A STUDY OF MAGNETIC INTERACTIONS IN HEXAGONAL IRON ALLOYS

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A STUDY OF MAGNETIC INTERACTIONS IN HEXAGONAL IRON ALLOYS

J. Williams, D. Pearson

To cite this version:

J. Williams, D. Pearson. A STUDY OF MAGNETIC INTERACTIONS IN HEXAGONAL IRON AL- LOYS. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-223-C2-225. �10.1051/jphyscol:1979280�.

�jpa-00218678�

JOURNAL DE PHYSIQUE Colloque C2, supplément au n° 3, Tome 40, mars 1979, page C2-223

A STUDY OF MAGNETIC INTERACTIONS IN HEXAGONAL IRON ALLOYS

J.M. Williams and D.I.C. Pearson

Department of Physios, The University, Sheffield, England

Résumé.- Des alliages de structure hexagonale à base de fer (phase e) contenant de 15 à 30 at. % de ruthénium ou d'osmium ont été étudiés par effet Mossbauer du 5 7Fe. Il apparaît un ordre antiferro- magnétique dans Fe-Ru et il en est probablement de même dans Fe-Os• Les valeurs mesurées du champ hyperfin à saturation et de la température de Néel ont été 15 kG et 210 k , 13 kG et 60 k , 3,3 kG et 30 K pour des alliages contenant 15 et 30 at. % de Ru, ou 15 at. % de Os respectivement. Dans tous les cas, la direction d'aimantation semble être orientée à environ 50° du gradient de champ électrique. L'alliage F e0 ) 70 s0 j 3 ne possède pas de température d'ordre. Les résultats obtenus sont extrapolés et discutés à partir des théories habituelles traitant de la mise en ordre magnétique dans le fer pur hep qui est seulement obtenu par application de pressions hydrostatiques élevées (>130 kbar).

Abstract.- Hexagonal (e-phase) alloys of iron with 15 and 30 atomic percent of ruthenium or osmium have been studied using the 57Fe Mossbauer resonance. The existence of antiferromagnetic ordering in the Fe-Ru and probably in the Fe-Os alloys is indicated. The saturated' hyperfine fields and Neel temperatures for the 15 and 30 at. % Ru and the 15 at. % 0s alloys were 15 kG and 210 K, 13 kG and 60 K, 3.3 kG and 30 K, respectively. In each case the direction of magnetization seems to be oriented at about 50 degrees to the e.f.g. The alloy Fe 0s showed no evidence of ordering at any tempe- rature. The results obtained have been extrapolated to those to be expected for pure hep iron and will be discussed in relation to current theoretical treatment of magnetic ordering in the e-phase of iron, normally realizable only by the application of large external pressures (P>130 kbar).

1. Introduction.- Three phases of pure iron (a, y and e) have been observed under different physical conditions and although widely studied using various techniques many of the microscopic properties are still not fully understood. In particular the magne- tic properties of such a transition metal are ob- viously of considerable interest. With reference to the phase diagram /l/ we see that under normal con- ditions at room temperature and atmospheric pressure, pure iron is stable in the bec a-phase. This phase is ferromagnetic, apart from a small region above

1000 K, where the Curie transformation takes place before the a-y (fee) transformation. The high tempe- rature/high pressure y-phase was found to be para- magnetic. However, a form of y-Fe which is stable at low temperatures can be produced by precipitation in copper. Gonser et al. Ill and Johnson et al. /3/

fo,und that this form ordered antiferromagnetically below ~ 70 K with a corresponding saturated hyper-

fine field ~ 24 kG.

Recently, much interest has been shown in the hep e-phase of iron, which is stable only at pres- sures in excess of ~ 130 kbars, particularly as re- gards its magnetic properties. This phase was origi- nally shown to be paramagnetic at room temperature by Nicol et al. /4/ and Pipkorn et al. 15/.

Mossbauer experiments have now been carried

out on pure hep Fe at low temperatures /I/ /6/ and rather surprisingly, showed little evidence of any magnetic ordering down to 2 K. The experimental ac- curacy lb I allowed an upper limit of 5 kG for the nuclear hyperfine field at 2 K.

