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AMORPHIZATION OF CRYSTALLINE COBALT
AND TIN MULTILAYERS BY SOLID STATE
REACTION AT ROOM TEMPERATURE
P. Guilmin, P. Guyot, G. Marcha
To cite this version:
Colloque C8, supplément au n°12, Tome 46, décembre 1985 page C8-485
AMORPHIZATION OF CRYSTALLINE COBALT AND T I N MULTILAYERS BY SOLID STATE REACTION AT ROOM TEMPERATURE
P . G u i l m i n , P . Guyot and G. Marchal
Laboratoire de Physique du Solide, (U.A. au C.N.R.S. N° ISS), Université de Nancy I, B.P. 239, S4S06 Vandoeuove les Nancy Cedex, France
Résumé - Des alliages amorphes C ox Snj_x sont fontes par diffusion à l'état solide, à température atrbiante, dens des échantillons multicouches de cobalt et d'étain. Quelques propriétés des alliages ainsi formés sont comparées à celles des alliages amorphes de même composition obtenus par coévaporation à 77 K.
Abstract - Amorphous COj. Sni_x alloys have been obtained, at room temperatu-re, by interdiffusion from initially crystalline multilayers. Ihe properties of these alloys (forming ability range, temperature variation of electrical resistance, crystallization temperature and magnetic behaviour) are compared to those of COx Snx-x amorphous alloys formed by co-evaporation at 77 K. INTRODUCTION
Multilayers of crystalline cobalt and tin (fig. 1) are obtained by sequential evapora-tion in an ultra-high vacuum chamber and condensaevapora-tion onto substrates held at 77 K. The respective thickness of cobalt and tin are calculated so as to obtain the expec-ted composition after complete interdiffusion. The typical thickness of a bilayer, A, is about 60 K. Details of the experimental procedure are given elsewhere /l/.
d Co 1 .... • ••:• ••••. •-. - i •V A .'.. •;.•"'..: sn \ I ..' ''• :'.' i Co
Fig. 1 - Schematic view of multilayer after evaporation at 77 K.
The films are heated up in situ to room temperature and then annealed isothermally at 290 K for times ranging up to 60 hours.
I - EXPERIMENTAL RESULTS
1 - Amorphous alloy forming ability range
The forming ability range of amorphous alloys is determined by electron diffraction on samples made of five doublets evaporated onto copper grids previously coated with amorphous carbon. Amorphous alloys are formed in a large composition range :
O.20 4 x 4 0.60. However, Sn crystallites are observed for x 4 0.24 and crystallites
C8-486 JOURNAL
DE
PHYSIQUEofa-Cofor x 20.55. This fo&g ability range is found to be roughly the same as the s t a b i l i t y range a t r m temperature of Cox Snl-x mrphous alloys prepared by co-evaporation a t 77 K /2/.
F'urthemmre, as for ~ o - e ~ p ~ r a t e d alloys and liquid Co-Sn alloys, the f i r s t -peak in the structure factor s p l i t s up into tsm separate components, one due to Sn-Sn corre- lations, while the other i s due to Sn-Sn and Co-Co interferences.
2
-
Tenrrperature variation of the electrical resistanceDuring the mrphization process, the electrical resistance of samples, made of four- ty doublets deposited onto thin glass slides, is masured. A typical variation of the resistance vs.tirne i s shawn i n fig. 2.
I L Time [hours] L .
Fig. 2
-
Variation of the resistance of a mltilayer vs time a t T = 290 K (x = 0.24) During the f i r s t stage of the annealing treatment a strong increase of the electrical resistance i s observed ; then its variation slaws d m and a maimalvalue is rea- ched for annealing times of a b u t fourty hours. We assume that hmgeneous amrpkiza- tion i s completed a t t h i s time.Then the samples are cooled d m to 77 K and heated a t the constant r a t e of 2 ~ . m - ' up to the crystallization t m p r a t u r e T,.
1 dR For a l l samples, the tesnperature coefficient of the resistance (KR) a = R i s measured a t T = 200 K. The TCR is negative and takes the same value (fig. 3 a) as in co-evaporated mrphous alloys of the same composition /3, 4/. By contrast the TCR
of crystalline multilayers or of i n q l e t e l y mrphized samples Is positive. 3
-
Crystallization temperatureThe crystallization temperature Tx, defined as the temperature a t which the resistan- ce exhibits a m r e or less strong decrease, mmpares also favorably (fig. 3 b) with that of m-evaporated alloys /2/.
The nature of crystallization products is studied on sanples heated i n the electron microscope. For x = 0.24, the Co Snn crystal phase is mainly observd ; for x = 0.50 the Co Sn canpund i s obtained while the Co3 Sn2 mmpound is forrned for x = 0.60. For intermdiate compositions a mixture of these canpunds is observed after crys- tallization. These results are similar to those obtained for m-evaporated mrphous alloys.
4
-
Magnetic behaviourco-evaporated films (dashed line)
(a) tenprature coefficient of resistance (TCR)
,
a , measured a t T = 200 K (b) crystallization temperature, TxFig. 4
-
Variation of the nagnetic m e n t per cobalt atcan during the amrphization process.A s for-the electrical resistance, two steps are observed : a relatively strong decrea- se of 11 i n the f i r s t period (t 6 10 h ) , folloived by a slmer variation of 11
.
