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Submitted on 1 Jan 1979
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EFFECT OF LOCAL ON FORMATION OF LOCAL MOMENTS IN BCC IRON ALLOYS
M. Shiga, Y. Nakamura
To cite this version:
M. Shiga, Y. Nakamura. EFFECT OF LOCAL ON FORMATION OF LOCAL MOMENTS IN BCC IRON ALLOYS. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-204-C2-206.
�10.1051/jphyscol:1979272�. �jpa-00218670�
C2-204 JOURNAL DE PHYSIQUE
EFFECT OF LOCAL ENVIRONMENT ON FORMATION OF LOCAL MOMENTS IN BCC IRON ALLOYS
M. Shiga and Y. Nakamura
Department of Metal Science and Technology, Kyoto University, Kyoto, 60S, Japan
Résumé.- L'analyse des spectres Mossbauer d'alliages BCC Fe enregistrés à la température de l'hélium liquide donne des informations sur les effets de l'environnement local sur l'apparition des moments locaux. Dans cet article, on présente les résultats relatifs à l'alliage Fe-Al désordonné.
Abstract.- By analyzing the Mossbauer spectra of some BCC Fe alloys at liquid He temperature, the effect of local environement on the formation of local moments has been investigated. In this report, the results of disordered Fe-Al alloys are presented.
Recent studies of the magnetism in alloys have revealed that the magnetic moment on atoms depend on their local environment rather than on average elec- tron properties. It is particularly interesting to study the effect of local environment near the cri- tical concentration for ferromagnetism. The Moss- bauer effect is an excellent tool to study this prob- lem in Fe alloys. We have already shown that in or- dered Fe-Al alloys large local moments are formed before ferromagnetism appears /l/. A similar result has been obtained for Fe-Cr alloys 12/. Contrary, in Fe-V alloys small local moments are formed on Fe atoms by the onset of ferromagnetism /3/. In this paper, we report the result of disordered Fe-Al al-
loys.
The Fe-Al phase diagram shows a fairly wide range of BCC solid solution up to 52 at.%Al. An Fe- Al superstructure is so stable for high Al contents that it cannot be suppressed by quenching from high temperatures. Previous studies /4,5/ have shown that the ordered structure can be suppressed by cold wor- king such as crushing or filling and that the decrease of magnetization with increasing Al content becomes less marked by cold working. In this study, we pre- sent microscopic information about the change in the atomic moment of crushed Fe-Al alloys as a function of Al concentration through the analysis of the hyperfine field distribution obtained from Mossbauer spectra at liquid He temperature where localized moments should be frozen.
Several Fe-Al alloys were prepared in an argon arc furnace by repeated melting. After a homogeniz- ing treatment, the ingots are crushed into 250-mesh powders. X-ray diffraction showed that the superlat- tice lines almost disappear except for the very weak Q o o ] line. The results of Mossbauer measurements
are shown in figure 1.
Fig. 1 : Mossbauer spectra of disordered Fe-Al al- loys at 5 K. Full curves are fitted from P(H) curves shown in figure 2.
The spectra were analyzed to yield both the hyper- fine field distribution (P(H) curve) and the histo- gram. The details of the analysis were given in pre- vious papers /1,3/. The difference in the isomer shift between a high field peak and a low field one was taken into account. The obtained P(H) curves and
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979272
histograms a r e shown i n f i g u r e 2.
Fig. 2 : Hyperfine f i e l d d i s t r i b u t i o n curves g i v i n g t h e f i t t e d curves i n f i g u r e 1. Histograms of t h e p r o b a b i l i t y of h y p e r f i n e f i e l d s a r e a l s o given.
The f i t t e d curves a r e shown-by s o l i d l i n e s i n F i g u r e 1 . As s e e n i n Figure 2 t h e P(H) curves a r e c h a r a c t e r - i z e d by two d i s t i n c t peaks f o r most of t h e a l l o y s . Using an e m p i r i c a l r e l a t i o n between t h e h y p e r f i n e
f i e l d and t h e atomic moment i . e .