In order to gain information about the e-phase of pure Fe whilst, at the same time, avoiding the high pressures involved for its stabilization, we may look at the properties of hexagonal phase alloys of iron in the high Fe concentration region. Early measurements by Ohno et al. Ill on such alloys sug- gested an antiferromagnetic coupling and an extra- polation to pure iron yielded a Neel temperature of ~ 100 K and a saturated internal field of ~ 16 kG, in clear disagreement with the results of /6/ and IM. Theoretical calculations carried out both by Fletcher and Addis /8/ and Madsen and Anderson (see Grimvall, 191) failed to predict any magnetically ordered state, on the basis of simple Stoner crite- ria, either for pure hep Fe or hep alloys. This see- med to contradict the alloy observations /7/ while

supporting the conclusions of the experiments on pu- re e-Fe.

In order to try to resolve some of the contro- versy regarding the magnetic properties of e-Fe, it was deemed that a detailed Mossbauer study of the more easily stabilized hexagonal Fe alloy systems

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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979280

JOURNAL DE PHYSIQUE

was r e q u i r e d .

2. Sample c h o i c e and p r e p a r a t i o n . - F u j i m o r i e t a l . / l o / s t u d i e d t h e d i f f e r e n t a l l o y s t r u c t u r e s s t a b i l i - zed u n d e r d i f f e r e n t p r e p a r a t i o n c o n d i t i o n s f o r g - R u a l l o y s and on t h e b a s i s of t h e i r r e s u l t s a l l o y s o f

15 and 30 a t o m i c p e r c e n t o f Ru (and 0 s ) i n Fe were chosen f o r o u r i n v e s t i g a t i o n of t h e &-phase. Appro- p r i a t e w e i g h t s o f t h e e l e m e n t s were t h o r o u g h l y mixed and a l l o y e d b e f o r e a n n e a l i n g , under s a f e hydrogen, a t 1400°C f o r a p e r i o d of 9 h o u r s and quenching i n t o w a t e r . The €-phase was confirmed by X-ray a n a l y s i s .

3. R e s u l t s . - 7 ~ e Mzssbauer a b s o r p t i o n s p e c t r a were o b t a i n e d f o r a l l f o u r a l l o y s a t t e m p e r a t u r e s ranging from 5 - 300 K. The h i g h t e m p e r a t u r e s p e c t r a were shown t o be q u a d r u p o l e d o u b l e t s w h i l s t a t low tem- p e r a t u r e s t h e r e was c o n s i d e r a b l e l i n e b r o a d e n i n g , p a r t i c u l a r l y f o r t h e Ru a l l o y s . (Williams and Pear- son, I l l / ) .

Various s o u r c e s of low t e m p e r a t u r e broadening were examined, namely, 1) i n c r e a s e d " t h i c k n e s s " e f -

f e c t s due t o an i n c r e a s e i n t h e r e c o i l - f r e e f r a c t i o n ; 2) inhomogeneous i n c r e a s e s i n t h e Q.S. due t o a n i s o -

t r o p i c l a t t i c e c o n t r a c t i o n ; 3) r e l a x a t i o n e f f e c t s and 4) t h e o n s e t of m a g n e t i c o r d e r i n g . We c o n c l u d e , however, t h a t t h e b u l k of t h e s p e c t r a l b r o a d e n i n g o b s e r v e d i s due t o t h e o n s e t of magnetic o r d e r i n g . The broadened s p e c t r a were t h u s assumed t o a r i s e from t h e c o - e x i s t i n g e l e c t r i c q u a d r u p o l e and magne- t i c h y p e r f i n e i n t e r a c t i o n s , b o t h having r o u g h l y t h e same o r d e r of magnitude. The f i t t i n g method sugges- t e d by van Dongen Torman e t a l . / I 2 1 was used and

ween t h e magnetic f i e l d v e c t o r and t h e e . f . g . s y m e - t r y a x i s i s ^" 50'.

