DISTRIBUTION OF ELECTRIC AND MAGNETIC HYPERFINE INTERACTIONS IN AN AMORPHOUS ALLOY

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DISTRIBUTION OF ELECTRIC AND MAGNETIC HYPERFINE INTERACTIONS IN AN AMORPHOUS

ALLOY

D. Price, S. Campbell, P. Back

To cite this version:

D. Price, S. Campbell, P. Back. DISTRIBUTION OF ELECTRIC AND MAGNETIC HYPERFINE

INTERACTIONS IN AN AMORPHOUS ALLOY. Journal de Physique Colloques, 1980, 41 (C1),

pp.C1-263-C1-264. �10.1051/jphyscol:1980190�. �jpa-00219783�

JOURNAL DE PHYSIQUE

Colloque C1, suppl&ment au no

1 ,

Tome

41,

janvier 1980, page

C1-263

DISTRIBUTION OF ELECTRIC ANB MGNETIC HYPERFINE INTERACTIONS IN AN AMORPHOUS

ALLOY

D.C. P r i c e , S.J. Campbell and P.J. Back

+

Dept. of Solid State Physics, Australian National University, Canberra, ACT, Australia.

+ Dept. of Physics, Royal Military College, Lhntroon, ACT, Australia.

1. I n t r o d u c t i o n

T h i s paper r e p o r t s some i n i t i a l r e s u l t s o f a Mtissbauer e f f e c t s t u d y aimed a t o b t a i n i n g informa- t i o n on t h e d i s t r i b u t i o n s o f e l e c t r i c h y p e r f i n e i n t e r a c t i o n s i n magnetic amorphous a l l o y s . Consider- a b l e e f f o r t h a s been made r e c e n t l y t o determine mag- n e t i c h y p e r f i n e f i e l d d i s t r i b u t i o n s i n amorphous m a t e r i a l s , b u t r e l a t i v e l y l i t t l e a t t e n t i o n h a s been p a i d t o e l e c t r i c h y p e r f i n e i n t e r a c t i o n s , and t h e i r dependence on l o c a l s t r u c t u r a l c h a r a c t e r i s t i c s o r t h e i r r e l a t i o n s h i p t o t h e i n t e r n a l magnetic f i e l d s .

The approach employed i n t h i s i n v e s t i g a t i o r . of t h e r e l a t i v e l y simple a:.loiqpho,s a l l o y FegOBgO

(Metglas 2605) h a s been t o p a r a m e t e r i z e t h e MBssbauer s p e c t r a by l e a s t - s q u a r e s f i t t i n g w i t h simple lirleshapes t h a t need n o t n e c e s s a r i l y have any p h y s i c a l s i g n i f i c a n c e , and then t o study t h e q u a l i t a t i v e (and e v e n t u a l l y q u a n t i t a t i v e ) f e a t u r e s o f t h e parameters o b t a i ~ e d .

2. Experimental D e t a i l s

The samples used were s t r i p s o f t h e -50 pm t h i c k r i b b o n produced by A l l i e d Chemical Corporation

(USA). S t r i p s were mounted side-by-side and glued a t each end t o a c i r c u l a r b r a s s sample h o l d e r u s i n g G.E. v a r n i s h . MBssbauer s p e c t r a were recorded w i t h t h e s e samples a t 4.2K and a t 295K with a small mag- n e t i c f i e l d o f up t o 100 mT a p p l i e d p a r a l l e l t o t h e r i b b o n d i r e c t i o n ( p e r p e n d i c u l a r t o t h e gamma-ray beam) i n o r d e r t o produce magnetic s a t u r a t i o n . The s p e c t r a o b t a i n e d i n d i c a t e d t h a t t h e samples were e f f e c t i v e l y s a t u r a t e d by a 50 mT f i e l d . I t was con- s e q u e n t l y assumed t h a t a l l h y p e r f i n e f i e l d s were p a r a l l e l t o t h e a p p l i e d f i e l d (and t h e r e f o r e perpen- d i c u l a r t o t h e gamma beam) and, because o f t h e amor- phous n a t u r e o f t h e a l l o y s , t h a t n u c l e a r e l e c t r i c f i e l d g r a d i e n t s would be a t a l l o r i e n t a t i o n s t o it w i t h e q u a l p r o b a b i l i t y .

