MÖSSBAUER STUDY OF THE MAGNETIC ORDERING IN THE RANDOM GARNETS Ca3(FexCr2-x)Ge3O12 (x=0.5, 1.0, 1.5)

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MÖSSBAUER STUDY OF THE MAGNETIC ORDERING IN THE RANDOM GARNETS

Ca3(FexCr2-x)Ge3O12 (x=0.5, 1.0, 1.5)

V. Povitskii, F. Litterst, E. Makarov

To cite this version:

V. Povitskii, F. Litterst, E. Makarov. MÖSSBAUER STUDY OF THE MAGNETIC ORDERING IN THE RANDOM GARNETS Ca3(FexCr2-x)Ge3O12 (x=0.5, 1.0, 1.5). Journal de Physique Colloques, 1980, 41 (C1), pp.C1-183-C1-184. �10.1051/jphyscol:1980153�. �jpa-00219722�

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JOURNAL DE PHYSIPUE Colloque C1, suppldment au n O 1, Tome 41, janvier 1980, page C1-183

I~SSBAUER STUDY OF THE MAGNETIC ORDERING I N M E RANDW4 GARNETS Ca3(~e,&r2-x)~e3012 (X = 0 . 5 , l . 0, 1-51 V.A. Povitskii, F.J. Litterst f and E.F. Makarov

I n s t i t u t e of ChemicaZ Physics, Academy of Sciences of the USSR, Moscow V-334.

+ Physik Department der Teehnischen Universitift Mi.lnchen, 0-8046 Garching, Fed. Rep. Germany.

The m a g n e t i c s t r u c t u r e s o f " s i n g l e s u b - l a t t i c e " g a r n e t s l i k e Ca3Fe2Ge3OI2 (FeGeG) a n d Ca3Cr2Ge3OI2 (.CrGeG) h a v e b e e n e x t e n s - i v e l y studied;e.g. b y n e u t r o n d i f f r a c t i o n ( s e e r;view .by B e l o v a n d S o k o l o v / I / ) . T h e m a g n e t i c i o n s i n FeGeG a n d CrGeG o c c u p y o n l y o c t a h e d r a l s i t e s a n d t h u s a r e s e b a r a t - e d b y a t l e a s t t w o o x y g e n i o n s . T h e i n d i r e c t e x c h a n g e i h t e r a c t i o n s f o r b o t h FeGeG a n d CrGeG a r e p r a c t i c a l l y i d e n t i c a l y i e l d i n g a n t i f e r r o m a g n e t i c o r d e r b e l o w TN *I 2 K.

The m a g n e t i c s t r u c t u r e s , h o w e v e r , a r e d i f f e r e n t w i t h t h e moments d i r e c t e d a l o n g t h e a x e s < 1 1 1 > ( K ~ < O ) a n d (100) (K1>O), r e - s p e c t i v e l y / 2 / . W i t h i n t h e c u b i c c r y s t a l - l i n e l a t t i c e o f t h e m i x e d g a r n e t s y s t e m C a 3 ( F e x C r 2 - x ) G e 3 0 1 2 o n e may e x p e c t a r a n - d o m l y o r i e n t e d s p i n a r r a n g e m e n t w i t h o n l y weak i n d i r e c t e x c h a n g e i n t e r a c t i o n s p o s s i b - l y s i m u l a t i n g a s p i n - g l a s s 1 i k e o r d e r . S p e c i f i c h e a t m e a s u r e m e n t s p e r f o r m e d o n C a 3 ( F e x C r 2 - x ) G e 3 0 1 2 ( x = 0 . 5 , 1 . O ,'I . 5 ) s u p p o r t t h i s v i e w / 3 / . T h e h e a t c a p a c i t i e s show b r o a d maxima n e a r TN=6-9 K. I t was p r o p o s e d t h a t t h e a n t i f e r r o m a g n e t i c o r d e r i s o f t h e c l u s t e r t y p e / 3 / .

