Mössbauer spectroscopy in amorphous metals: Failures and successes

Journal of Non-Crystalline Solids 106 (1988) 395 398

395

North-Holland, Amsterdam

MOSSBAUER SPECTROSCOPY IN AMORPHOUS METALS: FAILURES AND SUCCESSES U. GONSER, C.T. LIMBACH, F. AUBERTIN U n i v e r s i t a t des Saarlandes, FR 12.1 W e r k s t o f f p h y s i k und W e r k s t o f f t e c h n o l o g i e , 6600 SaarbrOcken, W-Germany M6ssbauer spectroscopy i s an " i d e a l " t o o l f o r probing the l o c a l surroundings of resonance atoms. Consequently i t was thought t h a t the a t o m i s t i c s t r u c t u r e of amorphous metals would be r e v e a l e d by the h y p e r f i n e s p e c t r a . However, the e f f e c t i v e l y i n f i n i t e number o f environments produces an i n f i n i t e number o f s p e c t r a l components. Thus, only d i s t r i b u t i o n s of h y p e r f i n e M6ssbauer parameters are observed. Despite c o n s i d e r a b l e e f f o r t s , M6ssbauer spectroscopy met w i t h the same f a t e as a l l o t h e r methods i n i t s f a i l u r e t o e l u c i d a t e the s t r u c t u r e of any o f the amorphous metals. However, M6ssbauer spectroscopy became the most p o w e r f u l t o o l f o r i n v e s t i g a t i n g or t r a c i n g any type of o r d e r i n g and also where and at what temperature c r y s t a l l i z a t i o n commences and i n what sequence c r y s t a l l i n e phases p r e c i p i t a t e . Very u s e f u l i s the combination o f conversion e l e c t r o n M6ssbauer spectroscopy (CEMS) and ~-ray t r a n s m i s s i o n M6ssbauer spectroscopy, a l l o w i n g the surface and the bulk t o be scanned s i m u l t a n e o u s l y . 1. INTRODUCTION By analogy w i t h " c e n t u r y v i n t a g e s " we might

1950 etecfro-deposition

Brenner,Couch.WiltiQms

1

1952 v~tpour deposition

Bucket, Hitsch

2,3

195B irrodiQtion

Gonser, Okkerse,FujitQ

I,,5

c a l l the amorphous metals and the h i g h - t e m p e r a t u r e (Tc) superconductors " c e n t u r y m a t e r i a l s " .

(d)

Both m a t e r i a l s are o f importance from a s c i e n -

1960 quenching from the melt Duwez. Willens, Klement

6

tific

1969 melt-spinning

7

and also from a t e c h n o l o g i c a l p o i n t of

view. They both r e p r e s e n t d e f e c t s t r u c t u r e s .

Pond, Maddin

1981 n~no-crystot-deposifion Gieiter

B

1983 solid sfo,te reoction

9

Whereas, i n the case o f the high-T c m a t e r i a l s , research i s concentrated on the mechanism o f s u p e r c o n d u c t i v i t y , the main aim of our work on amorphous metals i s s t i l l

focused on the atomic

Yeh. SoJl~wer, Johnson

FIGURE 1 Methods of producing amorphous metals

structure. - t h e small mean-free path of the e l e c t r o n s , r e s u l t i n g i n high e l e c t r i c a l

2. PROBLEMS OF STRUCTURE

resistivity.

Various methods have been used t o produce amorphous metals 1-9 , ( f i g .

1). A major r o l e

has been played by the T80M20 a l l o y s (T = Fe, Co, Ni, C u . . . ;

M = B, P, C, S i . . . )

because they

3. MOSSBAUER SPECTROSCOPY From the beginning M6ssbauer spectroscopy was considered t o be an i d e a l method f o r i n -

are easy t o produce, c o m p a r a t i v e l y s t a b l e and

v e s t i g a t i n g amorphous metals because the hyper-

mostly f e r r o m a g n e t i c . In g e n e r a l , the amorphous

f i n e i n t e r a c t i o n s probe the immediate surround-

metals can be c h a r a c t e r i z e d by the f o l l o w i n g

ing of the resonance atom. Most o f the more

three features:

i n t e r e s t i n g amorphous metals c o n t a i n the i s o -

-their

tope 57Fe which i s the best s u i t e d f o r M6ss-

lack o f m a g n e t o c r y s t a l l i n e a n i s o t r o p y ,

l e a d i n g t o magnetic s o f t n e s s , -their

lack o f d i s l o c a t i o n s and unique Burgers

v e c t o r s , l e a d i n g t o mechanical hardness,

0022 3093/88/$03.50 ~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

bauer spectroscopy. Moreover the specimens obtained by the quenching process have the d e s i r e d t h i c k n e s s and geometry. I t was t h e r e -

