SHORT-RANGE ORDER IN AMORPHOUS Co-Sn ALLOYS THROUGH NMR AND MÖSSBAUER SPECTROSCOPIES

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SHORT-RANGE ORDER IN AMORPHOUS Co-Sn

ALLOYS THROUGH NMR AND MÖSSBAUER

SPECTROSCOPIES

H. Nabli, M. Piecuch, J. Durand, G. Marchal

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, suppl6ment a u n012, Tome 46, d6cembre 1985 page C8-229

SHORT-RANGE ORDER I N AKORPHOUS Co-Sn ALLOYS THROUGH NMR A N D K ~ S S B A U E R SPECTRDSCOP I E S

H . N a b l i , M . P i e c u c h , J. Durand and G . tiarcha1

Laboratoire de Physique du S o l i d e , (U.A. au C.N.R.S. n o 1 5 5 / ,

U n i v e r s i t h de Nancy I , B.P. 239, 54506 Vandoeuvre l e s Nancy Cedex, France

R S d

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La distribution des champs hyperfins sur l e cobalt obtenue par RMN

dans l e s alliages m r p h e s Co-Sn ferramgn6tiques e s t analys6e en relation avec l'environnement en Sn des atomes r 6 s o r m t s de Co. Les valeurs myennes de l l B c l a t e m n t q u a d r i p l a i r e e t du dBplacemnt i s d r i q u e sur l ' b t a i n , obte- nues par spectroscopic PGssbauer dans l e s alliages arorphes Co-Sn pararm%Bti- ques, sugggrent que l e s a t m s d'Ctain dans ces alliages sont situ6s au centre de p r i s m s trigonaux construits avec l e s a-s de cobalt.

Abstract

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The hyperfine f i e l d distribution on 5 9 ~ o obtained by NMR in ferro- magnetic m r p h o u s CoSn alloys is related to the distribution of Sn environ- mt around the Co resonant nuclei. The man values of the quadrupole s p l i t -

tin and of the i s m e r s h i f t f o r tin i n paramagnetic CoSn alloys, as obtained by ?19Sn 6 s s b a u e r ~ p c t r u s m ~ , suggest t h a t the tin at- in these alloys are located a t the center of trigonal prisms of d a l t atcans.

Hyperfine interactions i n m t a l l i c glasses, in addition t o the informtion t h a t they can provide on electronic and magnetic structure, have proved a useful t o o l to study the local e n v i r o m t around the nucleus probe. In ferramgnetic alloys, the distri-

bution of the hyperfine f i e l d s mainly r e f l e c t s the distribution of the co-ordirlation numbers in t h e f i r s t atomic s h e l l around the nucleus probe. In paramagnetic alloys, the q-uadnqmle interactions r e f l e c t the e l e c t r i c f i e l d gradients (EFG) and, thus, the average synm?try about the nucleus probe.

We report on an attennpt to study the local e n v i r o m t through hyperfine interzctions on both constituents of binary amrphous C q , Snl-x alloys. Hyperfine f i e l d distribution on 5 9 ~ o was determined by NMI in femomgnetic alloys. Quadruple e f f e c t on tb w a s maswed by Essbauer spectroscclpy (ME) in paramagnetic alloys.

Arrorphous Cox Snl-x alloys were prepared by vacuum deposition ontc a liquid N2+cooled substrate /I/. These alloys were found to be s t a b l e a t

roam

temperature wiL& respect

to crystallization over a wide concentration range (0.18 x ~ 0 . 8 0 ) . Results of bulk

magnetization measurements together with the mgnetic phase diagrm, f o r these alloys were published elsewhere /2/.

5 ' ~ o NMR spectra i n zero external f i e l d with the spin-echo mthcd were recorded a t 1 . 6 K on fermmgnetic C q , Snl-x alloys (0.68 ( x

5

0.80)

.

Typical spectra a r e shown on fig. 1.

M. E. spxk.ra on l19sn were W e n a t rm t a p r a t u r e with a standard constant acce- leration spectrometer on pararmgnetic C q , Snl-, alloys (0.25 ( x

1

0.62)

.

