EXPERIMENTAL EVIDENCE FOR TWO KINDS OF DEFECTS IN AMORPHOUS Fe80 B20

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EXPERIMENTAL EVIDENCE FOR TWO KINDS OF

DEFECTS IN AMORPHOUS Fe80 B20

E. Nold, S. Steeb, P. Lamparter, G. Rainer-Harbach

To cite this version:

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JOURNAL DE PHYSIQUE CoZZoque C8, suppZdrnent au n08, Tome 41, aotit 1980, page C8-186

EXPERIMENTAL EVIDENCE FOR T W O KINDS OF DEFECTS

IN AMORPHOUS

Fe

B

80 20 E. Kold, S. Steeb, P. Lamparter and G. Rainer-Harbach

Max-Planck-Institut fiir MetaZZforschung, Institut fur Werkstoffwissenschaften, Stuttgart, R.F.A.

Abstract.- The scattering of cold neutrons

(A

= 1 nm) with amorphous Fe "B yields a strong con-

sin 0 8 0 L O

rribution at small q=4n

-

A

(0.07 (nm)-I

5

q 5 2.4 (nm)-') arising from two kinds of defects of different size :

i) local defects with an extent of about 1.5 nm

ii) Extended defects with an extent of about 40 to 60 nm, which are interpreted as quasi-dislocations in the amorphous material.

Experimental procedure and results Amorphous ribbons with the composition FegO1 1 ~ 2 0 were produced by melt spinning using as starting materials the Boron isotope I1B together with iron of natural isotopic abundance for specimen A and with iron enriched in the 57~e-isotope for specimen B. The ribbons were cut to pieces of 25 mrn length and packed together parallel to one another, thus forming a flat speci-

3

men 2 5 x 8 ~ 2 mm

.

This specimen was orientated with its area perpendicular to the primary beam. By a cadmium frame placed before the specimen the irradiated area on the flat specimen was defined to 8

x

8 mm'. Using these specimens, scattering experiments were performed at the cold source of the High Flux Reactor at the ILL, Grenoble,using the small angle scattering apparatus Dl7 working with slow neutrons

( 2

= 1 nm). Two arrangements of the counter were chosen: For larger q-values the distance specimen

-

counter amounted to 140 cm and the center

of the counter was arranged at an angle of 10 degrees to the primary beam. For smaller q-values the distance specimen-counter a- mounted to 280 cm with the center of the counter being placed in the direction of the primary beam.

The measured intensities were corrected for background contribution and absorption. Con- cerning the scattering lengths and isotopic composition of the iron components see _{I ] .} From the corrected intensities the coherent scattered intensity in absolute units was obtained by the method of the vanadium- calibration.

Fig. 1 shows the differential cross section for coherent scattering versus

d&oh,

q = Iq

1

= 4 t . w as obtained with specimen B. The scaling of the upper curve is different from that of the lower curve by a fac- tor of 30.

Discussion

The curves in Fig. 1 show rather strona neutron scattering at small q-values, which normally is caused by one or more of the following effects, namely magnetic scattering, scattering caused by concentration fluctuations, and scattering caused by density fluctuations.

i) Magnetic scattering

It should be stressed that the coherent differential cross section of Fig. 1 proved to be independent of the orientation of an applied external magnetic field H, which was orientated parallel to the length of

-b

.+

the ribbons, however, once with ~ / / q and

+ +

once with ylq. Also no influence of the strength of the external magnetic field which was varied between zero and 0.5 T could be recognized in the q-region of Fig.1, namely for 0.07 nm-lS q S 2.4 nm-

'

.

From this fact we conclude the small angle scattering being caused by inhomogeneities of the nuclear scattering length density with- in the specimens and not by magnetic scattering. This conclusion is supported by X- ray small angle scattering experiments performed recentlyL21.

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ii) S c a t t e r i n g from c o n c e n t r a t i o n f l u c t u a t i o n s C o n t r i b u t i o n s t o t h e s c a t t e r e d i n t e n s i t y which a r e c a u s e d by c o n c e n t r a t i o n f l u c t u a - t i o n s a r e p r o p o r t i o n a l t o t h e s q u a r e of t h e d i f f e r e n c e (

A

b ) of t h e s c a t t e r i n g l e n g t h s o f t h e components. The c o m p a r i s o n between

/

a s o b t a i n e d from specimen A a n d from

dRc

oh

specimen B shows t h e same q-dependency. How- e v e r , c u r v e A l i e s below c u r v e B i n s p i t e o f t h e f a c t , t h a t ( A b ) 2 f o r specimen A i s l a r g e r t h a n f o r specimen B. From t h i s f a c t we c o n c l u d e t h a t t h e i n t e r - p r e t a t i o n o f t h e p r e s e n t e x p e r i m e n t a l re- s u l t s i s n o t p o s s i b l e i n t e r m s o f concen- t r a t i o n f l u c t u a t i o n s . iii) S c a t t e r i n g from d e n s i t y f l u c t u a t i o n s Based on t h e f a c t s m e n t i o n e d a b o v e , w e d i s - c u s s i n t h e f o l l o w i n g t h e e x p e r i m e n t a l re- s u l t s from F i g . 1 i n t e r m s o f d e n s i t y f l u c - t u a t i o n s .

