SECOND ORDER PHASE TRANSITION IN THE FERRIMAGNETIC THIOSPINEL FeCr2S4

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SECOND ORDER PHASE TRANSITION IN THE FERRIMAGNETIC THIOSPINEL FeCr2S4

L. Brossard, J. Dormann, L. Goldstein, P. Gibart, P. Renaudin

To cite this version:

L. Brossard, J. Dormann, L. Goldstein, P. Gibart, P. Renaudin. SECOND ORDER PHASE TRAN-

SITION IN THE FERRIMAGNETIC THIOSPINEL FeCr2S4. Journal de Physique Colloques, 1979,

40 (C2), pp.C2-301-C2-301. �10.1051/jphyscol:19792105�. �jpa-00218474�

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JOURNAL DE PHYSIQUE

Colloque

C2,

supplkment au n o

3,

Tome 40, mars

1979,

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C2-301

SECOND ORDER PHASE TRANSIT1O;d IN THE FERRIMAGNETIC THIOSPINEL F e C r 2 S 4

L. B r o s s a r d , J.L. Dormann, L. G o l d s t e i n , P . G i b a r t , P. Renaudin

Laboratoire de Magndtisme, C . N . R. S . , I , Place Aristide Briand, 92190 Belleuue, fiance.

R6sumC.- FeCr,S,, d e s t r u c t u r e f e r r i m a g n d t i q u e s p i n e l l e , r e s t e c u b i q u e j u s q u ' l 4 , 2 K. I1 p r 6 s e n t e une t r a n s i t i o n de p h a s e du second o r d r e v e r s 13 K q u i a f f e c t e l e s s p e c t r e s M6ssbauer : l e s i n t e r - p r d t a t i o n s a n t C r i e u r e s 12-51 s o n t b a s 6 e s pour l a p l u p a r t s u r un modEle d e champ c r i s t a l l i n l un i o n s u p p o s a n t l e s 6 l e c t r o n s d du Fe2+ A l o c a l i s 6 s .

Une d 6 c o m p o s i t i o n p r 6 a l a b l e de n o s s p e c t r e s e n t r e 1 , 5 K e t 300 K e n s i n g u l e t s montre que l e s s o l u - t i o n s B 1 s i t e a n t C r i e u r e m e n t s u g g 6 r 6 e s ne s o n t p a s u n i q u e s . De p l u s , l e s i n t e n s i t 6 s e x p 6 r i m e n t a l e s d e s r a i e s d i f f e r e n t d e s i n t e n s i t d s t h 6 o r i q u e s e t l e s l a r g e u r s d e s r a i e s p r e s e n t e n t d e s v a r i a t i o n s i n c o h g r e n t e s a v e c T. Une r e p r o d u c t i o n c o h 6 r e n t e n c c e s s i t e deux s i t e s Mossbauer : 2 T > 13 K , l e s deux s i t e s d 1 i n t e n s i t 6 d g a l e s o n t c a r a c t 6 r i s C s p a r V z Z > 0 , rl = 0 , 8 =

q

⁼⁰e t p a r V Z Z < 0, q = I , 8 = J, =

n

A T< 13 K , l e s i t e l e p l u s i n t e n s e ( 0 . 7 5 ) e s t t e l que V,, < 0 , q ?. 0 . 2 5 , 8 =

n ,

J, = 0

T 1

e t l ' a u t r e t e l que V z Z < 0 , q ^?.0 . 2 5 , 8 = J, =

E.

Les d l e c t r o n s 3d du Fe IIA s o n t mieux d 6 c r i t s i a n s un modsle de bande 6 t r o i t e 1 6 1 doublement dCgdnd-

rCe : l T > 13 K , l e 6Pme C l e c t r o n du f e r r e l a x e e n t r e l e s deux d t a t s d e b a n d e , l ' u n de s y m d t r i e

( x 2

-

y 2 ) , l ' a u t r e d e s y m 6 t r i e ( 3 z 2

-

r 2 ) h y b r i d 6 a v e c l ' d t a t e x c i t d du C r I I I e n B . A T< 13 K , s e u l ce d e r n i e r 6 t a t e s t occup6. C e c i t r a d u i t l e p a s s a g e d ' u n paramagngtisme o r b i t a l ( T > 13 K) l un f e r r o m a g n g t i s m e o r b i t a l (T < 13 K) p a r a n a l o g i e a v e c 171.