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JOURNAL DE PHYSIQUE

I11

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AbI?LYSIS AISD DISCUSSION

1

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5 9N M R spectra ~ ~

m e 5 9 ~ o WlR spectra exhibit a very broad distribution of the h m i n e field on the Co s i t e s , spanning from 40 t o 240 kOe. The mean hyperfine f i e l d , a s taken from the center of gravity of the spectra, decreases rather linearly w i t h Sn content. m a - p l a t i n g t o pure mrphous mbdlt, one obtains a value of 215

a,

slightly lower than the hyperfine field for fcc Co (fig. 2) . Such a mall discrepancy (about 5 %) l i e s within experimental uncertainty. E m n g use of the reported values from bulk

magnetic masurenents /2/, the average hyperfine field is plotted versus the average mt for Co atcnns on fig. 3, yielding a value of about 130 kOe/pg for the h p r f i n e coupling constant. Such a value is very close t o the value &tained for pure Co and for m y Co based mrphous alloys /3/.

MHZ

Fig. 1

-

5 9 ~ 0 spectra taken a t 1.6 K in zero-field f o r several amrphous Co-Sn

alloys in the f e r r m g n e t i c state.

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We t r i e d t o make a quantitative correlation between the hyperfine f i e l d d i s t r i - bution and the Sn a-ordination number around the Co atoms. To do this, we assumed that the Co hyperfine f i e l d is p r i m r i l y determined by the number of Sn a t m in the f i r s t atcanic shell around the Co reference atoms. Such an hypothesis has been found reasonable in m y concentrated Co based alloys /4/. FrcnnNMR study of a Co 9 8 Sn 02 crystalline alloy, the s h i f t i n Co resonanefrecpency due t o one Sn atom £&st neigh- b u r of the Co at- is known t o be about 25-30 MHz /5/. Our experimental spectra

were analysed as the sum of Gaussian-like subspectra centered a t 195, 170, 140 and 110 MHz and corresponding t o Co a t a w having 1, 2 , 3 and 4 Sn f i r s t neighbours, respectively. Such an analysis i s independent of the f a c t that some structures are ob-

served in the experimental spectra a t about the sarre frequencies, the existence of these structures being possibly related to the excitation conditions. Provided that the wall enhancmnt factor of the resonant signal is frequency independent, the ana- l y s i s of the NMR spctrum for e@ alloy yields the average cc-ordination of Sn at- around the Co reference atoms (%n)

.

In f i g . 4 , the vGues for

Zsn

obtained from analysis of our N M R spectra are cam

pared with values for Z S ~ calculated from a binomial distribution (with a t o t a l co- ordination number of 12 o r 13 around the Co atoms) taking into account only the-Co atans having 0, 1, 2, 3 and 4 Sn f i r s t neighbours. The e x p e r b t a l values for

Zsn

l i e well below the calculated ones, suggesting the existence of a chemical short- range order in our moorphous alloys.

Fig. 3

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Average 5 9 ~ o h y ~ e r f i i ~ e f i e l d Versus mgnetic m m e t ~ t p Co a t m in m r p h o u s CoSn alloys.

.FROM ANALYSIS OF oFROM BINOMIAL LAW

2

0 '.L> I

0 0.20 0.26 0.30

(1 -X) [Sn at.conc.1

Fig. 4

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Number of Sn atams around a Co reference atom in amorphous Co-Sn alloys as

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C8-232 JOURNAL DE PHYSIQUE

Incidentally, it shouldbemtioned t h a t the absence of NMR signal. belaw 60 MHz indicates t h a t Co atoms w i t h m r e than 5 Sn f i r s t neighburs are not magnetic, accor- ding t o our andlysis.This is i n rather good agreement w i t h previous analysis of bulk magnetization data /6/.

2

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%ssbauer spectra

mid l 1 9 ~ n spectra in p a r m g n e t i c CoSn alloys a r e sham on fig. 5 . To account

f o r such spectra, one should consider a correlated distribution of quadruple i n t e r actions and i s m s h i f t s . A s a f i r s t order appmxbation, we assumed t h a t the spectra were made of two Lorentzian l i n e s convoluted by a distribution. The width of the distribution

(r)

is determined by ccxqxdng the broadening of the experimental l i n e s w i t h the minimdl width of our spzkomter (0.7 m.s) / 7 / . A detailed account of our analysis w i l l be given in a-forthcaning paper /C/. We w i l l 2 c u s here on the mean

valxes f o r the i-shift 6 and the quadruple s p l i t t i n g

%.