We h a v e s t u d i e d t h e same s a m p l e s a s u s e d i n / 3 / b y 5 7 ~ e M 6 s s b a u e r s p e c t r o s c o p y b e t w e e n 4.2 a n d 3 0 0 K. The s p e c t r a o b t a i n e d a t 3 0 0 K f o r x=0.5,1.0 a n d 1.5 show t h e same A E -

Q

-

=0.39 mm/s a s f o r p u r e FeGeG / 4 , 5 / . T h u s

t h e 0 2 - o c t a h e d r a w h i c h s u r r o u n d t h e ~ e ~ + a r e n o t d i s t u r b e d b y t h e c r 3 + a d m i x t u r e .

T h e o n s e t o f m a g n e t i c o r d e r b e l o w 9 - K , 8 K a n d 1 1 K f o r t h e c o m p o s i t i o n s x=0.5,1.0 and 1.5,resp.,shows u p i n b r o a d e n e d m a g n e t i c h y p e r f i n e ( h f ) p a t t e r n s ( f i g . 1 ) . F o r x s 1 . 5 w e l l - r e s o l v e d 6 - l i n e p a t t e r n s a r e o b t a i n e d b e l o w 6 K . T h e s p e c t r a f o r x=0.5 a n d 1.0 a r e s t i l l b r o a d e v e n a t 4.2 K. R e s o l v e d - y e t u n p o l a r i z e d s p e c t r a a r e a c h i e v e d b y t h e

a p p l i c a t i o n o f a n e x t e r n a l m a g n e t i c f i e l d . T h e l o # f i e l d dc s u s c e p t i b i l i t i e s

x

^show

C u r i e - W e i s s b e h a v i o u r w i t h n e g a t i v e 8. Weak maxima o f a r e f o u n d a t 8 K, 7 K a n d 8 K f o r x=0.5,1.0 a n d 1.5. T h e s e t e m p e r a t u r e s a g r e e w i t h t h o s e w h e r e t h e m a x i m a o f t h e s p e c i f i c h e a t w e r e o b s e r v e d . The s l i g h t l y h i g h e r t r a n s i t i o n t e m p e r a t u r e s f o u n d f r o m t h e M o s s b a u e r d a t a c a n b e c o n n e c t e d w i t h t h e f r e q u e n c y d e p e n d e n c e o f t h e b l o c k i n g o f t h e m a g n e t i z a t i o n o f m a g n e t i c c l u s t e r s . S i m i l a c r e s u l t s a r e known f o r t h o s e t y p e s o f s p i n - g l a s s e s w h i c h a r e s u p p o s e d t o c o n - t a i n m a g n e t i c c l u s t e r s / 6 , 7 / . T h e M o s s - b a u e r p a t t e r n s c o u l d b e s u c c e s s f u l l y f i t - t e d a s s u m i n g s u p e r p a r a m a g n e t i c r e l a x a t i o n o f t h e c l u s t e r m a g n e t i z a t i o n . We u s e d a r e - l a x a t i o n m o d e l / 8 / w h i c h i s a p p r o p r i a t e for o u r c u b i c s y s t e m . I t a l l o w s f o r a f l i p p i n g o f t h e h f f i e l d i n t o a r e v e r s e d d i r e c t i o n w i t h a r a t e q a n d i n t o o n e o f t h e f o u r p e r p e n d i c u l a r d i . r e c t i o n s w i t h a r a t e p. F r o m t h e r e s o l v e d s p e c t r a f o r x=1.5 i t i s a p p a - r e n t t h a t c l u s t e r s w i t h d i f f e r e n t r e l a x a - t i o n f r e q u e n c i e s m u s t b e p r e s e n t . A t 4.2 K t h i s shows u p o n l y i n a s y m m e t r i c l i n e s h a p e s ( s e e f i g . 1 ) . B e t w e e n 6 - 8 K, h o w e v e r , t h e r e a p p e a r s a c e n t r a l r e s o n a n c e l i n e near z e r o r e l a t i v e v e l o c i t y d u e t o f a s t e r r e l a x - a t i n g c l u s t e r s . T h e f i t p a r a m e t e r s d e r i v e d f o r t h e 4.2 K s p e c t r a a r e c o l l e c t e d i n t a b l e 1. O p t i m u m f i t s w e r e o b t a i n e d f o r p = q . I t was s u f f i c i e n t t o assume t h a t t w o t y p e s o f c l u s t e r s w i t h d i f f e r e n t r e l a x a - t i o n f r e q u e n c i e s p 1 ,p2 b u t i d e n t i c a l h f f i e l d s a r e p r e s e n t . T h e r e l a t i v e i n t e n s i - t i e s ( I N T I f r o m t h e " s l o w e r " ( p l ) a n d fast- e r " ( p 2 ) r e l a x a t i n g c l u s t e r s i n e a c h sample a r e c o n s t a n t o v e r t h e s t u d i e d t e m p e r a t u r e r a n g e . O u r d a t a s u p p o r t t h e v i e w o f a c l u s t e r - t y p e m a g n e t i c o r d e r i n t h e s e g a r -

