Influence of conduction electrons in a heavily-doped magnetic semiconductor : a 15Eu Mössbauer study of europium hexaboride

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Influence of conduction electrons in a heavily-doped magnetic semiconductor : a 15Eu Mössbauer study of

europium hexaboride

J. M. D. Coey, Olivier Massenet, M. Kasaya, Jean Etourneau

To cite this version:

J. M. D. Coey, Olivier Massenet, M. Kasaya, Jean Etourneau. Influence of conduction electrons in a

heavily-doped magnetic semiconductor : a 15Eu Mössbauer study of europium hexaboride. Journal de

Physique Colloques, 1979, 40 (C2), pp.C2-333-C2-336. �10.1051/jphyscol:19792118�. �jpa-00218488�

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I N F L U E N C E OF CONDUCTION ELECTRONS I N A H E A V I L Y - D O P E D MAGNETIC SEMICONDUCTOR

:

A l5lEu MOSSBAUER STUDY O F EUROPIUM H E X A B O R I D E

X +

J.M.D. Coey, 0 . Massenet, M. Kasaya and J . Etourneau*

Groupe des T r a n s i t i o n s de Phases, C.N.R.S., B.?. 166, 38042 Grenoble Ceder, France

' ~ a b o r a t d r e de Chimie du S o l i d e du C.N.R.S., Universitd de Bordeaux I , 33405 Talence, France

RCsw16.- Dans c i n q C c h a n t i l l o n s d i f f Q r e n t s d ' h e x a b o r i d e d'europium, EuB6, on a observC que l ' e u r o - pium Q t a i t d i v a l e n t e t aucune Evidence de v a l e n c e i n t e r m B d i a i r e n ' a Ct6 o b s e r v Q e . L ' o r d r e magnEtique Evolue d e p u i s l ' o r d r e f e r r o m a g n e t i q u e dans l e matCriau p r e s q u e s t o e c h i o m B t r i q u e 5 l ' o r d r e a n t i f e r r o - magnetique d a n s EuBs,sCo,i p a s s a n t p a r une rEgion i n t e r m e d i a i r e mictomagnetique a v e c un Lchange i n h o m g s n e a u t o u r de EuBg,9C0,1. On s u g g s r e q u e d e s amas f e r r o m a g n C t i q u e s d'europium a u t o u r d e s a t o - mes de c a r b o n e d o n n e u r s s o n t c o u p l Q s a n t i f e r r o m a g n g t i q u e m e n t 1 ceux e n t o u r a n t un carbone a d j a c e n t p a r Qchange d i r e c t B l e c t r o n i q u e .

A b s t r a c t . - The europium i s d i v a l e n t and no e v i d e n c e o f mixed v a l e n c e i s found i n any o f f i v e d i f f e - r e n t samples o f europium h e x a b o r i d e . The m a g n e t i c o r d e r e v o l v e s from f e r r o m a g n e t i c i n n e a r - s t o i c h i o - m e t r i c m a t e r i a l t o a n t i f e r r ~ m a ~ n e t i c i n EuBs 8 C 0 , 2 p a s s i n g t h r o u g h a m i c t o m a g n e t i c r e g i o n w i t h inhomogeneous exchange around EuBs s C o , l . There :t i s s u g g e s t e d t h a t f e r r o m a g n e t i c europium c l u s t e r s a r o u n d t h e c a r b o n donors a r e c6upled a n t i f e r r ~ m a ~ n e t i c a l l ~ by d i r e c t exchange o f e l e c t r o n s from a d j a c e n t d o n o r s .

