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Submitted on 1 Jan 1981
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INFLUENCE OF STRONG MAGNETIC FIELDS ON
FERROELECTRIC PHASE TRANSITIONS
L. Brunel, G. Landwehr, A. Bussmann-Holder, H. Bilz, M. Balkanski, M.
Massot, M. Ziolkiewicz
To cite this version:
JOURNAL DE PHYSIQUE
CoZZoque C6, supptlrnent au n012, Tome 42, dlcernbre 1981 page C6-412
INFLUENCE O F STRONG MAGNETIC FIELDS O N FERROELECTRIC PHASE TRANSITIONS
L . C . Brunel, G. ~andwehr*, A. Bussmann-~oldep*, H. ~ i l z * * , M. Balkanski***,
M. Massot*"" and M . K . Ziolkiewicz***
SNCI-CNRS, 38000 GrenobZe, France
*
Max-PZanck-Institut EVL, 38000 GrenobZe, France**
Max-PZanck-Institut, 7000 Stuttgart, F. R. G.**
*
Universitl Paris V I , Paris, FranceAbstract.- The f e r r o e l e c t r i c s o f t node i n SbSI has been i n v e s t i g a t e d by Ranan s c a t t e r i n g i n s t r o n g magnetic f i e l d s up t o 1 4 Tesla. The code frequency s h i f t s towards lower frequencies f o r a magnetic f i e l d perpendicular t o t h e f e r r o e l e c - t r i c c-axis w h i l e t h e mode frequency i s unaffected f o r a f i e l d o r i e n t e d p a r a l - l e l t o the c-axis The e f f e c t i s exp'lainec' i n terms o f a r e c e n t t h e o r e t i c a l model which i s based on t h e anomalously s t r o n g electron-phonon c o u p l i n g o f t h e 3p e l e c t r o n s l o c a t e d a t the sulphur i o n s .
I.
I n t r o d u c t i o n . - SbSI c r y s t a l s e x h i b i t a t 2. 2'10 K a t r a n s i t i o n f r o n a h i g h tempera- t u r e p a r a e l e c t r i c phase t o a f e r r o e l e c t r i c phase which i s t r i g g e r e d by t h e s o f t e n n i n g o f a transverse o p t i c a l phonon( I ) .
W i t h i n a recent microscopic model Sased on t h e h i g h l y a n i s o t r o p i c non l i n e a r temperature dependent p o l a r i z a b i l i ty of the chalcogeni- de i o n i t was p o s s i b l e t o c a l c u l a t e w i t h i n t h e s e l f c o n s i s t e n t phonon approximation (SPA) t h e temperature dependence o f t h e s o f t aode i n KTa03 c r y s t a l s ( 2 ) . eecause o ft h e h i g h p o l a r i z a b i l i t y of t h e 3p o r b i t a l s o f
s2-
i t seemed i n t e r e s t i n g t o study t h e e f f e c t o f a high magnetic f i e l d ( > 13 Tesla) on t h e dynamics o f t h e s o f t phonon. The e f f e c t o f a f i e l d should l e a d t o a shrinkage o f t h e 3p wave f u n c t i o n s as i t has been c a l c u l a t e d by Larsen f o r H- ions ( 3 ) . The shrinkage w i l l a f f e c t the p o l a r i z a b i l i t y o fs2-
and c o n c l u s i v e l y t h e f e r r o e l e c t r i c s o f t mode frequency.2 . Experimental
.-
The needle 1 i ke samples were i n s i d e a v a r i a b l e temperature c r y o s t a t and surrounded by t h e gas a t atmospheric pressure. The temperature monitored by a P t0
r e s i s t a n c e was s t a b l e t o 1 0.02 degree. The 6754
P.