For al- loys with x = 0,50, the magnetic mment per cobalt atan (for t s 40 h) is still aboutl . o p ~ , whereas 11 vanishss i n co-evaporated alloys of sam? ccanposition /5/. This can
be taken as an indication that i n alloys formed by solid s t a t e reaction, cobalt-rich clusters subsist after the mrphization process.Acr=ordinqto fig. 4 haever, p s t i l l changes slightly with t, for t = 40 h, and
it
can be w c t e d that after longer times perfect homgeneity would be achieved, with p = 0, as in co-evaporated samples. I1-
DISCUSSIONThe variation with time of the resistance of the samples, the values registered for the TCR and the concentration dependence of the crystallization temperature indicate unambiguously that mrphous films are formed. Previous authors / 6 , 7/ have proposed
C8-488 JOURNAL DE PHYSIQUE
These c r i t e r i a are actually f u l f i l l e d in systems where m r p h i z a t i o n by solid s t a t e reaction has been reported (Au-La, Au-Y, N i - Z r , Ni-Hf, Co-Zr). In the particular ca-
se of the Co-Sn system, Co is an anamlous f a s t diffuser in Sn /8/ and the heat of f o m t i o n of the equiatamic oampound Co Sn i s slightly negative :
-
7 kcal/mole /9/.Hence our experiments confirm the validity of the above semi-eqirical c r i t e r i a In mst systems, however, the amorphization occurs a t 200-3C0° C , whereas anorphi- zation of Co-Sn m l t i l a y e r s is observed a t r m temperature i n about 20 h. This can
be attributed to the small thickness of the doublets we used and t o the large value of the diffusion coefficient of Co in Sn.
'L
In order t o determine the interdiffusion coefficient D of Co and Sn a t r m t a p x a t u - re, we use the r e s u l t s of resistance roeasurements given in fig. 2. A s a function of x and t the cobalt concentration is given by the appropriate solution to FICK's equation.
where A is the mcdulation length, k = 2 a/A and
sin (n k d/2) 'I,
(t) = 2 (n d/2) ~ X P (- n2 k2 D t)
To calculate the resistance of the samples for a given concentration profile, we as- sume that the r e s i s t i v i t y of a layer of concentration c (x) a t point x i s identical t o that of co-evaporated amorphous alloy of same composition. This alloy r e s i s t i v i t y has been previously measured in hcmgenmus samples /3/. Using the expression (1) f o r c (x, t ) , it is possible to f i t the theoretical variation of the resistance R t o the, experimental data i n the necond nAage (t 10 h)
.
This procedure leads to values of Dbetween 0.5 and 1.5. .an: s-l in samffles investigated i n the present work. These data are close t o the d u e D = 1
.o. lom2
un2.s-l obtained. by BRUSON /13/ a t rm tempe- rature from the decay of the intensity of X-ray diffraction s a t e l l i t e s in Co Sn mul-tilayers.
In contrast, it is impossible t o obtain a good f i t to the experimental variation of R in the ~Jht n k g e , using the diffusion &el with a constant value of
B.
If we use q. (1) to interpret the resistance data i n t h i s stage, we would have t o assume t h a t
8
decreases quickly with t for t < 10 h, from an initial value(t=O) t o about 30
a
(t > 10 h).
The i n i t i a l value of
8
is in reasonable agreement with the value D = 10-l8 a$ .s-I obtained by extrapolation to room temperature of high temperature roeasurmts of the diffusion constant of Co in crystalline Sn/lo/.
This behaviour of the e l e c t r i c a l resistance of m l t i l a y e r s seems to confirm the &el proposed by DOLGIN and JOHNSON 1 6 , 11/ who'split up the m r p h i z a t i o n process in two
d i s t i n c t stages :
i) a f i r s t stage where the atomic transport is controlled by interfacial reaction barriers,
ii) a second stage due to bulk diffusion.
The t i m e variation of the average magnetic manent per cobalt atom, i l l u s t r a t e d i n fig. 4 , also confirms this mcdel. Fram the interdiffusion coefficient determined f r a ~ resistance data, and using the J a ~ ~ l n o - W a l k e r &el /12/, it is possible t o d e t e r mine the theoretical variation of p in the second stage. I n t h i s calculation we assu- W, a s suggested by TEIRLINCK /5/ in homgeneous samples, t h a t a cobalt atom carries a magnetic -t when it is surrounded by a t l e a s t nine Co atcsns. The average map-
t u r e a t which t h e amorphization occurs. The ccanparison between physical p r o p e r t i e s of
C* Snl-x co-evaporated a l l o y s and amorphous a l l o y s obtained by s o l i d - s t a t e reaction suggests that t h e two kinds of a l l o y s are l a r g e l y s i m i l a r .
/1/ Guilmin, P., Guyot, P. and Marchal, G., Phys. L e t t . A 109 (1985) 174. /2/ Geny, J. -F.
,
Thesis Nancy (1981).
/3/ Geny, J.-F., Marchal, G., Mangin, Ph., and Piecuch, M., Phys. Rw. B
2
(1982) 7449. /4/ Korn, D., Phys. L e t t A108
(1985) 214./5/ Teirlinck, D.
,
Thesis Nancy (1981).
/6/ Johnson, W. L., Dolgin, B. P. and Van Rossm, M., in " G l a s s Current Issues" A.S.I. Nato School, Teneriffe ( a p r i l 1984) to be published.
/7/ Schrcder,H., Sanwer, K., and Koster, U., Phys. Rev. L e t t = (1985) 197.
/8/ Le C l a i r e , A. D., in "Properties of Atomic Defects in Metals" &IPeterson, . N. L. and Siegel, R. W. (1978) 70.
/9/ Barin, I., Knacke, O., and Kuboschiwski, O., i n " T h m c h e m i c a l Properties of Inor- ganic Substances", Springer-Verlag, Berlin (1977) 189.
/lo/
Brik, V. B., Phys. M e t . Metall.,53
(1982) 176./11/ Dolgin, B. P. and Johnson, W. L., Phys. Rev. B t o b e published.
