MÖSSBAUER STUDY OF THE SHORT RANGE ORDER IN IRON BASED AMORPHOUS METALS USING THE RF COLLAPSE EFFECT

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MÖSSBAUER STUDY OF THE SHORT RANGE

ORDER IN IRON BASED AMORPHOUS METALS

USING THE RF COLLAPSE EFFECT

M. Kopcewicz, H.-G. Wagner, U. Gonser

To cite this version:

JOURNAL

DE PHYSIQUE

Colloque C8, suppl6rnent au n012, Tome 46, d6cernbre 1985 page C8-151

MOSSBAUER STUDY OF THE SHORT RANGE ORDER IN IRON BASED AMORPHOUS METALS USING THE RF COLLAPSE EFFECT

M. Kopcewicz, H.-G. wagnerr and U. ~ o n s e r *

I n s t i t u t e o f Experimental Physics, Warsaw University, Hoza 69,

00-681 Warsaw, Poland

*FB Angewandte Physik, Universitat des Saarlandes,

D-6600 Saarbriicken, F.R.G.

Resume

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Les m o d i f i c a t i o n s d ' o r d r e ?I c o u r t e distance dans l e s a l l i a g e s amorphes Fe S i B s o n t observees ?I 1 'a i d e de 1 ' e f f e t quadrupolaire des spectres Missbauer. Dans l ' e t a t ferromagnGtique, l ' e f f e t Zeeman magnPtique e s t e l i m i n P par a c t i o n d'une onde electromagn@tique RF.

A b s t r a c t

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Changes i n s h o r t range o r d e r o f amorphous FeSiB a l l o y s a r e ob- served through the quadrunole s p l i t t i n g i n Flossbauer spectra o f these m a t e r i a l s . The quadrupole s p l i t t i n q s are determined d i r e c t l y i n the f e r r o - magnetic s t a t e by u s i n g the r a d i o frequency c o l l a p s e o f t h e magnetic s p l i t -

t i n g . I

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INTRODUCTION

Our knowledge about the s t r u c t u r e o f amorphous m e t a l l i c a1 l o y s i s based m a i n l y on measurements i n v o l v i n g d i f f r a c t i o n methods. However, such methods p r o v i d e o n l y i n - f o r m a t i o n about t h e r a d i a l d i s t r i b u t i o n f u n c t i o n b u t n o t about l o c a l atomic arrange- ments. I n f o r m a t i o n about s h o r t range order i s obtained by methods such as blossbauer spectroscopy, NMR, PAC, from magnetic d i p o l e and e l e c t r i c quadrupole h y p e r f i n e i n - t e r a c t i o n s . E s p e c i a l l y the quadrupole i n t e r a c t i o n i s u s e f u l f o r probing the s h o r t range order because t h i s i n t e r a c t i o n i s determined by t h e l o c a l symmetry around the probe n u c l e i . For a long time the quadrupole i n t e r a c t i o n was neglected when ana- l y z i n g Mossbauer spectra o f ferromagnetic amorphous metals. I n t h e l a s t years t h e importance o f quadrupole i n t e r a c t i o n f o r the c o r r e c t i n t e r p r e t a t i o n o f iqossbauer r e s u l t s was recognized. Since the conclusions about the l o c a l s t r u c t u r e are based on the shapes o f the d i s t r i b u t i o n s o f h y p e r f i n e parameters (magnetic f i e l d , quadru- p o l e s p l i t t i n g ) , i t i s i m p o r t a n t t o o b t a i n these d i s t r i b u t i o n s w i t h a h i g h accuracy. However, t h e coexistence o f e l e c t r i c quadrupole and magnetic d i p o l e i n t e r a c t i o n s leads t o v e r y complicated Mossbauer spectra which a r e d i f f i c u l t t o i n t e r p r e t . The best way t o overcome t h i s problem i s t o determine one o f these i n t e r a c t i o n s sepa- r a t e l y . For some ferromagnetic amorphous metals i t was p o s s i b l e t o measure t h e quadrupole s p l i t t i n g i n t h e paramagnetic s t a t e . I n most cases t h i s i s n o t p o s s i b l e because o f the onset o f c r y s t a l l i z a t i o n close t o the Curie p o i n t .

