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ELECTRICAL TRANSPORT AND STABILITY OF TMxSn1-x AMORPHOUS ALLOYS (TM = Fe, Co, Ni)
G. Marchal, J. Geny, Ph. Mangin, Chr. Janot
To cite this version:
G. Marchal, J. Geny, Ph. Mangin, Chr. Janot. ELECTRICAL TRANSPORT AND STABILITY OF
TMxSn1-x AMORPHOUS ALLOYS (TM = Fe, Co, Ni). Journal de Physique Colloques, 1980, 41
(C8), pp.C8-477-C8-480. �10.1051/jphyscol:19808119�. �jpa-00220215�
ELECTRICAL TRANSPORT AND STABILITY OF TMxSnl-x AMORPHOUS ALLOYS (TM = F e , C o t ~ i )
G. Marchal, J . F . Geny, Ph. Hangin and Chr. J a n o t
Laboratoire de Physique du SoZide ( L A 155) FacuZtE des Sciences, C.O. 140, 54037 Nancy Cedex, France
A b s t r a c t . - A v a i l a b i l i t y o f TtlxSn amorphous a l l o y s (TM = F e , Co, Ni) have b e e n s t u d i e d i n r e l a t i o n w i t h r e s i s t i v i t y measurements. ~ k z e a l l o y s have b e e n found m o s t l y s t a b l e i n c o m p o s i t i o n r a n g e s c o r r e s p o n d i n g t o n e g a t i v e v a l u e s o f t h e r e s i s t i v i t y measurements. These a l l o y s h a v e been found m o s t l y s t a b l e i n c o m p o s i t i o n r a n g e s c o r r e s p o n d i n g t o n e g a t i v e v a l u e s o f t h e r e s i s t i v i t y t e m p e r a t u r e c o e f f i c i e n t a and whose w i d e n e s s i s maximum f o r t h e a l l o y w i t h t h e l a r g e s t s i n g l e - s i t e t m a t r i x f o r s c a t t e r i n g o f e l e c t r o n s . However t h e c r y s t a l l i z a t i o n t e m p e r a t u r e i s n o t s i m p l y r e l a t e d t o a and r o u g h l y f o l l o w s t h e l i q u i d u s c u r v e o f t h e e q u i l i b r i u m p h a s e d i a g r a m s i n which t h e e x i s t e n c e o f a number o f i n t e r m e t a l l i c compounds seems t o b e a s t a b i l i t y f a c t o r f o r t h e c o r r e s p o h d i n g amorphous s y s t e m .
I - INTRODUCTION.
During the past few years a number of papers have been mainly devoted t o the question of the stability of amorphous systems against crystallization. The underlying purpose was of course strong wishes of criteria for selec- ting good candidates t o the development of amorphous sys- tems. At least good empirical rules have been selected from what has become a wealth of experimental works in the field.
- On the one hand good glass formers are usually found in alloys MXMel-, (where M is a transition o r noble metal and Me an element of group IV or V) with the composition near an eutectic or an intermetallic stable or metastable compound, far from the solid solubility range. Such correla- tions between amorphous system stability and the corres- ponding equilibrium phase diagram have suggested a Hume- Rothery-like approach in an attempt t o understand the aval- lability of amorphous materials in terms of size, electronega- tivity or valence effects [ I ] . As a consequence structural models have incorporated what could be described as local chemical ordering of each atomic species 121.
On the other hand according t o a nearly-freeelectron model [ 3 ] the maximum stability of an amorphous system corresponds t o the composition for which the Fermi mo- mentumlies at kF = k d 2 , where kp is the position of the first peak in the structure factor [4]. Indeed, it is well known that in a crystal a gap opens up in the energy band at the zone boundary. For a liquid or a glass such a real gap is not expected but a substantial decrease in the density of states should occur a t the energy E = pr2/2m (kp/2) 2 since isotropy results in all the shates at ( k J =
1
k being2 p affected. Any devlation from isotropy introduces direction of dependency in
S(a,
the minimum in state density flattens and broadens resulting in an increase of the system0.20 which corresponds to effective valence Z* between I and 2 and 2 kF
-
kp, though XPS studies have glven contradictory results regarding the existence of a minimum in state density at the Fermi level 151 [ 6 ] . Secondary effects of the Nagel and Tauc rnodel [4], as deduced from an ex- tension to alloys [ 7 ] o f the Ziman theory 181, that is a ma- ximum of electrical resistivity and negative values of the temperature coefficient of resistivity near kp, have actually been observed in amorphous materials with composition cor- responding to 2 kF-
k In an other approach, Hafner er al. [9], and Beck et dl. P' [ 101 have shown that Ca-Mg, Mg-Zn and Ca-A1 metallic glasses are stabili~ed by the close mat- ching between the minima in the pair potentials and the maxima of partial distribution functions.In this paper stability against crystallization of Fe-Sn, Co-Sn and Ni-Sn amorphous alloys is analysed in combina- tion with their electrical transport properties and features deduced from the equilibrium phase diagrams of the corres- ponding systems. The main points that have made the study of these alloys fairly attractive are :
(i) they all include the same IV group element (Sn) which behaves here like a metal.