,
Hi = ap + b?I
Fe
w i t h a = 65 kOe/pB and b = 85 kOe/uB [ I ]
, uFe
cor- responding t o t h e high f i e l d peak i s roughly estimat- ed a s 2pB. The displacement of t h e peak p o s i t i o n t oa lower f i e l d w i t h i n c r e a s i n g A 1 c o n t e n t can be a t - t r i b u t a b l e t o t h e d e c r e a s e i n
.
Therefore, i t can be concluded t h a t t h e high f i e l d peak o r i g i n a t e s i n t h e magnetic Fe w i t h a moment of approximately 2yB ' The f r a c t i o n of t h e magnetic Fe atoms i s e s t i m a t e d from t h e i n t e g r a t e d i n t e n s i t y of t h e two peaks. The r e s u l t i s shown i n f i g u r e 3 t o g e t h e r w i t h t h a t ob- t a i n e d from t h e m a g n e t i z a t i o n measurements / 5 / . The agreement between b o t h v a l u e s c l e a r l y i n d i c a t e s t h a t t h e r e a r e no antiferromagnetically coupled moments i n d i s o r d e r e d Fe-A1 a l l o y s . I n t h e ordered Fe-A1 alloys,
however, t h e d e c r e a s e i n m a g n e t i z a t i o n beyond about 30 a t . % A l has been a t t r i b u t e d mainly t o antiferromag- n e t i c a l l y couuled moments from t h e f a c t t h a t t h e
f r a c t i o n of magnetic Fe atoms determined from t h e P(H) curves i s much l a r g e r than t h a t determined from t h e b u l k m a g n e t i z a t i o n / I / .
at. % AL
Fig. 3 : The f r a c t i o n of magnetic Fe atoms. Closed c i r c l e s a r e t h e p r e s e n t r e s u l t s . Open c i r c l e s a r e o b t a i n e d from t h e m a g n e t i z a t i o n 151. Crosses w i t h t h e e r r o r b a r i n d i c a t e t h e c a l c u l a t e d v a l u e s ( s e e t e x t ) .
F i n a l l y , we d i s c u s s t h e f r a c t i o n of magnetic Fe atoms on t h e b a s i s of a l o c a l environment model.
According t o Besnus e t a l . , Fe atoms i n t h e d i s o r - dered Fe-A1 a l l o y s become magnetic when they have more t h a n 8 o r 9 Fe atoms w i t h i n t h e i r next n e a r e s t neighbor s h e l l s 151. On t h e o t h e r hand, we have shown i n t h e p r e v i o u s s t u d y of t h e ordered Fe-A1 a l l o y s / 1 / t h a t t h e c r i t i c a l number of Fe atoms on t h e n.n.
s h e l l should be 4 o r 5 . I n t h e d i s o r d e r e d Fe-A1 a l - l o y s , t h e d i s t r i b u t i o n s of Fe and A 1 atoms a r e n o t completely random b u t t h e s u p e r s t r u c t u r e may be de- veloped t o some e x t e n t . It i s v e r y d i f f i c u l t t o de- termine t h e c r i t i c a l number c o r r e c t l y because t h e a n a l y s i s depends p r i m a r l y on t h e degree of o r d e r . N e v e r t h e l e s s , we have t r i e d t o i n t e r p r e t our r e s u l t w i t h t h e l o c a l environment model by t a k i n g account of t h e o r d e r parameter which i s roughly e s t i m a t e d from t h e i n t e n s i t y of t h e
~ o O I [
peak of X-ray d i f - f r a c t i o n . The c a l c u l a t e d f r a c t i o n of magnetic Fe atoms f o r t h e c r i t i c a l number of nc = 4 i n t h e n.n.s h e l l a r e shown a s c r o s s e s i n Figure 3. A l l t h e ex- p e r i m e n t a l v a l u e s a r e w i t h i n t h e e r r o r b a r s of t h e c a l c u l a t i o n which i n d i c a t e t h e e r r o r s due t o e s t i - mation of t h e i n t e n s i t y of s u p e r l a t t i c e l i n e s .
JOURNAL DE PHYSIQUE
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