The m a g n e t i c p r o p e r t i e s of europium h e x a b o r i - d e a r e t h e s u b j e c t o f some c o n t r o v e r s y . I n t h e o r i - g i n a l work, G e b a l l e e t a 1 r e p o r t e d EUBB a s a f e r r o - magnetic s e m i c o n d u c t o r w i t h T = 8 . 5 K / l / . Subse- q u e n t measurements on o t h e r p o l y c r y s t a l l i n e samples y i e l d e d v a l u e s i n t h e range 8-15 K. The f i r s t s i n g l e c r y s t a l 1 2 1 however d i d n o t o r d e r f e r r o m a g n e t i c a l l y d e s p i t e a p o s i t i v e p a r a m a g n e t i c C u r i e t e m p e r a t u r e , B p = 9 K / 3 / . I s i k a w a e t a 1 i n t e r p r e t e d t h e i r d a t a a s showing t h a t t h e c r y s t a l was a n t i f e r r o m a g n e t i c and s u g g e s t e d t h a t t h i s i s t h e i n t r i n s i c b e h a v i o u r o f t h e compound / 2 / . Very r e c e n t l y , Kasaya e t a 1 have s t u d i e d c r y s t a l s and powders o f EuBs-,Cx w i t h c o n t r o l l e d c a r b o n c o n t e n t , 0 < X < 0.21 / 4 / . Carbon a c t s an e l e c t r o n donor i n t h e B g framework, s o X i s a p p r o x i m a t e l y t h e c o n d u c t i o n ' e l e c t r o n c o n t e n t . 0

P was found t o d e c r e a s e r e g u l a r l y from + 15 K t o

-

7 K w i t h i n c r e a s i n g X , c r o s s i n g z e r o a t ^X= 0 . 1 3 . I t a p p e a r s t h a t c o n d u c t i o n e l e c t r o n s c o n t r i b u t e a n a n t i f e r r o m a g n e t i c exchange i n t e r a c t i o n . A t t h e same time t h e r e i s a s m a l l d e c r e a s e i n l a t t i c e p a r a m e t e r from 4.1855 f o r undoped samples t o 4.1695

A

f o r

X : 0 . 2 1 .

I n t h i s p a p e r , we r e p o r t 1 5 ' ~ u l 6 s s b a u e r d a t a on f i v e samples o f europium h e x a b o r i d e produ- ced i n d i f f e r e n t ways, i n c l u d i n g two i n t e n t i o n a l l y doped w i t h c a r b o n . We were p a r t i c u l a r l y i n t e r e s t e d +permanent a d d r e s s : Department o f Physics,Tohoku

U n i v e r s i t y , S e n d a i 980, J a p a n .

t o f i n d o u t i f samples w i t h 0 ^%0 were magnetically P

o r d e r e d , and w h e t h e r t h e exchange i n t e r a c t i o n s i n doped samples were s p a t i a l l y homogeneous.

D e t a i l s o f t h e samples a p p e a r i n t h e t a b l e . I t w i l l be c o n v e n i e n t t o t a k e t h e l a t t i c e c o n s t a n t a a s t h e c h a r a c t e r i s t i c p a r a m e t e r of e a c h sample a s t h e work o f Kasaya

st

s u g g e s t s t h a t a. de- c r e a s e s u n i f o r m l y w i t h i n c r e a s i n g c o n d u c t i o n e l e c - t r o n c o n t e n t , n. Although c a r b o n was n o t d e t e r m i n e d i n sample 111, comparison w i t h t h e d a t a i n r e f e - r e n c e 4 s u g g e s t s X 2. 0.05.

MEssbauer s p e c t r a were o b t a i n e d on a b s o r b e r s w i t h 25 mg/cm2 of e a c h sample a t 296 K and a t s e - v e r a l t e m p e r a t u r e s i n t h e r a n g e 1.6

-

20 K. A t room-temperatures a s i n g l e l i n e w i t h a t y p i c a l l y d i v a l e n t isomer s h i f t ,

-

1 2 . 7 ( . 1 ) m / s r e l a t i v e t o t h e s o u r c e lS1Sm i n SmF,, was o b s e r v e d i n a l l f i v e samples. No s i g n of a Eu3+ l i n e was s e e n a t any t e m p e r a t u r e , s o a l i m i t of 3 % i s t h e r e b y s e t on any p o s s i b l e inhomogeneous v a l e n c e m i x t u r e . Nor i s t h e r e any s i g n i f i c a n t v a r i a t i o n of t h e i s o m e r s h i f t between 20 and 296 K (<0.2 m / s ) , a r e s u l t which p l a c e s a l i m i t o f 2 % on any change i n homogeneous v a l e n c e a d m i x t u r e w i t h t e m p e r a t u r e . These r e s u l t s were c o n f i r m e d by measurements o f t h e s u s c e p t i b i -

l i t y which gave a t e m p e r a t u r e - i n d e p e n d e n t C u r i e c o n s t a n t o f 7 . 7 ( 2 ) a t t e m p e r a t u r e s up t o 800 K, a s e x p e c t e d f o r Eu2+. I t f o l l o w s t h a t t h e gap between t h e 4f l e v e l and t h e c o n d u c t i o n band i n EuBs i s

