufour et al. / Profiles in Fe / Si multilayers

551

5. Conclusion 1

In summary, the interface between iron and silicon is 18A with a composition gradient. Crystalline iron with silicon in substitution, magnetic amorphous and non-magnetic amorphous layers are successively found. The magnetic and structural profile are explained by a Jaccarino-Walker model. The effect of the crystallisation at low temperature or during the warming revealed by electrical measurements has not been systematically studied. It could produce an asymmetry in the interfaces.

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