396

U. Gonser et al. / M6ssbauer ,spectroscopy in amorphous metals

microcrystalline n a ~ n e 0 97 0 9~ 00

p

~s94

~

097

~0

94

7

c)

unit cell "molecules"

increasing chemical short range order

Bernat {RDP)

1 01~

0 91

(~/$) 6

3 0 3 VELOCITY (mm/s)

bsBOB20

FIGURE 3 Schematic r e p r e s e n t a t i o n of t h r e e a l t e r n a t i v e models used t o i n t e r p r e t experiments on TsoM20 amorphous a l l o y s

6

spectra and t h a t t h e r e i s a r e a l danger o f obFIGURE 2 M6ssbauer spectra of FeRflBpn produced by the m e l t - s p i n n i n g t e c h n i q u e ~ t - V d e c r e a s i n g wheel surface v e l o c i t i e s ((a) 25 m/s, (b) 21.9 m/s, (c) 18.7 m/s (d) 12.5 m/s) and thus decreasing cooling rates

t a i n i n g a r t e f a c t s 10 . In the past, the i n t e r p r e t a t i o n was based e s s e n t i a l l y

o f the data

along t h r e e d i f f e r e n t

l i n e s of assumptiQn: c r y s t a l l i n i t y

(in micro-

or nano-dimensions), elementary u n i t s ,

or ran-

f o r e expected t h a t the a t o m i s t i c s t r u c t u r e o f

dom dense packing of hard spheres (Bernal n~del).

the amorphous metals would be r e v e a l e d by the

For the s e l e c t i o n of the a p p r o p r i a t e model con-

h y p e r f i n e s p e c t r a . However, the spectra o b t a i n -

s t r u c t i o n we might place the extreme views at

ed e x h i b i t e d very broad l i n e s and were also

the corners of a t r i a n g l e

rather similar.

The r e a l i t y

As an example, the M6ssbauer

as shown in f i g u r e 3.

might then be found somewhere i n -

spectrum o f amorphous Fe8oB20 i s shown i n f i -

side the t r i a n g l e .

gure 2a. Contrary t o c r y s t a l l i n e

proceeded by assuming a model which a l l o w s the

materials,

Up t o now s c i e n t i s t s

where we expect uniqueness i n the values of

data t o be f i t t e d .

their

t o reverse the procedure, t h a t i s t o a r r i v e a t

s p e c t r a l p a r a m e t e r s , f o r amorphous systems

the i n f i n i t e infinite

number o f environments produces an

number o f s p e c t r a l components. Thus,

So f a r ,

have

no one has been able

the model by making a unique i n t e r p r e t a t i o n the data. Despite c o n s i d e r a b l e e f f o r t ,

M6ssbauer

i n M6ssbauer spectroscopy, we face d i s t r i b u t i o n s

spectroscopy met w i t h the same f a t e as a l l

of hyperfine fields,

o t h e r methods i n i t s f a i l u r e

interactions and p r i n c i p a l field ral it

isomer s h i f t s ,

quadrupole

(magnitude, asymmetry parameters axes o r i e n t a t i o n s of the e l e c t r i c

gradient).

It

can be shown t h a t i n gene-

i s i m p o s s i b l e t o deduce the c o r r e c t d i -

stribution

f u n c t i o n s from the e x p e r i m e n t a l

of

t o e l u c i d a t e the

s t r u c t u r e of any of the amorphous metals. The problem we are r e a l l y have r e p r o d u c i b i l i t y ibility.

f a c i n g i s t h a t we

as opposed t o i r r e p r o d u c -

Seen m a c r o s c o p i c a l l y , c e r t a i n p r o p e r -

t i e s are r e p r o d u c i b l e , but on the l o c a l or

U. Gonser et al.

/

397

MOssbauer spectroscopy m amorphm~s metal~

&00

80-

crysfo[[ine / ~

~morphous

60 I

40" 20-

0

~0

"

2~

3~

,e[ocdy of the roller (m/s)

He,Ar,...