Recently, we i n t r o d u c e d a method f o r d i r e c t determi n a t i o n o f quadrupole s p l i tti nq o f amorphous metals i n t h e ferromagnetic s t a t e /1,2/. I t makes use o f t h e r a d i o frequency ( r f ) induced c o l l a p s e o f t h e magnetic h y p e r f i n e s t r u c t u r e . When the sample i s exposed t o the rf f i e l d which i s stronger than the l o c a l a n i s o t r o p y f i e l d , maq- n e t i z a t i o n r e v e r s a l occurs i n response t o the e x t e r n a l rf f i e l d o s c i l l a t i o n s . This leads t o a r e d u c t i o n o f the observed magnetic h y p e r f i ne f i e l d s seen by t h e "lssbauer n u c l e i , and when the magnetization r e v e r s a l i s f a s t e r than t h e Larmor precession of t h e nuclear magnetic moment i n the l o c a l f i e l d the magnetic h y p e r f i n e f i e l d averages t o zero. The remaining spectra show quadrupole s p l i t t i n g (QS) and isomer s h i f t ( I S ) only. The d i s t r i b u t i o n o f QS can be e x t r a c t e d d i r e c t l y from such spectra by using, e.g.

,

the constrained Hesse Rubartsch method / 3 , 4 / .

C8-152 JOURNAL DE PHYSIQUE

isomer s h i f t i n Feloo-x-ySixBy and i n Feloo-xBx amorphous a l l o y s . The observed changes o f QS as a f u n c t i o n o f t h e composition o f FeSiB a1 low us t o d e t e c t the changes i n the s h o r t range o r d e r due t o t h e replacement o f i r o n atoms by s i l i c o n as p r e d i c t e d i n one o f the r e c e n t s t r u c t u r a l model /5/.

I 1

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EXPERIMENTAL

Amorphous r i b b o n s of Feso-xSixBlo, where 9 4 5 2 1 , and Fes4-xSixB16, where 05x518, and F ~ I O O - ~ B ~ (145x520) were meltspun i n a i r . Samples o f approx. 14x20 mm were used as Mossbauer absorbers. The sample thickness was about 25 pm. The absorbers were i n - s e r t e d i n t o the c o i l o f the r f generator and measured i n transmission geometry. The rf f i e l d o f 62 MHz whose s t r e n g h t could be changed from 0 t o 25 Oe was a p p l i e d i n the plane o f t h e absorber. To a v o i d excessive r f heating t h e absorbers were water cooled d u r i n g rf exposure. .Mossbauer spectra were recorded using a 57Co i n C r source. A l l isomer s h i f t s are given r e l a t i v e t o an a-Fe standard.

I 1 1

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RESULTS AND DISCUSSION

A t y p i c a l Miissbauer spectrum obtained i n our experiment i s shown i n F i g . 1A.

Fig. 1

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(A) R f c o l l a p s e d spectrum o f amorphous Fe74Si10B16 ( 6 2 MHz, 20 Oe). ( 6 ) D i s t r i b u t i o n o f quadrupole s p l i t t i n g s found from the f i t i n F i g . 1A.

1 0- c - "7 - E 0 9 - ' c'

C

+ z 0.8- P:

The c e n t r a l p a r t o f t h e collapsed spectrum o f Fe74SiloB16 exposed t o t h e rf f i e l d o f 62 MHz and 20 Oe i s shown. The s o l i d l i n e represents a f i t o f a d i s t r i b u t i o n o f QS t o the experimental p o i n t s . The f i t t i n g procedure was a constrained Hesse Rubartsch method /3,4/. To reproduce t h e observed asymmetry i n the QS d o u b l e t i t was necessary t o i n t r o d u c e a l i n e a r c o r r e l a t i o n between I S and QS:

I S = a

.

