ELSEVIER Journal of Non-Crystalline Solids 192& 193 (1995) 355-359
J O U R N A L O F
An AWAXS study of Co-Er-B ternary amorphous metallic alloys
B. Bouchet-Fabre a,*, A. Kebab b, j. Dixmier c, H Lassri c, R. Krishnan c
a LURE, b?lt 209d, Universit~ Paris-Sud, F-91405 Orsay c~dex, France
b Laboratoire de M~tallurgie Physique, Universit~ de Rouen, F-76130 Mont-Saint-Aignan, France c LPSB, CNRS Meudon Bellevue, l pl. Aristide Briand, F-92190 Meudon, France
Abstract
Amorphous Co79_x_yErxB21+y alloys, with x < 10 at.% and y < 3 at.%, belong to the series of ternary transition metal-rare earth-metalloid glasses of great interest for their high coercivity and magnetization. An anomalous wide angle X-ray scattering study has been performed on a series of metallic glasses in order to relate their magnetic behaviour to their local and medium range order. Due to the large difference in size and electronic number, Z, between the three components, this technique allows the determination of the surroundings of the cobalt and erbium atoms and the discrimination of the Co-Co, Er-Er and Co-Er correlations in the first coordination shell. A competition between the topological disorder and a chemical ordering in the short and medium range occurs when the Er content increases, which may influence their magnetic and thermodynamic properties.
1. Introduction
The magnetic properties of amorphous ferromag- nets are dominated by the local and medium range orders in two ways: first, the exchange interaction varies for the different pairs of neighbouring spins and depends on the distances between them; second, the spin directions are defined by the local field determined by the short-range structural order [1].
The magnetization of melt-spun C o 7 9 _ x _ y E r x B 2 t + y ribbons with x < 10 at.% and y < 3 at.%, has al- ready been studied: it was shown that under intense magnetic field the antiparallel arrangement of Co and Er spins breaks down, and that the field needed to reach this situation depends strongly on the Er
* Corresponding author. Tel: + 33-1 64 46 80 00. Telefax:
+33-1 64 46 41 48. E-mail: bouchet@lure.u-psud.fr.
content beyond 4 at.% Er [2,3]; this behaviour re- veals the existence and evolution of the exchange
J E r - E r in these diluted glasses, where the ferrimag- netic coupling Jco-Er normally dominates.
A thermal study performed on Fes0_xRxB2o, where R is a rare earth, also shows a strong depen- dence on x around x = 6 at.%, where the crystalliza- tion temperature exhibits a maximum value, leading to the precipitation of the tetragonal RB 4 phase; at higher concentration of R, the Fe3R phase first ap- pears [4]. In a previous structural study on Co 79 - x EL B21, the presence of first-neighbour E r - E r pairs was confirmed and a change starting at x = 7.5 at.% was observed in the cobalt surrounding [5]. This paper reports a detailed structural study based on anomalous wide angle X-ray scattering (AWAXS) of the same melt-spun ribbons series that have been employed for the magnetic measurements, at compo-
0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 3 7 6 - 2
356 B. Bouchet-Fabre et al. /Journal of Non-Crystulline Solids 192 & 193 (1995) 355-359
Table 1
Weights of the pairs at k = 3 ,~ i in S~(k) and Sx(k) as defined in Section 2
Co75.1Er3.9B21 Co70.9Er5.1 B24 Co71.5Er7.5 B21 Co69.sEr9.5 B21 Differential structure factor S Z : without Co-Er contributions
C o - C o 0.96 0.95 0.99 1.03
Er-Er - 0.01 - 0.02 - 0.05 - 0.09
C o - B 0.05 0.07 0.06 0.07
Differential structure factor S x : without Co-Co contributions
E r - C o 0.84 0.80 0.77 0.73
Er-Er 0.10 0.13 0.18 0.22
E r - B 0.04 0.05 0.04 0.04
C o - B 0.02 0.02 0.01 0.01
sition C o 7 9 _ x E r x B 2 1 with x = 3.9, 7.5 and 9.5 at.%
and Co70.9Er5.1B24 . The X-ray measurements have
been performed with a two-circle goniometer on the wiggler beam line at LURE (France), equipped with a solid multidetector [5,6]. A new way to explore the differential surrounding from the total structure fac- tors has been developped in order to eliminate either the C o - C o correlations which are dominant, or that from the C o - E r pairs.