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792118

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JOURNAL DE PHYSIQUE

Q w l e I& Description L a t t i c e Parameter Carriers/Eu Ordering Temperature tlhf (1.6) tlhf ( 4 . 2 ) Ulhf ( 4 . 2 ) Referrnce

n Tc (K) (koe) ( k k ) ( t o e )

I S t o i c h i m t r i c powder 4.1855 ( 3 ) ^2-0 12.5 ( . 5 ) 332 308 2 . 0 4

I 1 1 , h e a t e d i n E u 4.1855 ( 3 ) 2 . 0 10.7 ( . 5 )

vapour 336 277 2 0

111 Single c r y s t a l 4.1810 ( 3 ) 0.03 6.3 ( . 3 ) 316 262 eo 2

I V Cdrbon-doped powder 4.1733 ( 3 ) 0.14

'

X = 0.14 7 ( 1 ) nd 270 25 4

k s u n e d equal t o x i n the f o m l a E I I B ~ - ~ C ~

Table I : D e t a i l s of the europium hexaboride samples.

l e a s t 0.2 eV, i n accord with r e s i s t i v i t y measure-

-

ments a t high temperatures which y i e l d e d an a c t i v a - t i o n energy of 0 . 3 eV 151.

The appearance of magnetic hyperfine s p l i t - t i n g i n d i c a t e s t h a t a l l our samples a r e magnetically ordered a t 4 . 2 K , r e g a r d l e s s of the value of 8

.

P Three t y p i c a l s p e c t r a a r e shown i n f i g u r e I .

and IV) a r e g r e a t l y broadened. The corresponding spread i n h y p e r f i n e f i e l d AHhf i s given i n t h e t a b l e . Line broadening i n sample I11 cannot be due t o macroscopic inhomogeneity because i d e n t i c a l s p e c t r a were o b t a i n e d f o r samples from d i f f e r e n t p a r t s of t h e c r y s t a l . Nor i s i t r e l a t e d t o t h e proximity of t h e o r d e r i n g temperature (by poor temperature c o n t r o l o r homogeneity) because i t i s absent i n V , whose o r d e r i n g temperature i s j u s t a s low a s t h a t of I 1 1 o r I V . The broadening disappears i n s p e c t r a taken a t 1.6 K , so i t must b e a s s o c i a - t e d with inhomogeneity of t h e exchange i n t e r a c t i o n s pn a m i c r o s c o ~ i c s c a l e . The b r e a d t h of t h e exchange d i s t r i b u t i o n can be e s t i m a t e d from l o c a l mole- c u l a r f i e l d theory 161 t o be roughly a s g r e a t a s i t s average v a l u e . The absence of any s i g n i f i c a n t broadening i n samples I , I1 and V shows t h a t t h e exchange i n t e r a c t i o n s t h e r e a r e e s s e n t i a l l y homogeneous.

Magnetic o r d e r i n g temperatures were d e r i v e d f o r a l l f i v e samples by e x t r a p o l a t i n g t h e h y p e r f i - ne f i e l d t o z e r o . For t h e ferromagnets, I and 11, t h e s e temperatures a g r e e f a i r l y well with those o b t a i n e d from A r r o t t p l o t s 171. For t h e o t h e r samples they correspond t o a cusp i n t h e s u s c e p t i - b i l i t y 171. V appears t o be a n t i f e r r o m a g n e t i c , from t h e c o n s i s t e n c y between 8 Tc and x(Tc). 111

P '

and I V however may b e s t be d e s c r i b e d a s mictoma- Fig. 1 : Miissbauer s p e c t r a a t 4 . 2 K of ferromagnetic g n e t s s i n c e t h e y c o n t a i n a s p a t i a l l y inhomogeneous ( I ) , mictomagnetic (111) and a n t i f e r r o m a g n e t i c (V) distribution o f exchange interactions of both samples of europium hexaboride. P a r t i c u l a r s of each

sample a r e given i n t h e t a b l e . s i g n s . The s u s c e p t i b i l i t y cusp i n t h e s e two samples i s e l i m i n a t e d i n f i e l d s of 2-3 kOe whereas t h a t of Both t h e nominally s t o i c h i o m e t r i c samples ( I and sample V p e r s i s t s i n f i e l d s g r e a t e r than 10 kOe.