~ b s t r a c t ' . - The s p i n e 1 f e r r i m a g n e t FeCr S e x h i b i t s a second o r d e r p h a s e t r a n s i t i o n around 13 K anomaly, i n t h e s p e c i f i c h e a t / l / . I t r g m k n s c u b i c down t o 4.2 K. P r e v i o u s i n t e r p r e t a t i o n s o f MESS- b a u e r s p e c t r a have s u g g e s t e d : a s t a t i c t o dynamic J a h n T e l l e r t r a n s i t i o n 1 2 1 , an a n t i f e r r o d i s t o r - t i v e t r a n s i t i o n 1 3 1 , an i n d u c e d E.F.G. by t h e m a g n e t i c o r d e r 141 o r by a l a t t i c e d i s t o r s i o n due t o m a g n e t o s t r i c t i v e e f f e c t s / S / . Most of t h $ s e i n t e r p r e t a t i o n s were b a s e d upon a s i n g l e i o n c r y s t a l f i e l d a p p r o a c h , t h e 3d e l e c t r o n s of Fe2 A were supposed t o be l o c a l i z e d .

Mossbauer s t u d i e s on s t o i c h i o m e t r i c p o l y c r y s t a l l i n e FeCr2S,were c a r r i e d o u t from 1.5 t o 300 K.

i n i t i a l d e c o m p o s i t i o n of t h e s p e c t r a i n t o s i n g l e t s shows t h a t t h e p r e v i o u s s i n g l e s i t e s o l u t i o n s 12-51 a r e n o t u n i q u e ; moreover t h e e x p e r i m e n t a l and c a l c u l a t e d l i n e i n t e n s i t i e s do n o t a g r e e and t h e d i f f e r e n t l i n e w i d t h s show i n c o h e r e n t v a r i a t i o n s w i t h T. A more r e l i a b l e two s i t e s s o l u t i o n i s sugges- t e d . A t T > 13 K , t h e two s i t e s have t h e same i n t e n s i t y and a r e c a r a c t e r i z e d by V Z Z > 0, q = 0, 8 =

$ = 0 and V,, < 0 , q = I , O = $ =

4.

At T < 13 K , t h e two s i t e s a r e c a r a c t e r i z e d by V Z z < 0 , q 0 . 2 5 , 8 =

4

b u t J, = 0 and J, = a r e r e s p e c t i v e l y r e l a t e d w i t h t h e h i g h e r ( 0 . 7 5 ) and t h e lower (0.25) s i t e i n t e n s ~ t i e s . Moreover, t h r e x p e r i m e n t a l mean l i n e w i d t h , t h e h y p e r f i n e m a g n e t i c f i e l d s and t h e i s o m e r s h i f t s show d i s c o n t i n u i t i e s a t 13 K. F i n a l l y , low t e m p e r a t u r e X r a y s p a t t e r n s show no d i l a - t a t i o n anomaly and no d i s c o n t i n u i t y of t h e a a / a T v a l u e s ( a : u n i t c e l l p a r a m e t e r ) .

These f e a t u r e s a r e e x p l a i n e d assuming a n a r r o w d o u b l y d e g e n e r a t e d band of F ~ I I A , owing t o t h e low m o b i l i t y o f 3d e l e c t r o n s 161. The s i x t h 3d e l e c t r o n o f F ~ I I A i s assumed t o occupy o r b i t a l band s t a - t e s w i t h ^{/ E >}o r (iO> + l $ c r T 1 ~ > ) symmetry, owing t o t h e h y b r i d i z a t i o n o f / o > w i t h t h e B s i t e e n e r g e - t i c a l l y c l o s e c r l s t a t e . A t T > 13 K , t h e s i x t h 3d e l e c t r o n r e l a x e s between t h e ) E > and t h e h y b r i - d i z e d 10> o r b i t a l band s t a t e s , l e a d i n g t o two e q u a l i n t e n s i t y s i t e s . A t T < 13 K , o n l y t h e h y b r i d i - zed o r b i t a l band s t a t e i s o c c u p i e d . T h i s c o r r e s p o n d s t o an o r b i t a l paramagnetism f o r T > 13 K and t o a f e r r o m a g n e t i c o r b i t a l o r d e r i n g f o r T < 13 K. T h i s i s r e l a t e d w i t h t h e f a c t t h a t a n Hubbard H a m i l t o n i a n i n t h e a t o m i c l i m i t l e a d s n o t o n l y t o s p i n o r d e r i n g b u t a l s o t o o r b i t a l o r d e r i n g / 7 / . Moreover, t h e t r a n s i t i o n shows a t T < 13 K a s l o w r e l a x a t i o n o f t h e E.F.G. The c o u p l i n g o f t h e band w i t h t h e phonons a t T c 13 K i s d i f f e r e n t from a l i n e a r and weak dynamic J a h n T e l l e r e f f e c t a s f o r example f o r t h e A s i t e d i l u t e ~ e ~

.

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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792105