SOLID-STATE REACTION OF Zr-Fe MULTILAYERED FILMS INVESTIGATED BY MÖSSBAUER SPECTROSCOPY

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SOLID-STATE REACTION OF Zr-Fe

MULTILAYERED FILMS INVESTIGATED BY MÖSSBAUER SPECTROSCOPY

C. Michaelsen, M. Piepenbring, H. Krebs

To cite this version:

C. Michaelsen, M. Piepenbring, H. Krebs. SOLID-STATE REACTION OF Zr-Fe MULTILAYERED

FILMS INVESTIGATED BY MÖSSBAUER SPECTROSCOPY. Journal de Physique Colloques,

1990, 51 (C4), pp.C4-157-C4-162. �10.1051/jphyscol:1990418�. �jpa-00230778�

COLLOQTJE DE PHYSIQUE

Colloque C4, suppl6ment au n014, Tome 51, 15 juillet 1990

SOLID-STATE REACTION OF Zr-Fe MULTILAYERED FILMS INVESTIGATED BY MOSSBAUER SPECTROSCOPY

C. MICHAELSEN''), M. PIEPENBRING and H.U. KREBS

Institut

fiir

Metallphysik, Hospitalstrasse 3-5,

0 - 3 4 0 0

Gdttingen,

F.R.G.

Abstract

-

Multilayered Z r

-

Fe films of average composition ZrSOFeS0 were prepared by sputtering.

The samples were solid

-

state reacted and subsequently investigated by Conversion Electron Mossbauer Spectroscopy ( CEMS ).

Already in the as -deposited multilayer an amorphous interlayer is detected. In the early stage of the interdiffusion reaction a Mossbauer spectrum is observed which is typical for an amorphous Zr

-

rich phase of concentration near Zr,Fe. After further annealing a completely amorphous film can be reached.

Differences between solid

-

state reacted, mechanically alloyed, melt -quenched and especially

CO -sputtered Z r

-

Fe samples can be observed in the middle of the concentration range. However, these differences can be reduced by a heat treatment, which changes the atomic arrangement around the Fe atoms and leads to the appearance of direct Fe -Fe contacts.

1

-

Introduction

Since it was discovered that amorphous phases can be formed by a solid-state reaction C 11, this process has developed considerable fundamental and technical interest. The glass formation ranges of these amorphous alloys can often be described by the assumption of a metastable equilibrium between the amorphous phase and the terminal solid solution C 23. It has, however, to be investigated whether the structure of the amorphous state derived by solid-state reaction is the same as the one obtained from a rapid quenching process.

Hereby, the 5 7 ~ e - Miissbauer spectroscopy yields useful informations about the local structures as well as the volume fractions of the resulting Fe

-

containing phases. In this paper we therefore report on a Mossbauer study on the amorphization of Z r -Fe multilayers by solid

-

state reaction and compare the results with amor- phous Z r - Fe samples, which were prepared by melt

-

quenching, CO

-

sputtering as well as mechanical alloying.

2

-

Experimental Methods

Z r

-

Fe multilayers consisting of 100 bilayers of 5 0 8 Fe and 1 0 0 8

Zr

were prepared by magnetron sputtering at a base pressure of I X 10-~mbar and a substrate temperature of about -50°C. A 1 5 0 8 chromium buffer- layer was predeposited on the substrate ( single

-

crystal silicon or AI2O3 ) and on top of the multilayer. For comparison, some amorphous Z r

-

Fe films were also prepared by CO

-

sputtering.

In order to characterize intermediate states of the reaction, the heat treatments were performed on a hot stage in a vacuum better than 4 X 10-=mbar while in

-

situ monitoring the X - r a y diffraction pattern as described in C 3 1.

( 4 ) Present address: GKSS

-

Forschungszentrum Geesthacht GmbH. Max

-

^Planck

-

^{StraBe, D}

-

2054 Geesthacht Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990418

COLLOQUE DE PHYSIQUE

The CEMS measurements were performed in the constant -acceleration mode. An electron counter was used with a constant gas flow of He

-

10%CH4. The velocity scale was calibrated relative to the center of an cc-Fe spectrum. The obtained CEM spectra were fitted by a set of lines with Lorentzian shape. For some spectra.

the quadrupole splitting distributions P ( Q S ) were determined as described in C4 l . All these spectra were recorded to approximately the same signal

-

^to

-

noise ratio, and the corresponding QS distributions were carefully extracted by using the same smoothing factor in all cases.

