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Submitted on 1 Jan 1979

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MÖSSBAUER SPECTROSCOPY OF AMORPHOUS Fe-B ALLOYS

R. Oshima, F. Fujita, K. Fukamichi, T. Masumoto

To cite this version:

R. Oshima, F. Fujita, K. Fukamichi, T. Masumoto. MÖSSBAUER SPECTROSCOPY OF AMORPHOUS Fe-B ALLOYS. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-132-C2-134.

�10.1051/jphyscol:1979245�. �jpa-00218643�

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JOURNAL DE PHYSIQUE Colloque C2, supplPment au n o 3, Tome 40, mars 1979, page C2-132

!IOSSBAUER SPECTROSCOPY OF AMORPHOUS Fe-B ALLOYS

R. Oshima, F. E. Fujita, K. Fukamichi* and T. Masumoto X

Faculty of Engineering Science, Osaka University, Toyonaka, Osaka, Japan

he

Research I n s t i t u t e for Iron, Steel and Other Metals, Tohoku ~ n i v e r s i t y , ~ e n d a i , Japan

RLsum6.- L1&tude par spectromdtrie MGssbauer du 5 7 ~ e dans les alliages amorphes Fe-B contenant 12 P 2OatZB rdvsle leur caraetsre ferromagngtique doux et une curieuse d6pendance du champ hyperfin moyen avec la concentration, qui a 6t6 interpr6tge par interactions fer-bore et la microstructure de la matrice amorphe. Le processus de cristallisation et la stabilit6 des phases concerndes ont aussi dtb 6tudi6s.

Abstract.- The 5 7 ~ e ~assbauer spectroscopy of Fe-B amorphous alloys containing 12-20atZB revealed the characteristic soft ferromagnetic properties and a peculiar concentration dependence of the mean internal field, which was interpreted by the interactions between iron and boron atoms in different ways depending on the boron content and the microstructure of amorphous matrix. The crystallization process and stabilities of concerned phases were also studied.

1. Introduction.- The ~Essbauer spectroscopy was first employed by Tsuei et al./l/ to examine an amorphous Fe-P-C alloy. They analyzed the broad six line spectrum on the assumption of disorderd near neighbor configurations of iron atoms in the amor- phous structure. On the other hand, Fujita et al.

/2/ investigated the amorphous Feso-PI7-Cs and

Feso-Pie-C7

alloys, and concluded that the amor- phous structure fairly maintained the bcc type near neighbor configuration, differing from either the conclusion by Tsuei et al. and so far proposed den- se random packing structures.

Recently binary Fe-B amorphous alloys have been prepared in the wide region of boron concen- tration by rapid cooling from the melt, and an in- var effect was found between 12'15atZB 131. The reduction of iron mment was also reported as the boron content was decreased below 15at%.

In the present paper we report the concentra- tion and temperature dependence of the internal field distribution and the preferred orientations of magnetization of Fe-12"2Oat%B amorphous alloys by the MGssbauer spectroscopy. The minimum of the mean internal field was found between 13-'15at%B, indica- ting that the boron atoms do not behave as an elec- tron donor, but interact with iron atoms in diffe- rent ways depending on the concentration and micro- structure of the amorphous matrix. The crystalliza- tion and phase separation process are also descri- bed.

2. Experimental.- The absorbers were made of rib- bons of amorphous iron alloys with 30vm thick and I*l.5m wide containing 12"20at%B. 5mm wide amor-

phous ribbons of Fe-17atZB alloy were also used.

Most of the measurements were carried for as-quen- ched specimens. The thermal stability of amorphous Fe-17%B alloys was examined in a specimen annealed at various temperatures up to 800°C.

The Mgssbauer spectroscopy was done between -190°C and 268°C using a 30mCi 5 7 ~ o source diffused in copper.

3. Experimental Results.- 3.1 Directions of-mgf?_e- tization and the internal field distribution.

...

The ~Essbauer patterns of amorphous Fe-B alloys at room temperature exhibited characteristic broad six peaks /2/. The second and fifth peaks, however, be- came remarkably small when measured at -190'~.

Figure I shows an example of the spectral change of an Fe-17%B alloy; the intensity ratio of the six peaks are 3:3.2:1:1:3.2:3,3:1.5:1:1:1.5:3 and 3:0.7:1:1:0.7:3 at room temperature, -130°C and -1 90°C respectively. When an amorphous Fe- 13%B alloy was measured above room temperature the second and fifth peaks grew as the temperature rose. The peak intensity ratio was 3: 3.8: 1: 1:3.8: 3 at 145'C, above which the spectrum became too broad to evaluate the exact ratios, arid weak pure iron peaks appeared at 260'~ due to phase separation.

The above results indicate that the iron moment in amorphous alloys are easy. to be flipped by the thermal stress due to the temperature change. That is, spin orientations are more or less in the spe- cimen plane at high temperatures and they turn to the normal to the plane at low temperatures.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979245

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highest at the boron concentration of 16%B and the lo-

,> <;A>;.-. ,*

i( _:.

. .

r . . . '?.d,j I

. . .

. .