k r y p t o n l a s e r l i n e was used t o e x c i t e t h e spectra, t h e power was c a r e f u l l y k e p t constant ( 8 mW on t h e sanple). The Raman s p e c t r a were recorded i n t h e back s c a t t e r i n g arrangement w i t h the l a s e r l i g h t i n c i d e n t on t h e 113 o r l I O c r y s t a l faces. The spectrometer was a J a r r e l - A s h double nonochromator. The experiments were performed e i t h e r w i t h a s p l i t c o i l ? i t t e r magnet (maximum f i e l d 13 Tesla) o r w i t h a conventional S i t t e r magnet (maximum f i e l d 1 4 T e s l a ) . 3. Results .-Spectra were recorded w i t h t h e magnetic f i e 1 d 3 para1 l e l o r perpendicular t o t h e c r y s t a l c-axis. The s o f t mode i s o n l y Raman a c t i v e i n t h e f e r r o e l e c t r i c phase. For each experiment t h e temperature o f t h e sample Ts was k e p t constant below t h e Curie p o i n t Tc and t h e f i e l d t u r n e d on. For Tc-
Ts l e s s than 4 degrees t h e s o f t mode frequency s h i f t s t o lower frequencies when t h e magnetic f i e l d i s perpendicular t o t h ef e r r o e l e c t r i c a x i s ( F i g . 1). I f t h e temperature of t h e sample i s t o o low Tc
-
TS < 5"t h e f i e l d does n o t a f f e c t t h e s o f t phonon frequency. The s o f t mode frequency
: Raman s p e c t r a o f SbSI, Tc
-
Ts = 3.25 K I n s e r t : frequency o f t h e s o f t mode Wo, s h i f t s CWF
observed a t 14 T e s l a f o r v a r i o u s sample temperatures. i s u n a f f e c t e d whatever t h e t e m p e r a t u r e o f t h e sample f o r a magnetic f i e l d o r i e n t e d p a r a l l e l t o t h e c - a x i s . The r e s u l t s a r e i d e n t i c a l f o r experiments c a r r i e d o u t w i t h t h e l a s e r Seam i n c i d e n t on t h e 110 f a c e o r on t h e 110 f a c e o f t h e c r y s t a l .
4. Discussion.- The dynamical p r o p e r t i e s o f SbSI c o u l d be w e l l d e s c r i b e d w i t h i n a s i m p l e d i a t o n i c model w i t h q u a r t i c non l i n e a r t e m p e r a t u r e dependent p o l a r i z a b i l i t y a t t h e c h a l c o g e n i d e i o n l a t t i c e s i t e ( F i g . 2 ) . The p o l a r i z a b i l i t y g(T) c o n s i s t s o f two p a r t s , a harmonic n e g a t i v e e l e c t r o n - i o n c o u p l i n g g2 w h i c h r e f l e c t s s t r o n g l y a t t r a c t i v e Coulomb f o r c e s and a non l i n e a r p a r t g4, w h i c h s t a b i l i z e s t h e s o f t mode i n t h e p a r a e l e c t r i c as w e l l as i n t h e f e r r o e l e c t r i c regime ( 4 ) .
As has been shown f o r p e r o v s k i t e s and I V - V I compounds o n l y t h e harmonic p a r t o f t h e p o l a r i z a b i l i t y i s a f f e c t e d b y a charge t r a n s f e r ( 5 ) . For SbSI t h e c a l c u l a t i o n s show t h a t g4 i s c o n s t a n t a t a l l f i e l d s t r e n g t h s w h i l e g2 decreases l i n e a r l y w i t h t h e f i e l d (39 % f o r 14 T e s l a ) . Close t o Tc, w i t h i n t h e model wf2 i s g i v e n by
2
W f Z x g2 + 3g4 "V > WoZ/2f
Experimental d a t a show t h a t f o r a l l f i e l d s wf as a f u n c t i o n o f T reaches t h e sane v a l u e a t Tc
-
Tg where Tg = 283 K ( F i g . 3). T h i s b a h a v i o u r can be expressed i n terms o f an e m p i r i c a l f o r m u l a f o r b o t h t h e m a g n e t i c f i e l d dependence and t h e t e m p e r a t u r e dependence o f wfZ ; wf2 = ( T-
T ) ~ + ~ . C X ~ ~ / ( T ~-
T3)€.The i d e n t i f i c a t i o n o f t h e e m p i r i c a l parameter w i t h t h e nodel parameter i s e a s i l y shown :
where E i s l i n e a r l y dependent on: as E = so + sI.5. The comparison o f e x p e r i m e n t a l d a t a w i t h t h e c a l c u l a t i o n s i s shown on F i g . 3.