-

1 I n F i g . 1 we o b s e r v e below q = 0.8 nm a v e r y s t r o n g r i s e t o w a r d s q-0, f o r q larger F i g . 1 5 7 ~ e g 0 1 'B 20. t h a n 1 nm-l a r a t h e r f l a t r u n i s o b s e r v e d . From t h i s b e h a v i o u r w e c o n c l u d e t h a t t h e s c a t t e r e d i n t e n s i t y i s c a u s e d by two k i n d s o f d e f e c t s o f d i f f e r e n t s i z e , which now w i l l b e d i s c u s s e d i n d e t a i l . L o c a l d e f e c t s From t h e q - r e g i o n a b o v e 1 nm-l i n F i g . 1 we o b t a i n a G u i n i e r - p l o t a s shown i n F i g . 2 . T h i s p l o t shows a l i n e a r r u n . From t h e g r a - d i e n t o f t h e s t r a i g h t l i n e a r a d i u s of gy- r a t i o n R = 0.53 nm was o b t a i n e d . R e c e n t l y g I F i g . 2 5 7 ~ e 8 0 1 G u i n i e r - p l o t

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c8-188 JOURNAL DE PHYSIQUE q L -+O and y i e l d s a r a d i u s o f g y r a t i o n o f a b o u t 2 7 nm. Another p o s s i b i l i t y t o y i e l d an i n f o r m a t i o n

_I

4

c o n c e r n i n g t h e e x t e n s i o n of t h e e x t e n d e d d e f e c t r e g i o n s c o n s i s t s i n p e r f o r m i n g a F o u r i e r t r a n s f o r m . I n F i g . 4 we show t h e F o u r i e r t r a n s f o r m G ( r ) which w a s o b t a i n e d from ( q ) f o r dJ'c 0 q s m a l l e r t h a n 0.62 nm-'

.

The G ( r f shows t h e r a d i u s of t h e e x t e n d e d d e f e c t r e g i o n s b e i n g a p p r o x i m a t e l y 40 nm. F i g . 4 5 7 ~ e g 0 1 ' B ~ ~ : F o u r i e r Transform of 11

S i n c e t h e G u i n i e r - p l o t shown i n F i g . 3 d i d F i g . 5 57Fe80 B ~ ~Double-logarithmic : n o t behave i n a l i n e a r way, t h e model of _{p l o t o f}

*/

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d i l u t e d homogeneous p a r t i c l e s d o e s n o t h o l d . dhcoh F i g . 5 shows an a l t e r n a t i v e r e p r e s e n t a t i o n of t h e d a t a , namely t h e p l o t l o g

*/

v s . da,oh l o g q . T h i s p l o t shows l i n e a r b e h a v l o u r i n t h e r e g i o n 0.075

5

q 5 0.6

he

g r a d i e n t of t h i s s t r a i g h t l i n e p r o v e s t o be -3. T h i s i s a c c o r d i n g t o r e f . r 6 ] a s t r o n g h i n t on s m a l l a n g l e s c a t t e r i n g c a u s e d by edge d i s - l o c a t i o n s . A s a r e s u l t we a r r i v e a t t h e f o l l o w i n g p i c t u r e of t h e e x t e n d e d d e f e c t r e g i o n s : They a r e r e g i o n s w i t h i n t e r n a l l o n g range s t r e s s which behaves a s t h e i n - t e r n a l l o n g r a n g e s t r e s s i n c r y s t a l s b e i n g caused by d i s l o c a t i o n s . A p p a r e n t l y t h e ex- t e n d e d d e f e c t r e g i o n s a r e produced by t h e r a p i d quenching p r o c e s s d u r i n g t h e pro- d u c t i o n of t h e amorphous r i b b o n s : The non u n i f o r m i t y o f t h e s o l i d i f i c a t i o n p r o c e s s w i t h i n t h e r i b b o n l e a d s t o d e n s i t y f l u c t u - a t i o n s .

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dipole scattering if the dipole distance would be smaller than 14 nm. At present we extend the measurements to lower q-values in order to study the q-dependency of the scattered intensity. This will allow to de- cide whether the extended defect regions are formed by single dislocations or quasi dislocation dipoles.

Finally we show in Fig. 6 an overview on the scattering behaviour of amorphous Iron- Boron-compounds. Besides the high angle region we recognize the small angle region

Bad Godesberg, for financial support, and to Dr. M. Wilkens for valuable discussions.

References

1 M. Weber, S. Steeb, 2. Nat.Forschung 33a (1978) 799

-

2 0. Kratky, Graz, private communication 3 T. Egami, K. Maeda, V. Vitek, Phil.Mag.

(1980) in press

4 T. Egami, private communication A. Guinier, G. Fournet, ,"Small Angle Scattering of X-Rays", Wiley, N.Y.,1955 H.H. Atkinson, P.B. Hirsch, Phil.Mag. 3 (1958) 213

-

W. Schmatz, Rivista Nuovo Cimento

5

(1975) 398

H. Kronmiiller, J. Ulner, J. Magnetism and Magnetic Mat.

5

(1977) 52

9 G. Giiltz, H. Kronmiiller, Phys-Letters A in press

Fig. 6 Amorphous Iron-Boron: Scattering behaviour

which up to now can be subdivided into four subregions. These are caused by:

i) small defects with diameters of approxi- mately 1.2 nm (present paper)

ii) quasi-dislocations (present paper) iii) quasi-dislocations as in ii) or quasi- dislocation dipoles as predicted in ref.181. This q-region is at present under investi- gation in this laboratory.

iiii) regions containing distorded spins and thus causing magnetic scattering. These regions are caused by the stressed regions containing quasi-dislocations (ii)) or quasi-dislocation dipoles (iii)) via the mechanism of elasto magnetic coupling.

(Ref. 191).

Acknowledgement