3 . Resu1.t~

The 4.2K and 295K s p e c t r a , f o r an a p p l i e d f i e l d of 50 mT, a r e shown i n F i g . 1. The s o l i d l i n e s through t h e d a t a p o i n t s a r e t h e b e s t f i t s t o s i x symmetric a b s o r p t i o n peaks: t h e i n d i v i d u a l peaks a r e a l s o shown a s s o l i d l i n e s . These l i n e s have t h e pseudo-lorentzian shape [l]:

where

r

is t h e f u l l width a t h a l f maximum and a i s t h e l i n e s h a p e exponent ( a = 2.0 f o r a l o r e n t s i a n ) . T h i s l i n e s h a p e h a s been shown [l] t o provide a good approximation t o t h e Voigt, o r Doppler, p r o f i l e and t o s a t u r a t e d M8ssbauer a b s o r p t i o n l i n e s .

The parameters corresponding t o t h e s o l i d l i n e s i n F i g . 1 a r e l i s t e d i n Table 1, i n which t h e a b s o r p t i o n peaks a r e numbered i n o r d e r o f i n c r e a s - i n g v e l o c i t y , i . e . i n each spectrum t h e l e f t - m o s t

a b s o r p t i o n peak i s no. 1 and t h e r i g h t - n o s t i s no.

E, The peaks were c o n s t r a i n e d i n t h e l e a s t - s q u a r e s C i t t 2 n g p r a c e d u r e . t o e n s u r e t h a t t h e two peaks ~f

each of t h r e e p a i r s ( o r d o u b l e t s ) ha3 equal apeas

!5u+ n e t shapes) i . e . P.(1) = A ( G ) . Ai2) = A ( 5 ) , A ( 3 )

= A ( 4 ) a s expected from t h e 5 7 ~ e n u c l e a r t r a n s i t i o n

V E L U C 1

T Y C

M M / ' S )

F i g . 1. Mtissbauer s p e c t r a o f amorphous FegOBp0 a t 4.2K (lower) and 295K ( u p p e r ) i n a magnetic f i e l d of 50 mT i n t h e p l a n e o f t h e sample ( p e r p e n d i c u l a r t o

t h e d i r e c t i o n of t h e y - r a y s ) . The s o u r c e was 5 7 ~ ~ ~ a t room temperature. The s o l i d l i n e s a r e e x p l a i n e d

i n t h e t e x t ( S e c t i o n 3 ) .

p r o b a b i l i t i e s when t h e dominant h y p e r f i n e i n t e r a c - t i o n i s t h e magnetic d i p o l e term. D e s p i t e , t h e f a c t t h a t t h e samples used were n o t i d e a l l y t h i n , t h i s c o n s t r a i n t was considered t o b e r e a s o n a b l e be- c a u s e t h e shapes and a b s o r p t i o n maxima o f t h e peaks of each d o u b l e t were n o t g r e a t l y d i f f e r e n t .

It i s i m p o r t a n t t o n o t e t h a t w h i l e t h i s para- m e t e r i z a t i o n scheme y i e l d s very good f i t s t o t h e e x p e r i m e n t a l s p e c t r a , t h e widths and shapes o f t h e peaks o f each e q u i a r e a l p a i r a r e s i g n i f i c a n t l y d i f - f e r e n t . Thus, t h e main q u a l i t a t i v e f e a t u r e s o f t h e r e s u l t s i n Table 1 a r e :

( i ) a l l peaks a r e very n e a r l y symmetric ( i i ) f o r each d o u b l e t , t h e widths and shapes of

t h e two peaks a r e s i g n i f i c a n t l y d i f f e r e n t ( i i i ) t h e c e n t r o i d s o f each o f t h e t h r e e d o u b l e t s

were n e a r l y i d e n t i c a l w i t h i n t h e u n c e r t a i n t y l i m i t s imposed by t h e l i n e a r i t y o f t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980190