(ii) changes in size effects are small and monotonous f r o n ~ Fe to Ni.
(iii) going from Fe to Co and Ni results in a progressive
filling of the 3d band, a decrease of the single-site t matri- ces for scattering of electrons [ I l ] with an evolution o f the d-wave phase shift ~ 7 2 (EF) toward n.
(iv) the effective valence Z* is about 1 in all three pure liquid metals (Fe, Co, Ni) [12].
(v) the three corresponding equilibriup phase diagrams looks quite similar, including a number of isotype phases [I31 having B 35, DO 19 or C 16 local structures [14].
energy. Then a ((return force, develops towards the amor- phous state : the deeper this minimum, the more stable against crystallization the amorphous material is expected.
Such a model has been experimentally supported by many MxMel-x metallic glasses having best stability at 1
-
x 2:Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808119
JOURNAL DE PHYSIQUE
2 - PREPARATION O F THE AMORPHOUS ALLOYS.
The TMxSnl-, amorphous alloys were obtained by a vapour quenching technique using separate crucibles from which the two component elements were evaporated and then condensed onto liquid nitrogen cooled substrates [I51 (glass plates or carbon coated grids). The deposition rate of each element was measured and controlled using two different quartz monitoring systems. A careful mass calibra- hon of the oscillating quartz was carried out by comparing their individual scales with the directly optically measured thickness of gold (on MT side) or tin (on the Sn side) films deposited onto liquid nitrogen cooled glass plates.
This is an improvement of the method previously described (161 1171 [18]. In fact the oscillating quartz being at room temperature and the substrates at the liquid nitrogen tempe- rature, different back diffusion contributions for tin deposi- tion must be taken into account as evidenced from atomic absorption of thick samples [19]. The direct recalibration procedure has led to correct tin concentration claimed in Ref. 1161-[I81 by about 5 .% towards the tin side. Back difcusion is not observed in deposition of Fe, Co, Ni, Si, Al, Au, etc
...
but clearly exists in tin deposition as shown by the presence of a tin film on both sides of a grid placed between the quartz afid the crucible. This parasite phenome- non is probably enhanced by the low melting point of tin and may be typical of the,atom behaviour when they hit the substrate : the quenching procedure might be less effi- cient as expected in giving a suddenly frozen jam of atoms.3
-
STABlLITY AND ELECTRICAL RESISTIVITY O F THE ALLOYS.3.1. - Stab~lity against crystallization.
Characterization of the deposited alloys has been canied out using electron micrography at room temperature and electrical resistivity behaviour in subsequent annealing at a rate o f 2 O per minute. Depending on MT and composition, three different cases have been observed :
Fig. 1 . - Composition ranges o f stability at room tempe- rature for MTxSnlsv alloys.
Full line : pure amorphous system..
Dashed line : microcrystals in an amorphous s a .
(i) crystallized alloys at room temperature.
(ii) amorphous material at room temperature as shown by electron microscopy and diffraction and typical sudden change in electrical resistivity at higher temperature corres- ponding to crystallization.
(iii) initial crystallized microislands embedded in an amor- phous sea.
Annealing the sample results in a progressive change of elec- trical resistivity without obvious stage transition, while more and more microcrystals are seen in EM pictures.
Fig. 2. - Composition dependence o f the crystallization temperatures.
Corresponding composition ranges are shown in figure 1 and the composition dependence of the crystallization tem- peratures is given in figure 2. It is worth noting that the MTxSnl-x alloys behave quite differently at crystallization temperature, depending of MT : in FeXSnl-, the electrical resistivity exhibits either a sharp increase (x
>
0.37) or a sudden decrease (x<
0.37) when crystallization takes place [16] while in CoXSnl-, and NixSn l-x the transition is shown by a resistivity fall whatever the composition (figure 3). Looking at figures I and 2, it is clear that :(i) the composition range of availability for MTxSnl-, amor- phous alloys is larger for MT = Fe (50 %) than for MT =
Co (40 %) or MT = Ni (20 %).