2. Anomalous wide angle X-ray scattering study
The scattered X-ray intensity per average atom at an energy, E, is usually written as
l( k, E) = ( f2) 4_ (f)2[ S( k ' E) - 1]
with
( r e ) = ]~.,xifi(k ' E ) f i ( k ' E),
( f ) 2 E X i X j f i ( k , g ) f j ( k , e ) .
The complex atomic scattering factor of atomic species j: f~(k, E) =fj0(k) + f ; ( E ) + / i f ( E ) , shows significative variation with E just near the absorp- tion edge of species j. The weighted differential structure factor AqS(k, EA, E u) is calculated by taking
A i j I ( k , E A , En)
= I ( k , Eh)Fq(k, EB) - I ( k , EB)Fij(k, EA) ,
Vo(k, E a ) = ( 2 - 8 i j ) / 2 [ f j ( k , Eu)fi(k, EB)
e.)],
then
Aijl(k, EA, EB)
= A i j ( f 2 ) + A i j ( f ) 2. [ A i j S ( k , EA, E B ) -- 1]
with
Aij(f2 > = ( fA2>Fij( k, EB) -- (f2>Fij( k , EA), Aij( f ) 2 = ( f A ) i F / j ( k , EB) - (fB)eFij( k, EA).
In the differential structure factor are eliminated either C o - E r correlation (called Sz(k), or C o - C o (called Sx(k)). f j ( E ) is calculated by Kramers- Kr6nig transformation of f ; ' ( E ) extracted from EX- AFS data; Sz(k) and Sx(k) have been extracted from the data taken at 8100 and 8355 eV, respectively.
The relative weights of the pairs are listed in Table 1.
3. Results
The curves, Sz(k), from which are removed the C o - E r correlations are drawn in Fig. 1. The oscilla- tions above 4 ,~-1 with two maxima at k 2 = 5.4 ,~-i and k 3 = 6.3 .~-1 are unchanged while the mean position of the first peak k I increases from 3.2 ,~-1 to 3.4 ~ - 1 together with the width, with in- creasing Er content. The ratio, k2/k x, decreases from 1.69 to 1.61 which is close to the golden number, r, when increasing XEr. These features reveal a surprising contraction and ordering of the Co subnetwork as the concentration of Er increases.
Caution has to be used in the interpretation of the
curves Sx(k) drawn in Fig. 2 since most of the signal
B. Bouchet-Fabre et al. /Journal of Non-Crystalline Solids 192& 193 (1995) 355-359 357
1 1
,.', 9
~ 7
i t 3
e~
- 1
. . . . . . . . . . . . ! . . . . I . . . . I . . . . |
i C o 6 9 . $ E r 9 . $ B 2 1
~: . . . . C o 7 1 , $ E r 7 . $ B 2 1
i ~ = = CoT0.9ErS.IB24
. . . C o 7 ~ . i E r 3 . 9 1 B 2 1
: ? *
... ~ ... ~... ... :,.~... ~..~.,~...~ .~..,, ~ .~.
• • ' i' ". " " , . - , ' "
... ~ ... ~:.-.,----~.----i ... ~ ... ::-~---:-~-'- . '., ' . . ~ . . , , , . '
. . . . ~ . . . , . /
... + ... ] - i ... ", ---....! ... i ... '.','-~,-*'-'."-., ;~.~
3 4
k ( A " )
Fig. 1. Differential structure factors, Sz(k), for Co79_xErxB21, with x = 3.9, 5.1, 7.5 and 9.5 at.% and Co70.9Ers.1B24.
1 2
m
• . 1 0
~ 6
4
- 2
. . . . . . . . . . . . ! . . . . ! . . . . i . . . .
. . . z . . . " " ' " F t " i . . . i" C o 6 9 . S E r 9 . $ B 2 1 . . . .
. ~ . . . . C o T I . ~ E r 7 . $ B 2 1
... T ... -[----~ ... --.. ~0,,.9~,.,~,, ----~
... ;---/--i-].]!1 ... - ... " ... c°~s"E'3"9sz' -- 4
i • i ¢r i l ~ ~ ' ' "
... ~-~' ... i . . . i~, .. . . ~ ~ ~ - . . ; " " " ~ ~ ...