'i 'at -190%

1

-

I I I I I I I I I

-8 -6 -4 -2 0 2 4 6 8 Doppler Veloc~ty (rnm/s)

1 1 1 1 1 1 1 , {

Fe-l7%B, A m o r p h o u s

Fig. 1 : ~Zssbauer spectra obtained with an Fe-17 at%B amorphous alloy at several temperatures.

west at around 14%.

The internal field distribution curves are obtained by the Fourier analysis of the observed spectra.

Figure 2 shows the analyzed curves thus obtained corresponding to each spectrum of figure 1.

I I

Fe-17x8, Amorphous - a t RT

---

at -130.C

~ ~

Internal Field ( kOe)

Fig. 2 : The internal field distribution curves corresponding to each spectrum of figure 1.

The above mentioned peak ratio and a small isomer shift distribution are carefully taken into consi- deration in the analysis. The line profile scarcely alters but shifts to the high field side when the temperature is lowered. 'The 12,13 and 14atXB alloys, which have an invar effect, exhibit a small bump

in the lower field side at room temperature, but otherwise the profiles look similar in all condi- tions and can be accounted for by the alloying effect in usual solid solution alloys.

The concentration dependence of mean internal field at -190°C is shown in figure 3. It is note- worthy that the averaged moment of iron atoms is the

Fig. 3 : The concentration dependence of the mean internal field at -190°C.

3.2 Thgsal stabi1i~y.- The thermal stability of the amorphous Fe-17%B alloy was examined. No change was observed in the specimen annealed at 200'~ for

10 days. When annealed between 250'~-300°C, the alloy exhibited an appreciable change in the peak intensity ratio and in the half width of each peak.

Around 325OC, an intermediate phase of cementite structure, FesB, appeared while a final product, Fe2B, was observed by annealing at 350'~

.

The phase

separation is completed in 12 hours at 8 0 0 ~ ~ .

2

a

- ,

300

b

4. Discussion.- In spite of the variety of dense random packing models for amorphous alloys, the structure has not yet been clarified experimentaly.

Since the iron moment is affected by nearby atoms, the internal field distribution revealed by the Yijssbauer spectroscopy is useful to examine the atomic arrangements in the amorphous structure. In a simplest model electrons of metalloid atoms, for instance, 2p electrons of boron in the present case are transfered to the unfilled d band of transition metals /4/. In addition, a strong covalent bonding is expected to form between the metalloid and nea- rest metal atoms, stabilizing the amorphous struc- ture and changing the magnetic interactions further.

Accordingly, the mean iron moment in the Fe-B amor- phous structure will be reduced in proportion to the boron concentration. However, the maximum and minimum of the mean internal field in figure 3 suggests that boron atoms are not simply an elec- tron donor but interact with iron atoms in diffe- rent ways depending on the concentration and micro- structure of the amorphous matrix. In order to interpret the above strange behavior, we assume that

I " ' ' l ' ' ' r l '

-

Fe-B Amorphous

- -

I 0

a L

IL

=

2 5 0

0 0 0 O O O O 0

- -

E

6

-

z

z

i # , * > l . l t . l *

10 1 1 12 13 14 15 16 17 18 19 20 21 B content ( a t . % )

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C2-134 JOURNAL DE PHYSIQUE

(a) Near neighbor configurations in the amorphous Fe-B alloys are not quite different from those in crystalline solid solution alloys.

(b) Boron atoms sit on the substitutional like si- tes in the amorphous structure, and the structure is stabilized accordingly.Below a certain critical value the structure becomes more ordered and boron atoms will begin to occupy the interstitial like sites.

(c) The interstitial like boron atoms reduce the moment of near neighbor iron atoms more than the substitutional atoms because of smaller interatomic distances.

With these assumptions, the above concentration dependence of the mean internal field can tentati- vely be explained in the following way. At high boron concentrations the amorphous structure may mainly contain substitutional like boron atoms, and the concentration dependence above 16%B can be explained by simple dilution. At low concentrations, the degree of order in the structure will become higher so that the boron atoms presumably have to enter the interstitial sites, giving rise to the concentration dependence in the left-hand side of figure 3. In the intermediate concentrations, the two states of boron will coexist and the reverse dependence with the minimum can be explained by the fractional sum of the internal fields of the two states.

In the crystallization and phase separation process of Fe-17%B alloy, the initial stage of the phase separation exists around 250

-

300°C, i m e - diately followed by the first precipitation of pure iron. Then an intermediate phase of cementire struc- ture, Fe3B is formed probably in the boron rich re- gion surrounding the iron precipitates. The Fe3B phase is metastable in a wide temperature range, and suppresses the formation of the final product, Fe2B.

References

/I/ Tsuei, C. C., Longworth, G. and Lin, S. C. H., Phys. Rev.

fi

(1968) 603.

/ 2 / Fujita, F. E., !*lasumoto, T., Kitaguchi, M., and

Ura, Y., Japan. J. Appl. Phys:

16

(1977) 1731.

/3/ Fukamichi, K., Kikuchi, M., Arakawa, S. and Masumoto, T., Solid .State Commun.23 (1977) 955.

/4/ Yamauchi, K. and Mizoguchi, T., J. Phys. Soc.

Japan

39

(1975) 541.

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