3

-

Results

In Fig. l b and lc, the CEM spectra of an as

-

prepared and a partially reacted Zr

-

Fe multilayer are presented, respectively. For comparison, in Fig.1a the CEM spectrum of a conventional a -Fe standard absorber, fitted by a six line pattern, is shown. For the multilayers, a line broadening and an asymmetry of the individual lines compared to the sextet of Fig. l a can be observed, suggesting a structural distortion of the a - F e layers possibly caused by internal stresses. A similar line broadening and asymmetry is also observed in mechanically alloyed Zr

-

Fe specimens C 5 1.

For the multilayers a second component at approximately Omm/s can be seen, which is described by an additional doublet. A small fraction of this paramagnetic subspectrum is already present in the as - prepared multilayer (Fig. l b ), and it becomes more dominant in the heat treated sample ( Fig. l c ). The relative area fractions of this component are calculated as 9 % for the as

-

prepared and 31 X for the partially reacted sample. This indicates that already in the unreacted film an amorphous phase with a thickness of about 10 - 15

X

exists.

- 8 - ^{4 0 4} 8

v e I o c i t y [mmls]

Fig. 1 : CEM spectra of ( a ) a

-

Fe standard foil, ( b ) Zr

-

Fe multilayer as -deposited, and

( c Zr -Fe multilayer heat treated at 230°C for 3.5 h. The amorphous fraction, fitted by a doublet, is hatched.

The CEM spectra of the multilayered films, measured within a smaller velocity range of t 2 mm/s, are shown in Fig. 2. Using this velocity scale a better resolution of the amorphous component within the two inner lines of a

-

Fe is obtained. Upon annealing, the intensity of this subspectrum, here described by two independent Lorentzian lines, is increasing. After a heat treatment of 1 h at 350°C the film is completely amorphous. This was also verified by X

-

ray and TEM measurements as earlier shown C 6 1. The observed Mossbauer spectrum is in agreement with that of an amorphous Zr 50Fe50 alloy C 5.7 l and further annealing at temperatures up to 450°C does not lead to a significant change of the CEM spectrum.

- 2 0 2

velocity [mmlsl

Fig. 2 : CEM spectra of Zr

-

Fe multilayers. ( a ) as

-

prepared, and ( b ) after 3.5 h at 230°C,

( c ) I h at 3 5 0 ~ ~ . and ( d 1 h at 4 5 0 ~ ~ .

From Fig. 2 it can be seen that the amorphous component of the spectrum of the as

-

prepared respectively the partially reacted sample exhibits a more asymmetric shape and a higher quadruple splitting than present in the final state. This is clearly demonstrated in Fig. 3a, where the

cc -

Fe component was numerically subtracted from the spectrum of Fig. 2b ( partially reacted film 1. The resulting spectrum is compared to the fully reacted sample, and for both measurements the P ( Q S ) curves were calculated. After partial reaction the amorphous phase exhibits a spectrum typical for an amorphous Zr

-

rich composition. Using the concentration dependence of the average quadrupole splittings as well the asymmetries of the spectra of amorphous Zr

-

Fe alloys

C

5,8 3 it can be concluded that a composition near Zre,Fea3 is formed at the beginning, whereas the final spectrum is typical for an amorphous alloy of composition near ZrS0FeSO. However, the spectra of the as

-

^deposited

C4-160 COLLOQUE DE PHYSIQUE

respectively the partially reacted sample can not be identified with those of the crystalline phases as a

-

Zr.

Zr3Fe. Zr2Fe or magnetically ordered ZrFe2. In particular, cl

-

Z r with metastably enhanced solubility of Fe.

which is expected to appear during solid-state reaction, is not detected in these experiments, probably because of the small Fe content and the small volume fraction compared t o the other occurring phases.

- 2 0 2 0 1 2

velocity [mmlsl QS t m m l s l

Fig. 3 : CEM spectra and distributions P of quadrupole splittings QS of Z r

-

Fe multilayers, ( a ) after 3.5 h at 230% ( a

-

Fe lines subtracted 1. and ( b ) after l h at 400°C.