. • ~ ' ',i I • N t i | '
_ . . . . . L . . . i~ . . . r. - . . . ~ . . . : . . . , ' . . .
, ' " ... ?,~ ... .1 " . . . " ' " ' - ~ . ' . : ' . . . - "
" L ; '~ /,. t ' ~ . ~ . " 7 '
!I!I!I!IIIEIIII!IE;IIEII iiilili i!i'iill IIIIIEIIIIIII iiiii iiiii iiiii iii;i IIIIIIIIIIIIIIIIE
, , , , I , , , , I , , , , L , , , , I , . . .
2 3 4 5 6
k ( A -])
Fig. 2. Differential structure factors Sx(k) for Co79_xErxB2], with x = 3.9, 5.1, 7.5, 9.5 at.% and Co70 9ErsAB2~
c o m e s from the C o - C o pairs, at least for the more d i l u t e s a m p l e s . T h e c u r v e s s h o w a v e r y s t r o n g o s c i l - l a t i o n around 3 A - 1 w i t h a d e e p m i n i m u m at 3.38
o 1
. ~ - 1 . T h e l a r g e p r e p e a k l o c a t e d at 1.6 A - , a l r e a d y described in Ref. [5] increases with XEr.
T h e c o r r e s p o n d i n g p a i r r a d i a l d i s t r i b u t i o n f u n c - tions, R z ( r ) and R ~ ( r ) , are p r e s e n t e d in Figs. 3 and 4. T h e n e g a t i v e c o n t r i b u t i o n o f the E r - E r pairs to
R z ( r ) is s m a l l but leads to a first d e e p m i n i m u m n e a r 3.3 A and a s e c o n d in the v i c i n i t y o f 5.4 A. A l l the s a m p l e s s h o w in the m e d i u m r a n g e the s a m e w e l l d e f i n e d distribution o f C o - C o pairs n e a r 6.3 ~,. A Gauss±an fit o f these c u r v e s up to 4.5 ~, is p r e s e n t e d in T a b l e 2. T h e first C o - C o distance, d 1, a r o u n d 2.5 exhibits a s m a l l c o n t r a c t i o n b e t w e e n 5.1 and 7.5 at.% Er. T h e fit leads to a v e r y h i g h n u m b e r o f
Table 2
Results of the best Gauss±an fit performed for each sample on Rz(r) and Rx(r)
dco_c o N/o" dEr_E r N/o" dEr_Co Nco/~r
Co751Er3.9B21 2.54 + 0.01 10 ± 0.3/0.28 3.62 ± 0.04 2.8 ± 0.3/0.16 3.05 ± 0.01 13 ± 1/0.20 0.087 at./A 3
4.09 = 18/0.38 5.53 4.70 = 23/0.28
Co70.9Er5,1B24 2.52 ± 0.0l 11.7 ± 0.5/0.18 3.46 ± 0.02 3.5 ± 0.2/0.10 2.96 ± 0.01 11 ± 1/0.21 0.086 at./,~ 3
4.10 = 15/0.25 4.9 = 20/0.35
Co71.sErT.sBel 2.49 + 0.01 9 ± 0.5/0.26 = 3.45 < 1 2.98 ± 0.01 15 ± 0.5/0.21
0.082 at./.~ 3
4.05 = 17/0.36 4.74 = 29/0.31
Co69.sErg.5B21 2.50 ± 0.01 8.2 + 0.3/0.20 3.53 ± 0.04 1.3 ± 0.3/0.16 3.00 + 0.01 15.2 ___ 0.5/0.215 0.079 at./A 3
4.20 = 19/0.4 5.54 4.76 = 27/0.32
3 5 8 B . B o u c h e t - F a b r e e t a l . / J o u r n a l o f N o n - C r y s t a l l i n e S o l i d s 1 9 2 & 1 9 3 ( 1 9 9 5 ) 3 5 5 - 3 5 9
.~ 1 7 0
1 5 0
. ~ 1 3 0
I
e 1 1 0
%, 1t
~ 9o
~ 7O
i .~ 3o
C o 6 9 . S E r g . S B 2 1 . . . i . . . i . . . T . . .
•