Air

PHe'PAr'PAir

FIGURE 4 Parameters influencing amorphization by the melt-spinning technique

FIGURE 5 Amorphous f r a c t i o n x versus surface v e l o c i t y , Vu of the wheel in the m e l t - s p i n n i n g process

the start of precipitation and crystallization atomistic scale, the number of environments is

(Fig. 2b). The new lines can be used to ident-

infinite, which quite naturally leads to irre-

ify the phases. In our case, ~-Fe and Fe2B are

producibility. This can be illustrated by a

the first crystalline phases to appear. A fur-

comparison with biology where a species as a

ther reduction of the velocity to 18.7 m/s

whole is reproducible, while, in atomistic

leads to the occurrence of the Fe3B phase (Fig.

terms, every individual within the species is

2c). The crystalline phases were identified

unique, and thus irreproducible.

from the well-known parameters of their M6ssbauer spectra. It is interesting to note that

4. PHASE ANALYSIS

the sequence of crystallization of the various

So far, we have failed to obtain conclusive information on the structure of amorphous metals

phases is different when the amorphous sample is subjected to an annealing heat treatment

by M6ssbauer spectroscopy. On the other hand,

(starting from lower temperatures) or produced

however, M6ssbauer spectroscopy has become the

by reducing the quenching rate (coming from the

most powerful tool for investigating or tracing

melt at higher temperatures). When amorphous

any type of ordering, and also where and at what

Fe8oB20 alloys are annealed the first phase to

temperature crystallization commences and in

crystallize is Fe38 followed by ~-Fe. The cry-

what sequence crystalline phases precipitate.

stallization of Fe2B requires prolonged anneal-

Some of the parameters which influence the

ing at elevated temperature. Contrary to this

amorphization by the melt-spinning technique are

behaviour, the lowering of the quenching rate

shown schematically in figure 4. Of course, the

immediately produces Fe2B and ~-Fe before the

most influential factor is the speed of the

Ee3B phase is formed.

roller surface V quenching

because it determines the

rate11~

The amorphous a l l o y Fe80820 i s a t y p i c a l

Our data allowed us to plot a phase diagram of the amorphous fraction x versus the surface velocity of the rollers (Fig. 5). The area of

example. When the surface v e l o c i t y of the wheel

the amorphous fraction represents effectively

drops below 25 m/s new l i n e s appear i n d i c a t i n g

"our failure", that is, we have no idea of the

398

U. Gonser et al. / MSssbauer ~pectroscopy in amorphous meta[~"

from the surface which was i n contact w i t h the 106•

wheel during p r o d u c t i o n (upper p a r t of Fig. 6).

contact surface

1.0z,

However, the f r e e surface was subjected t o a

~ 1.02.

somewhat lower quenching r a t e and p r e c i p i t a t i o n

1.00= 1.00

o f ~-Fe occurred (lower p a r t of Fig. 6).

"~

In c o n c l u s i o n , i t

bulk

.> ~ 0.90

might be s t a t e d t h a t the

g r e a t success o f M6ssbauer spectroscopy seems

0.80

t o be i n d e t e r m i n i n g where, when and what

v- 0.70-

o r d e r i n g i s found in amorphous metals. • "

'

free

surface

= 1.01o

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-..

E

1.00I

)_

,

,

I

I~-~,

1. A. Brenner, D.E. Couch and E.K. W i l l i a m s , J. Res. N a t l . Bur. Stand. 44 (1950) 109.

Vet0cify [ turn/s]

2. W. Buckel and R. H i l s c h , Z. Phys. 132 (1952) 420.

FIGURE 6 y - r a y a b s o r p t i o n spectrum and conversion e l e c t r o n emission spectra of the two surfaces of quenched Fe91Zr 9 a l l o y s

3. W. Buckel and R. H i l s c h , Z. Phys. 138 (1954) 109. 4. U. Gonser and B. Okkerse, J. Phys. Chem.

S o l i d s 7 (1958) 55. amorphous s t r u c t u r e .

However, the c r y s t a l l i n e

area i s extremely w e l l known w i t h regard t o the identity

o f the phases, the sequence of t h e i r

appearance and t h e i r r e l a t i v e i s where our success l i e s ,

abundance. Here

and M6ssbauer spec-

t r o s c o p y has h a r d l y any c o m p e t i t o r s . Very u s e f u l i s the combination of conversion e l e c t r o n M6ssbauer spectroscopy (CEMS) and yray t r a n s m i s s i o n M6ssbauer spectroscopy, a l l o w ing the surface and the bulk t o be scanned s i multaneously. The e m i t t e d conversion e l e c t r o n s can escape from a depth o f about 1000 R. Experiments w i t h the a l l o y Fe91Zr9 might demons t r a t e the s e n s i t i v i t y

within a critical

range

o f the quenching r a t e . The y - r a y a b s o r p t i o n spectrum o f the paramagnetic a l l o y i s shown in the middle p a r t o f Fig. 6. An i d e n t i c a l

con-

v e r s i o n e l e c t r o n emission spectrum i s obtained

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