0 Co

Sn,

*

Sn11

Fig. 5

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Typical 9 ~ n %ssbauer spectra a t roam t a p r a t u r s i n paramagnetic CoJn

m r p h o u s alloys

CRYSTAL STRUCTURE OF CoSn [hcp 8351 Fig. 6

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Tin sites i n the c r y s + d l i n e Co Sn ccm&xmId

(ilfter ref. 9 )

.

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The mean iscsraershift

7

increases s l i g h t l y with tin content. This a r i s e s from the fact t h a t the charge transfer of s electrons from tin +ac o b i l t is a decreasing function of tin content /6/. The mean p d r u p o l e s p l i t t i n g is nearly corstant. W e compared t h i s value w i t h t h a t chtained f o r the crystalline Co Sn ampund. I n t h i s l a t t e r c a v

pound, there a r e two c r y s t a l s i t e s f o r Sn, namely the SnI s i t e s (see f i g . 6) located i n hexagonal planes having a

%

of 3.06

mn/s

and a 6 of 2.04

mn/s,

and t h e Sn11 s i t e s located a t the center of Co trigonal prisnr; and having values of 1.56 and 1.84

mo/s

f o r

hEq

and 6, respectively /9/. Wssbauer s p c t r a f o r crystalline and m r p h o u s 20 Sn are a m p x e d on f i ~ 7 . One can see t h a t the Sn1 contribution is lacking in

the an-oxphous spectrum, i n amorphous phase being close t o the f o r Sn11

in the c ~ y s t a l l i n e

.

The 6 value in the mrphous phase is 2.04

Ws

wfiich is the value f o r 6

on

Sn1. This apparent discrepancy is explained in t h e follmingway :

XI enhancement of 6 would r e s u l t f r m a srraller covalency in amorphous rretals, a s dxeady observed m r p h o u s Fe Sn alloys /6/. Thus, hypothetical pure amorphous t i n LEIS s y ~ l a t e d t o have a 6 of 3.27 rm/s and then an electronic configuration of 5 s2 5 p 1 6 1 .

rv

-

CONCLUSION

Main conclusions of this study can be summxized a s follaws :

1) the tin atoms around cobalt reference s i t e s in Co r i c h amorphous Co Sn alloys

are not distributed r a n d d y

,

ii) the tin a m in m r p h o u s Co Sn allays around the equiatcmic composition and the Sn r i c h side a r e m s t l y located a t the center of trigonal prisms of cobalt atom.

Such a s i t e selectivity which i s widely doament-ed for amorphous alloys of tran- s i t i o n metals with small elements (B, P, C) was rather unexpected for mrphous al-

loys of transition metals w i t h a large element such a s Sn.

REFERENCES

/1/ Piecuch, M., Geny, J.-F., and ~ c h a l , G., in An~~rphous Metals and Non-Equilibrium Processing (Edit. de Physique, Paris, 19841, p. 79

/2/ Teirlinck, D., Piecuch, M., Geny, J.-F., M a r c h a l , G., Janot, C h r . and Mangin, Ph.,

J. Appl. Phys.

2

(1982) 7734

/3/ See a review by Panisscd P., Durand, J. and Budnick, J. I., Nucl. Instr. Meth.,

199 (1982) 99.

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/4/ See, f o r exanp?le, Shavishvili, T. M., and X i l i p t a r i , I. G., Phys. Stat. Sol. (b) 92 (19791 39 ; Kiliptari, I. G. and Shavishvili, T. M., Phys. M e t . Metall.

50

(1980) 99

-

/5/ Oono, T. and Itoh, J., J. Phys.Soc. Japan

27

(1969) 1359

/6/ Piecuch, H., Janot, Chr., and Marchal, G., J. Physique C 1

41

(1980) 251-

/7/ Eibsc!!utz, M., Lines, M. E., Chen, H. S. and

l&mmto,

T., J. Phys. F

14

(1984) 505.

/8/ Nabli, H., l i e d M., Delcmix, P., Durand, J.and b-chal G., (to be published)

.