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980153

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C1-184 JOURNAL DE PHYSIQUE

n e t s . A l t h o u g h t h e i r c r y s t a l l o g r a p h i c struct- u r e i s s i n g l e - p h a s e / 3 / , t h e b r e a k - u p o f t h e m a t e r i a l i n t o m a g n e t i c c l u s t e r s w i t h d i f f e r e n t r e l a x a t i o n f r e q u e n c i e s may b e i n d u c e d b y f l u c t u a t i o n s i n t h e d i s t r i b u t i o n o f C r a n d F e o v e r t h e o c t a h e d r a l l a t t i c e s i t e s . We c o u l d show t h a t t h e c o l l e c t i v e s p i n d y -

n a m i c s o f t h e c l u s t e r s i n t h e p a r t i a l l y f r o - z e n m a g n e t i c s t a t e c a n b e q u a n t i t a t i v e l y e v a l u a t e d f r o m t h e M o s s b a u e r d a t a . T h i s i n - f o r m a t i o n o n t h e t i m e s c a l e o f t h e 5 7 ~ e n u - c l e a r L a r m o r p r e c e s s i o n t i m e i s s u p p l e m e n - t a r y t o t h a t f r o m t h e s p e c i f i c h e a t a n d m a g n e t i z a t i o n m e a s u r e m e n t s w h i c h a r e s e n s i - t i v e ta\ t h e m a g n e t i c c l u s t e r d y n a m i c s o n 1 y o n a f a r s l o w e r t i m e s c a l e b e c o n d s ) . B o t h m i c r o s c o p i c a n d m a c r o s c o p i c p r o b e d a t a a r e v e r y s i m i l a r t o t h o s e o b t a i n e d f o r s p i n - g l a s s e s w h i c h a l s o e x h q b i t a g r a d u a l f r e e z - i n g o f t h e c l u s t e r m a g n e t i z a t i o n b e l o w t h e i r " s p i n - g l a s s t e m p e r a t u r e " .

T a b 1 e 1 : Ca3(FexCr2-x)Ge3012

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R e f e r e n c e s :

/ 1 / PELOV K.P a n d SOKOLOV V . I . , Sov.Phys.

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/ 3 / BE,LOV K.P., VALYANSKAYA T.V., M I L L ' B.V.

SOKOLOV V . I . a n d SOLOV'EVA N.A., JETP L e t t .

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/4[ S A K I C K I J . a n d HAFNER S.S., P h y s . L e t t . 68A ( , I 9 7 8 1 8 0

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and PESHKOV V.P., Sov.Phys. JETP

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161 VIOLET G.E. a n d BORG R.J., Phys.Rev.

14'9 ( t a 6 6 ) 5 4 0 a n d

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M U R A N I A.P., J.Magn.Magn.Mat.

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181 AFANAS'GV A.M. a n d ONISHCHENKO E.V., S . a v , ~ h i . s ; 'JETP

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F i g . l : M o s s b a u e r a b s o r p t i o n s p e c t r a o f C a 3 ( F e x C r 2 - x ) G e 3 0 1 2 hx=0.5,1.0,1.5).

S o u r c e : 5 7 ~ 0 ~ .

T h i s w o r k was s u p p o r t e d b y a n e x c h a n g e p r o g r a m b e t w e e n t h e Academy o f S c i e n c e s o f t h e USSR a n d t h e D e u t s c h e F o r s c h u n g s g e - m e i n s c h a f t .