A complete a n a l y s i s o f t h e I5 and 30 a t . % Ru and t h e 15 a t . % 0 s a l l o y s y i e l d e d s a t u r a t e d hyper- f i n e f i e l d s and N6el t e m p e r a t u r e s of 15 kG and 2101<,

13 kG and 60 K and 3 , 3 kG and 30 K r e s p e c t i v e l y . 4.- D i s c u s s i o n . - Our E - R u r e s u l t s when e x t r a p o l a t e d t o t h o s e e x p e c t e d f o r p u r e € - i r o n s u g g e s t an orde- r i n g t e m p e r a t u r e

-

" 17 kG.

The low t e m p e r a t u r e t h i g h p r e s s u r e ~ E s s b a u e r s t u d i e s of p u r e i r o n , showed no such e v i d e n c e of m a g n e t i c o r d e r i n g . These r e s u l t s seem, a t f i r s t , t o

c o n t r a d i c t t h e e x p e c t e d b e h a v i o u r of p u r e E-Fe from o u r s t u d y of t h e €-phase a l l o y s . However, we might c o n s i d e r o u r e x t r a p o l a t i o n t o p u r e E-Fe and t h e r e - s u l t s f o r p u r e Fe a t h i g h p r e s s u r e w i t h r e f e r e n c e t o t h e p l o t of atomic moment v e r s u s a t o m i c volume i n f i g u r e 2 191.

( v - v ~ ) / v ~ ( % ) Relative atomic volume t h e r e s u l t i n g h y p e r f i n e f i e l d dependence on tempera-

t u r e f o r t h e F e o . 8 5 R u o . 1 5 a l l o y i s shown i n f i g u r e 1.

F i g . 2 : The f e r r o m a g n e t i c moment ( s p i n s p e r atom) f o r b c c , f c c and hcp i r o n a s a f u n c t i o n of a t o m i c volume (from r e f e r e n c e 1 9 1 ) . The arrows show t h e volume change under a p r e s s u r e of 100 k b a r s and t h e t h e r m a l e x p a n s i o n up t o 1800 K. Dashed l i n e s i n d i - c a t e t h a t t h e s p i n was n o t u n i q u e l y o b t a i n e d .

Temperature ( K

F i g . 1 : The measured h y p e r f i n e f i e l d - t e m p e r a t u r e dependence f o r t h e F e o . 8 5 R u o . l s a l l o y .

A comparison o f t h e f i t t e d s p e c t r a w i t h Kundig's 1131 c a l c u l a t e d p l o t s s u g g e s t e d t h a t t h e a n g l e b e t -

We n o t e , i n g a r t i c u l a r , t h e form o f t h e &-phase cur-.

ve which i s c l o s e t o z e r o ( < 0 . 1 ~ ) f o r volumes below

-

t o b e c l o s e t o z e r o , a s was indeed observed. However, i t i s l i k e l y t h a t an e x t r a p o l a t i o n t o z e r o p e r c e n t Ru ( o r indeed 0 s ) would imply a p u r e Fe a t o m i c v o l w me w e l l above t h a t o b s e r v e d u n d e r t h e n e c e s s a r y con- d i t i o n s f o r i t s s t a b i l i z a t i o n . T h i s b e i n g t h e c a s e , i t i s q u i t e p o s s i b l e t h a t o u r e x t r a p o l a t i o n would

f a l l above z e r o on t h e volume a x i s , and o u t v a l u e s would a p p l y t o a " t h e o r e t i c a l &-phase1' of Fe, physi-

c a l l y u n r e a l i z a b l e . Assuming p r o p o r t i o n a l i t y between t h e Fe m a g n e t i c moment and t h e observed i n t e r n a l hy- p e r f i n e f i e l d , o u r e x t r a p o l a t i o n y i e l d s a v a l u e of

0 . 1 ~ f o r t h e E-Fe a t o m i c moment, and from f i g u r e B

2 t h i s would imply t h a t o u r e x t r a p o l a t i o n f a l l s a t a volume

-

We t h u s conclude t h a t i n t h e &-phase E - R u a l l o y s t h e r e i s Mzssbauer e v i d e n c e f o r magnetic o r - d e r i n g a s s u g g e s t e d by e a r l i e r r e s i s t i v i t y ( S a r k i s s a n and Coles 131) and s p e c i f i c h e a t ( C l a u s s /14/) d a t a .

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