MOSSBAUER STUDIES OF AMORPHOUS FeSi COMPOSITIONALLY MODULATED THIN FILMS

Conversion electron Mossbauer spectra of composition modulated FeSi thin films have been analysed within the framework of a quasi shape independent model in which the distribution function for the hyperfine fields is assumed to be given by a binomial distribution. Both the hyperfine field and the hyperfine field distribution depend on the modulation characteristic length.


Introduction
Thin solid films are of active scientific interest as well as of great technological importance and considerable theoretical and experimental works t o understand the electronic and magnetic structures of surfaces and interfaces have been undertaken in recent years'"'z).Several Mossbauer works"-".'have been carried out with the 'aim to study the iron "in contact with" diverse material by using the hyperfine magnetic fields as a test for the interfacial effects.
The main conclusions af t h e Xossbauer s t u d i e s r e f e r t h a t t h e moment of the "interfacial" Fe is different from t h e bulk value and depends on the coating material":'.
-Our FeSi composition modulated thin films were prepared in a special triode-sputtering system onto glass substrates held a t room temperature.Films were grown by codeposition from two independently polarized Fe and S i cathodes The resulting films were magnetically s o f t with an induced uniaxial anisotropy axis in their plane, which was deduced from transverse biased initial susceptibility (TBIS) measurements In t h i s work w e have studied single layers of an amorphous alloy of composition Fe76Sizs with different thicknesses separated by amorphous single layers of Si.The total thicknesses of the samples, of about lOOOA, were determined by using a Tolonsky interferometer.The amorphous, character of t h e films was determined by X-ray diffraction.By means of low angle X-ray scattering we have tested the modulating structure of the multilayers.
X-ray results agree well with the A values predicted by the preparation method.
In Table I we sumarize the thickness and composition of the samples.

Table I:
Thickness and composition of the different samples I F e 7 5 S i 2 5 s i n g l e Amorphous Si s i n g l e C h a r a c t e r i s t i c layer thicknesses layer thicknesses modulation For the analysis of the Xossbauer spectra we have assumed that: 1) the distribution functions for the isomer s h i f t and quadrupole interaction are s o narrow compared with the distribution function for the magnetic hyperfine field (P(Il) 1 that they can be approximated with

6-
functions, 2) P(H) is given by a binomial distribution": with H(n)= HDtnAH.W e have fixed the number of f i e l d s t o 21 and consequently 2=20 and n varies between 0 and z.Furthermore we have assumed t h a t t h e i n t e n s i t y r a t i o f o r a l l the samples is 3:4:1, as indicate magnetic m e a s ~r e m e n t s ~-~ >.The least-squares fitting procedure run t o f i n d t h e optimun values of the shape-parameter x, the internal width AH, the lower limit H e and the line width of the basic sextuplet4'.
W e have also introduced a pure quadrupole doublet in order to take account f o r t h e existence of non magnetic Fe atoms.
In Table I1 we summarize the hyperfine parameters obtained from t h e above exposed f i t procedure and in Figure 2 4.
The magnitudes of the hyperfine fields of the interfacial irons are smaller than those of the irons located at the bulk which agrees well with the fact that the iron magnetic moment decreases when increases the number of S i f i r s t and second neighbours.Increasing the characteristic modulation length, A, the P(H) distribution of the "bulk" iron moves to higher fields (see Figures 2 and  4 ) .This may not be attributed to variation in the magnetic moments but t o an increase of the exchange interactions and consequently in the Curie ternperatureslo'.
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Figure 4 :
Figure 4: deconvolution of hyperfine field distribution for sample 3 and 4. hsehd lines correspond to distributions of Figure 2.

Table 111 .
Hyperfine parameters of the quadrupole doubletSample Isomer s h i f t (mm/s) Q S .(mm/s) Intensity