C6-4 14 JOURNAL DE PHYSIQUE F i g . 3 : S o f t mode f r e q u e n c y v e r s u s sample In w temperature f o r v a r i o u s f i e l d s ( d o t s : 3 e x p e r i m e n t a l d a t a
-
l i n e s : c a l c u l a t i o n s ) . OT- TO g e t a more m i c r o s c o p i c i n s i g h t i n t o t h e 4T- 6T- m a g n e t i c f i e l d e f f e c t we may compare o u r c a l c u - IOT- l a t i o n s w i t h t h e c a l c u l a t i o n s by Larsen ( 3 ) on 14T- t h e e f f e c t o f a magnetic f i e l d on t h e wave f u n c t i o n s o f t h e H- i o n . These c a l c u l a t i o n s have shown t h a t when a magnetic f i e l d i s1 1:s
. .
in( T~-- T! a p p l i e d t h e wave f u n c t i o n s s h r i n k and change i n shape t o something r e s e m b l i n g an e l l i p s o i d . T h i s e f f e c t i s s t r o n g l y r e l a t e d t o t h e magnetic f i e l d s t r e n g t h . As we a r e i n t h e l o w f i e l d l i m i t ( y = 3 . 1 0 - ~ see r e f . 3) t h e s h r i n k a g e s h o u l d b e o f t h e o r d e r o f 1.10-~% a t -6-.
1 4 Tesla, which would correspond t o a d i r e c t i o n n a l charge t r a n s f e r o f 1.10 e,
t h i s i s t o o low a v a l u e t o e x p l a i n t h e s h i f t o f t h e s o f t mode we observed.
From t h e change o f g2 w i t h f i e l d i t can be concluded t h a t t h e charge t r a n s f e r i s
- 3
-
o f t h e o r d e r o f 2 . 5 . 1 ~ 1 - ~ e - , which corresponds t o a s h r i n k a g e o f
2.5.10
% I n t h e wave f u n c t i o n s o fs2-.
As has been shown f o r K Ta O3 ( 2 ) , a charge t r a n s f e r o f 0 . 5 1 f r o m t h e oxygen t o t h e Ta i o n l e a d s t o a s h i f t o f 150 cm-' o f t h e s o f t mode frequency. T a k i n g t h e same v a l u e f o r SbSI t h e r e o r i e n t a t i o n o f charge b y a 14 T e s l a m a g n e t i c f i e l d would s h i f t wf by x 1 c f l , which i s i n good agreement w i t h e x p e r i m e n t a l data. I t i s concluded t h a t t h e s t r o n g enhancement o f charge t r a n s f e r , more t h a n two o r d e r s o f magnitude g r e a t e r t h a n t h e charge t r a n s f e r c a l c u l a t e d f o rH-
( 2 ) , i s due t o t h e h i g h l y a n i s o t r o p i c non 1 in e a r p o l a r i z a b i 1 i t y o f t h e c h a l cogenide i o n .References
1. For p e r t i n e n t r e f e r e n c e s see :
a) M. K. Teng, 11. B a l k a n s k i and M. Massot, Phys. Rev. 5,
2,
1031 (1972)b) J.P. Pouget, S. W. S h a p i r o and K. Nassan, J. Phys. Chem. S o l i d s ,
40,
267 (1979) 2. A. Bussmann and H. B i l z ( t o be p u b l i s h e d )3. D. M. Larsen, Phys. Rev. B,
20,
5217 (1979)4. H. B i l z , A. Bussmann, G. Senedek, H. B U t t n e r and D. Strauch, F e r r o e l e c t r i c s ,