From thermodynamics one would expect a large concentration gradient in the amorphous phase during its formation by a solid-state reaction. Therefore, the detection of only a Z r - rich amorphous phase at the beginning of the reaction is quite surprising. Otherwise, if a considerable amount of any Fe

-

rich amorphous alloy would also be present, it would clearly dominate the Mossbauer spectrum because of its higher visibility in the Mossbauer effect. The result that the spectrum is dominated by a Z r -rich concentration therefore indicates, that much more Z r - r i c h than Fe-rich amorphous phase is formed. An amorphous phase of composition richer in Zr than the final product and a concentration gradient much smaller than expected has also been observed in partially reacted Z r - C O multilayers C 9.10 1.

For comparison, CEM spectra of some CO - sputtered Z r

-

Fe samples were also recorded ( see Fig. 4 ). As shown in a previous work C 11.12 l , detectable differences exist between the spectra of meltspun, mechanically alloyed and ^CO

-

sputtered samples at an intermediate concentration range between 33 and GO at. % Fe. For instance, the average quadrupole splittings

TS

( see Fig. 5 ) are always higher for the sputtered than for the other samples. However, a heat treatment at e.g. 370°c ( Fig. 4 b ) leads t o a clear decrease of

m,

^resulting

in a CEM spectrum which looks similar t o an amorphous phase which is more rich in Fe (Fig. 4c ). It is not evident so far i f only a topological relaxation or, in addition, a phase separation in the amorphous state earlier found in this Z r -Fe system t 63 is responsible for this result. However, the differential distribution functions DDF derived from anomalous X - r a y scattering on similar samples f 11 l have shown that for Fe concentrations above 33 at. % the first peak position decreases during annealing, caused by the appearance of Fe

-

Fe neighbors in direct contact. These direct Fe-Fe contacts are obviously not present in a Zr50Fe50 alloy, which was

CO

-

sputtered on a liquid nitrogen cooled substrate. But an additional heat treatment leads to a topological and probably also a chemical relaxation, resulting in direct Fe

-

Fe contacts. This would explain that the Fe surroundings detected by Mossbauer effect become richer in Fe.

v e l o c i t y [rnm/sl QS [ m m l s l

Fig. 4 : CEM spectra and quadrupole splitting distributions of CO

-

sputtered Z r - Fe alloys, ( a ) Z r 6SFe35 ( liquid nitrogen cooled substrate 1, ( b 1 Zr65Fe35 after l h at 370°c, and ( c 1 ZrS9Fe4, ( substrate not cooled )

Fig. 5 : Average values of quadrupole splittings

TS

of amorphous Zrloo-x Fex samples versus Fe content.

mechanically alloyed ( from Ref. C 5 1 1, A melt -spun ( from Ref. C 5 1 ),

o CO

-

sputtered ( from Ref. C 7 l 1,

V ^CO

-

sputtered on a liquid nitrogen cooled substrate,

V

same sample as above additionally heat treated at 370°c for l h, V CO

-

sputtered without cooling the substrate,

A multilayer, solid

-

state reacted at 350°C for I h.

COLLOQUE DE PHYSIQUE

4 -

Conclusions

From the CEMS measurements we can conclude that already in the as

-

deposited sputtered Z r

-

Fe multilayer a disordered interface of about

10 - ¹⁵ 1

thickness exists. In the early stage of the reaction the Mossbauer spectrum exhibits a shape typical for an amorphous phase of an unexpected composition near Zr,Fe. After complete reaction the Mossbauer spectrum indicates the formation of an amorphous phase of the expected concentration ZrsoFe5,.

The comparison of amorphous Z r

-

Fe specimens prepared by different techniques shows, that CO

-

^sputtered

alloys in the middle of the concentration range appear to be different from melt

-

quenched or solid

-

state reacted samples. However, a heat treatment ( e.g. for

l

h at

370%

) causes a clear change of the Fe surrounding, leading to similar Mossbauer spectra as in the other samples. This change can be related t o the occurrence of Fe

-

Fe neighbors in direct contact.

Acknowledgements

One author ( H.U.K. 1 likes to thank David Webb for his help in the sample preparation. This work was supported by the Deutsche Forschungsgemeinschaft, via the Sonderforschungsbereich

126.

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