QS

+

b,

where a and b a r e c o r r e l a t i o n parameters. The QS d i s t r i b u t i o n r e s u l t i n g from t h e f i t i s shown i n Fig. 16. From t h i s d i s t r i b u t i o n t h e average values o f and

E

were ex- t r a c t e d . The dependence o f

=

on t h e i r o n content, CFe, o f the a1 l o y s i s shown i n Fig. 2. As can be seen t h e r e a r e c l e a r changes i n the slope o f (c,,). They a r e found a t C F ~ = 73 and CFe = 8 1 f o r Fes4-~SixB16 and a t c,,

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76 f o r F e s o - x S i x B ~ ~ . These values agree v e r y w e l l w i t h those determined from s h o r t time measurements above the Curie temperature /5/.

Such behaviour o f QS (cFe) r e f l e c t s changes i n s h o r t range o r d e r as i r o n i s replaced by s i l i c o n , as p r e d i c t e d by a r e c e n t s t r u c t u r a l model 151. S i l i c o n w i l l take up Fe s i t e s w i t h no boron nearest neighbours. When these a r e f i l l e d t h e r e must be an over- a l l s t r u c t u r a l change t o c r e a t e more Fe s i t e s w i t h no B nearest neighbours. This w i l l l e a d t o d i f f e r e n t l o c a l symmetries and hence t o d i f f e r e n t quadrupole spl i t t i n g s . These changes i n s h o r t range order are n o t r e f l e c t e d as c l e a r l y i n t h e v a r i a t i o n o f -

I S (cpe) b u t t h e r e i s a l s o a change o f slope close t o the values found f o r (cFe) (Fig. 3).

!+,+++,

+

+

:::::

-2 -1 0 1 2 0 0 5 1 0 1 5 2 0

F i g . 2

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V a r i a t i o n o f the average quadrupole s p l i t t i n g ,

v,

w i t h i r o n c o n c e n t r a t i o n ( q F e a ~ - ~ S i ~ B l s , @ F e s o - X S ~ ~ B ~ O ) .

F i g . 3

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V a r i a t i o n o f the average isomer s h i f t ,

x,

w i t h i r o n c o n c e n t r a t i o n ( q FesQ&i xB16, @ Fe90-XSiXBT0).

U n f o r t u n a t e l y t h e number o f d i f f e r e n t sample compositions f o r amorphous FeToo-xBx i s t o o low t o be c e r t a i n about trends. I t must be p o i n t e d o u t t h a t almost no data f o r

@ have been published f o r concentrations x<45 /6/. F i g . 4 suggests t h a t again t h e r e may be a drop i n

QS

as the Fe content increases. The isomer s h i f t increases w i t h Fe c o n t e n t (Fig. 5 ) and thus reverses the t r e n d observed a t l o w e r Fe c o n c e n t r a t i o n

(Fig. 3). I n f a c t Fig. 5 can be seen as a c o n t i n u a t i o n o f Fig. 3. The f l a t r e g i o n between c = 75 and 80 may be connected w i t h t h e p o s i t i o n o f the e u t e c t i c .

JOURNAL

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PHYSIQUE

F i g . 4

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Average q u a d r u p o l e s p l i t t i n g ,

v,

v e r s u s i r o n c o n c e n t r a t i o n , cFe, f o r F ~ I O O - ~ B ~ .

F i g . 5 - Average isomer s h i f t ,

x,

v e r s u s i r o n c o n c e n t r a t i o n , CFe, f o r Fe,oo-xBx. ACKNOWLEDGEMENTS

REFERENCES

/1/ Kopcewicz, M., Wagner, H.-G. and Gonser, U., S o l i d S t a t e Commun. 48 (1983) 531. /2/ Kopcewicz, M., Wagner, H.-G. and Gonser, U., H y p e r f i n e I n t e r . 1 5 m (1983) 729. /3/ Hesse, J. and Rubartsch, A., J. Phys. E7 (1974) 526.

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/4/ Le Caer, G. and Dubois, J.M., J. P h y s . 7 1 2 (1979) 1083.

/5/ Dubois, J.M. and Le Caer, G., Acta M e t a r 32 (1984) 2101.