(ii) when comparable, crystallization takes place at higher temperature in Co compounds than in Fe and Ni alloys : Tcr reaches 530 K with MT = Co o r Fe but is lower than 400 K if MT = Ni.
thetinsideavailabilityofamorphousmaterialseems : t o be l i m i t e d by T c r going down t o t h e d e p o s i t i o n temperature. On t h e MT s i d e quenching e f f i c i e n c y might questionable.
3.2. - Electrical resistivity data in the amorphous states.
The electrical resistivity po as measured at T = 80 K on as-quenched samples have quite a similar composition dependence in FexSnl-x and CoxSnl-x amorphous alloys, that is bowler hat shaped curves reaching maxima of 250 pncm at x
---
0.45*
0.05 and being reduced down to 200 pncm a n i to 150 pncm by compositional shifts of Ax = -+ 0.15 and+
0.25 respectively.In NixSnl-, po continuously increases from 100 i n c m at x = 0.3 to 125 pncm at x = 0.5. Detailed analysis of resistivity data will be performed in a next future after achie ving accurate mass density measurements which are presently in progress.
Temperature coefficients of resistivity a =
1 9
=1 a
p dT R dT are not affected by imperfect knowledge of the geometry sample. They have been measured after stabilizing the alloys by annealing up to 300 K [I71 and their composition depen- dences are shown in figure 4. Except for the iron richest FexSnl-, alloys, a is mostly negative, with a minimum near x
-
0.28 in FexSnl-, and near x = 0.32 in CoxSnl-,, and behaves like in MTxGel-, liquid alloys [20] (MT = Fe, Co, Ni). In those liquid alloys po is maximum near x = 0.6 ( p t = 190 pWcm in Fe Ge and Co Ge, 120 pS2cm in Ni Ge), a is negative around a minimum value near x-
0.4 (shifted by about Ax-
0.2 with respect to the po maximum like in the present work) and becomes posi- tive'for both the smallest and largest x values.Thus the Ziman model seems to be as relevant to the qualitative understanding of electrical properties of the amorplious MTxSnl-, alloys as it is to liquid alloys. Howe- ver orders of magnitude are badly interpreted and from changes in the single-site t matrices a larger resistivity in Fe-Sn than in Co-Sn would have been expected [ I l l . A more realistic model, as kuggested by Esposito et al. [12],
Fig. 4. - Composition dependence of the temperature coefficient o f resistivity in MTxSnl, amorphous alloys compared to liquid MTxGel, data.
0 Fe-Sn A Co-Sn Ni-Sn liquid Co-Ge
should take into account influence of short range order in scattering of electrons.
4 - DISCUSSION AND CONCLUSION.
Among the features observed in the present work two of them may give significant support to the stability electronic model of Nagel and Tauc [4] :
(i) composition ranges of availability correspond most'ly to negative value of the temperature coefficient of resisifivity.
Near pure element sides ar might become positive lilte in MTxGel-x liquid alloys.
(ii) MT with the weakest single-site t matrix for scatte- ring of electrons resulting in the less deep minimum in the nearly-free electronic state density [4] gives amor- phous alloys with narrowest stability range.
However some conclusions allow to question this elec- tronic model :
(i) the crystallization temperature does not follow the composition dependence of a and Tcr is even the highest in FexSnl.x alloys for the few x values correspbnding t o positi- ve OL Relations between a and To, as observed by Hillman et al.
[3] in Ni PB, do not apply in the present cases.
(ii) In Ni-Sn and Co-Sn amorphous alloys the crystalliza- tion temperature Tcr roughly follows the composition de- pendence of the liquidus temperature Tliq as observed in the equilibrium phase diagrams (see table I in which values of the TcdTliq ratios are reported).
Table I
JOURNAL DE PHYSIQUE
REFERENCES
Like Tliq, Tcr might rapidly decrease on the Sn rich side because of the low melting point of tin. Such a com- parison is more difficult with the Fe-Sn system which exhibits a broad miscibility gap in the liquid state.
(iii) the availability range is wider for the Fe-Sn system whose equilibrium phase diagram shows a great number of intermetallic compounds.
Mass density measurements and structure factor deter- mination are in progress to complete the investigation of those MT,Snl, amorphous systems and critically compare the various stability criteria.
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