11) and t h e most carbon r i c h one (V) show unbroade- F i g u r e 2 shows t h e magnetic phase diagram of t h e ned l i n e s i n t h e magnetic s p e c t r a , but t h e l i n e s of europium hexaboride system a s a f u n c t i o n of l a t t i - samples w i t h i n t e r m e d i a t e l a t t i c e parameters (111 ce parameter. The second s c a l e r e l a t e s a. t o t h e

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F ! g . 2

:

Magnetic phase diagram of europium hexabo- ride showing paramagnetic (P), ferromagnetic (F), mictomagnetic (M) and antiferromagnetic (AF) re- gions. The ordering temperatures

0

and

+

are deri- ved respectively from Mijssbauer and magnetic 171 data.

0

indicates the paramagnetic Curie temperatu- res 141.

The small difference

(%

20 kOe) between the hyperfine fields, extrapolated to T

=

0, in ferro- magnetic samples represents the sum of the contri- bution of the conduction electron spin polarisation to the latter and the supertransferred hyperfine in- teraction.

While the basic ferromagnetic exchange cou- pling in near-stoichiometric EuB6 can be understood in terms of the mechanism proposed for EuO 181, it is puzzling that the addition of conduction elec- trons can give an antiferromagnetic contribution.

The

RKKY

interaction is positive at short distances

and low conduction electron densities.The first ne- gative region occurs between 4.5

X 102'

and 2.6

X

1oZ2 per cc at the nearest neighbour distance of EuBs. It is impossible to obtain antiferromagnetic nearest-neighbour coupling at concentrations of order

0.05

electrons/Eu (6.8

X

lo2

^O

per cc) even taking account of the fact that the electron densi- ty is not uniformly distributed, but peaked near the donor sites. In the most favourable case, assu- ming hydrogenic wave functions and a dielectric constant

E

4, the electron density at europium sites adjacent to a carbon impurity is only 2.0x10~?

take place at much longer distances. Figure 3 shows how it may come about. There is a high density of states in the impurity band close to a carbon donor and the added electron there couples ferromagneti- cally to the surrounding group of europium ions.

Fig

3 :

Schematic representation of mictomagnetic EuB6. Ferromagnetic clusters formed around the do- nor centres are coupled antiferromagnetically by direct exchange between electrons from adjacent donor

S.

Mid-way between the donors however the electron den- sity is much lower and direct overlap of their hy- drogenic wave functions gives antiferromagnetic coupling between the electrons from adjacent donors.

In this way, the inhomogeneous exchange in the mic- tomagnetic region of EuB6-,Cx finds a natural expla- nation. At higher electron concentrations the densi- ty in the impurity band becomes uniform and the exchange homogeneous.

Finally we note that there are some signs of competing exchange interactions in nominally stoichiometric, ferromagnetic EuB6. The rise in the magnetization and hyperfine field below the Curie point is much less abrupt than that fiven by the Brillouin function. Also the specific heat anomaly is broad and complex

/ 1 , 9 / .

These effects probably stem from the presence of some l ~ ~ ~ c a r r i e r s , even in the best materials prepared so far. If these carriers turn out to be extrinsic, due to defects or impurities such as trivalent rare earths, then EuBs, with its simple structure and almost rigid Boron lattice, should be an ideal compound for stu- dying the influence of conduction electrons on the magnetism and transport properties for a magnetic semiconductor.

We are grateful to N.V.

Dang

for performing some of the magnetization measurements and to

J.

Chappert for the G ~ b a u e r data at 1.6 K. The au-

thors have also benefitted from a discussion with

Professor N.F. Mott.

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JOURNAL DE PHYSIQUE

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