C1-264 JOURNAL DE PHYSIQUE s p e c t r o m e t e r and t h e s t a t i s t i c s of t h e

s p e c t r a

( i v ) t h e reZative s p l i t t i n g s o f t h e t h r e e doub- l e t s a r e , w i t h i n experimental u n c e r t a i n t y , e q u a l t o t h o s e observed by V i o l e t and Pipkorn 121 i n t h e M8ssbauer s p e c t r a o f a-Fe.

Table 1

parameters Derived from t h e S p e c t r a of F i g u r e 1.

Sample Absorption peak no.

Temperature 1 6 2 5 3 4

-

*6 i s c e n t r e s h i f t o f d o u b l e t ( r e l a t i v e t o Fe f o i l a t room t e m p e r a t u r e ) .

t A i s s p l i t t i n g o f d o u b l e t .

Units of 6 , A and

r

a r e m m / s . ^ci i s dimensionless.

4 Q u a l i t a t i v e I n t e r p r e t a e

The following q u a l i t a t i v e c o n c l u s i o n s can be drawn from t h e parameters l i s t e d i n Table 1:

( i ) t h e s p e c t r a cannot be i n t e r p r e t e d i n terms o f a d i s t r i b u t i o n o f magnetic h y p e r f i n e f i e l d s p(Hhf) only, a s has been attempted p r e v i o u s l y 131.

( i i ) t h e q u a l i t a t i v e f e a t u r e s of t h e parameters can be e x p l a i n e d by a model i n which t h e r e a r e d i s t r i b u t i o n s both o f t h e e l e c t r i c quad- r u p o l e c o u p l i n g c o n s t a n t e Q V z z and of t h e magnetic h y p e r f i n e f i e l d Hhf, i n which both d i s t r i b u t i o n s a r e symmetric about c e n t r o i d s and, most i m p o r t a n t l y , i n which t h e d i s t r i - b u t i o n s a r e mutually dependent i n t h e s e n s e t h a t f o r each v a l u e of Hhf t h e r e corresponds a p a r t i c u l a r v a l u e of V,, t h a t i s l i n e a r l y r e l a t e d t o it. The c e n t r o i d of t h e VZZ d i s t r i b u t i o n must be a t , o r very c l o s e t o ,

v,,

= 0 .

( i i i ) I f t h e r e i s a d i s t r i b u t i o n o f isomer s h i f t s w i t h i n t h e sample it must e i t h e r be very narrow ( 5 0.05 mm/s w i d t h ) , o r be independent o f Hhf and Vzz and be a symmetric d i s t r i b u - t i o n .

5. C a l c u l a t i o n s

S p e c t r a have been c a l c u l a t e d u s i n g t h e assum- p t i o n s o u t l i n e d i n S e c t i o n 2. For s i m p l i c i t y i n t h e s e p r e l i m i n a r y c a l c u l a t i o n s t h e e l e c t r i c f i e l d g r a d i e n t s were assumed t o have a x i a l symmetry (n=O), and t o be s m a l l enough t o t r e a t a s f i r s t - o r d e r p e r - t u r b a t i o n s on t h e m a g n e t i c a l l y - s p l i t n u c l e a r l e v e l s . Le Caer and Dubois [4] have independently considered t h e e f f e c t on a M8ssbauer sp-ectrum o f such randomly o r i e n t e d E .F.G. I S .

Figure 2 shows a spectrum c a l c u l a t e d i n t h i s way f o r a d i s t r i b u t i o n c o n s i s t i n g o f two d i s c r e t e , and e q u a l l y probable, v a l u e s of Hhf. The c o r r e s p -

onding v a l u e s o f Vz, a r e e q u a l i n magnitude b u t of

o p p o s i t e s i g n : t h i s is t h e s i m p l e s t case of t h e s o r t of d i s t r i b u t i o n d e s c r i b e d i n S e c t i o n 4 above.

I t w i l l r e a d i l y be seen t h a t t h i s spectrum has a l l of t h e q u a l i t a t i v e f e a t u r e s o f t h e e x p e r i m e n t a l s p e c t r a of Fe80B20 a s l i s t e d i n S e c t i o n 3 .

-.-

^{23.0 T;}

-

0.6 MS

?7,O T ; + 0.6 nn/S TOTAL SPFCTRUN

-

^{6 -Lf}

-

^{2 B}

+

^{2 +Lf}

+

⁶

V E L D C I T Y < M M / S )

F i g . 2 . M8ssbauer spectrum c a l c u l a t e d f o r two d i s c r e t e p a i r s o f Hhf, eQV,, v a l u e s a s d e s c r i b e d i n S e c t i o n 5. The parameter v a l u e s f o r t h e two compon- e n t s p e c t r a were Hhf= 23.0 T, &eQVZz = - 0 . 6 0 mm/s and Hhf'27.0 T, &eQVzz=t0.60 m m / s . The E.F.G.

asymmetry parameter 0 was t a k e n a s z e r o . 6 . Discussion

These r e s u l t s i l l u s t r a t e t h e importance of accounting f o r t h e e l e c t r i c quadrupole i n t e r a c t i o n s i n t h e a n a l y s i s o f t h e MBssbauer s p e c t r a o f amor- phous m a t e r i a l s . I t has a l s o been shown r e c e n t l y [5] t h a t isomer s h i f t d i s t r i b u t i o n s a r e important i n some c a s e s .

The mutual dependence of t h e h y p e r f i n e f i e l d and e l e c t r i c quadrupole i n t e r a c t i o n s i n d i c a t e s e i t h e r t h a t b o t h q u a n t i t i e s a r e s i m i l a r l y i n f l u e n c e d by t h e l o c a l arrangement o f both m e t a l and m e t a l l o i d atoms, o r perhaps more l i k e l y , t h a t t h e l o c a l d i s - t r i b u t i o n s o f m e t a l and m e t a l l o i d atoms a r e c o r r e - l a t e d a s , f o r example, i n t h e dense random packing o f hard s p h e r e s s t r u c t u r a l model [ 6 1 o r q u a s i - c r y s t a l l i n e models [?].

More q u a n t i t a t i v e c o n s i d e r a t i o n s t h a n t h o s e p r e s e n t e d above a r e under way. It i s expected t h a t t h e y w i l l show t h a t t h e h y p e r f i n e f i e l d d i s t r i b u - t i o n s deduced from t h e MBssbauer s p e c t r a a r e s i g - n i f i c a n t l y modified when quadrupole i n t e r a c t i o n s , a r e t a k e n i n t o account. I t i s hoped some l i g h t may b e shed on t h e s t r u c t u r e of t h e s e amorphous a l l o y s . References

[l] P r i c e , D . C . , "An E m p i r i c a l Lineshape S u i t a b l e f o r F i t t i n g a Wide V a r i e t y o f . S p e c t r a 1 Data", Research Report No. 5 , Dept. of S o l i d S t a t e P h y s i c s , ANU (1976).

[2] V i o l e t , C.E. and P i p k o r n , D . N . , J . Appl. Phys.

42 (1971) 4339.

-

[3] Chien, C . L . , Phys. Rev. B E (1978) 1003.

[4] Le Caer, G . and Dubois, J . M . , "Evaluation o f Hyperfine Parameter D i s t r i b u t i o n s from Overlapped Mtlssbauer S p e c t r a o f Amorphous Alloys" ( t o be p u b l i s h e d ) .

[5] Chien, C.L. and Chen, H.S., J . Physique Colloque

10

(1979) C2-118.

[6] C a r g i l l , G.S. i n "Rapidly Quenched Metals"

ed. N . J . Grant and B.C. Giessen (MIT, 1976).

[7] e.g. Kem6ny, T . , Vincze, I . , Fogarassv, B.

and A r a j s , S . , Phys. Rev.B

i n press(1979)