FORM FACTOR MEASUREMENT IN FERROMAGNETIC COBALT ORTHOVANADATE

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FORM FACTOR MEASUREMENT IN

FERROMAGNETIC COBALT ORTHOVANADATE

H. Fuess, R. Müller, D. Schwabe, F. Tasset

To cite this version:

H. Fuess, R. Müller, D. Schwabe, F. Tasset. FORM FACTOR MEASUREMENT IN FERROMAG-

NETIC COBALT ORTHOVANADATE. Journal de Physique Colloques, 1982, 43 (C7), pp.C7-253-

C7-256. �10.1051/jphyscol:1982736�. �jpa-00222343�

JOURNAL DE PHYSIQUE

Colloque C7, supplément au n°12, Tome 43, décembre 1982 page C7-253

FORM FACTOR MEASUREMENT IN FERROMAGNETIC COBALT ORTHOVANADATE H. Fuess , R. Miiller , D. Schwabe and F. Tasset

*Institut fur Kvistallographie und tttneralogie der Universitdt Senokeriberg- anlage 30 , D-6000 Frankfurt/Main 1, F.R.G.

**I. Physikalisches Institut dei> Justus-Liebig-Univevsitdt, Heinrich Buff-Ring 16, D-6300 Giessen, F.R.G.

+

Inetitut Laue-Langevin, 1S6 X, Centre de Tri, F-38042 Grenoble Cedex, France

Résumé. - Des monocristaux de 1'orthovanadate de cobalt ont été tirés selon la méthode de Czochralski. Le composé est orthorhombique avec deux sites cristallographiques différents de cobalt dans la maille.

Des mesures magnétiques dans des champs d'une intensité jusqu'à 15 Tesla ont montré une anisotropie prononcée et un ordre ferromagnétique au—dessous de T

= 6j3(3)K. Des spectres sur poudre aux neutrons et des mesures aux neutrons polarisés ont mis en évidence la présence des moments magnétiques différents dans les deux sites de cobalt.

L'analyse des facteurs de forme magnétiques a élucidé la distribution des électrons entre les niveaux t2g et e

pour les deux ions de co- balt et a permis de reproduire 1'anisotropie observée.

Abstract. - Single crystals of cobalt orthovanadate CO3V2O0 were grown by the Czochralski method. The substance is orthorhombic with two dif- ferent crystallographic sites for the cobalt ions. Magnetization mea- surements in fields up to 15 Tesla show pronounced magnetic anisotropy and a ferromagnetic ordering below T

= 6.3(3)K. Neutron powder pat- tern and polarized measurements on a single crystal revealed diffe- rent magnetic moments for the two cobalt ions. The analysis of the magnetic form factors gave the electron distribution between t2g and eg levels for both cobalt ions and reproduced the observed anisotropy.

Introduction. - Polarized neutron studies reveal the distribution of d and f electrons in solids. The 3d electrons of transition elements are involved in chemical bonding and their distribution is a direct indication of the degree of covalency. A spin transfer between iron and oxygen was observed in the studies of yttrium iron garnet /I/ and Fe3<D4 / 2 / . In order to determine a possible covalent contribution to the chemical bond a detailed study of the ternary oxide cobalt ortho- vanadate has been started. C03V2O3 is orthorhombic (space group Cmca)

and contains Co2+ ions in two crystallographically different sites /3,4/. The substance orders ferromagnetically below T

= 6.3(3)K in contrast with other 3d vanadates / 5 / . The neutron diffraction study should elucidate the surprising ferromagnetic behaviour.

Experimental. - The phase diagram of the CoO - V20

system was studied by Brisi / 6 / . It indicates that the growth of C03V2O8 is not possible from the stoichiometric melt of composition (CoO)3V

0

due to incon- gruent melting of the compound.

Solid cobalt orthovanadate coexists, however, with liquid (CoO)

V

0

for 1.3 S x S 1.5 in the temperature range between 830 C and 1000 °C.

The growth of Co

V

0g from nonstoichiometric melt with x ^ 2.4 was therefore attempted after the conventional Czochralski method.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982736

C7-254

DE PBYSIQUE

The a p p a r a t u s f o r growth was a Malvern C z o c h r a l s k i s y s t e m MSR4 w i t h i n d u c t i o n h e a t i n g from an i r i d i u m c r u c i b l e ( = 36 mm, h = 38mm) un- d e r dynamic argon atmosphere. The a p p r o p r i a t e amount of V205 and C O ~ ( O H ) ~ C O ~ - powder (MERCK) was mixed. The v e r y voluminous m i x t u r e w a s c o n t i n u o u s l y f i l l e d i n t o t h e h e a t e d c r u c i b l e (*13000C) t o s i n t e r ,

t o r e a c t and t o m e l t . The w a t e r was blown o u t w i t h t h e argon stream.

The m e l t was n o t c o o l e d down i n t h e c r u c i b l e t o a v o i d d e s t r u c t i o n a s t h e m a t e r i a l s t u c k e x t r e m e l y w e l l t o t h e w a l l and seemed t o expand w i t h d e c r e a s i n g t e m p e r a t u r e . Growth proceeded t h e r e f o r e d i r e c t l y one hour a f t e r f i l l i n g and t h e m e l t was poured o u t a f t e r e a c h run. A t y p i - c a l c h a r g e c o n s i s t e d of 80 g m e l t from which a s p h e r e of 4 g - ^{6 g}

was grown. M e l t and s p h e r e a p p e a r t o t a l l y b l a c k .

S u c c e s s f u l growth was a c h i e v e d o n l y w i t h a growth speed l e s s t h a n 3.8 mm h-1 under t y p i c a l l y 60 rpm. S e e d i n g w i t h a p l a t i n u m w i r e i n v a r i - a b l y r e s u l t e d i n p o l y c r y s t a l l i n e m a t e r i a l , whereas s e e d i n g w i t h a v e r y t h i n s i n g l e c r y s t a l ( c l e a v e d from a p o l y c r y s t a l l i n e s p h e r e i n which n o r m a l l y a l l g r a i n s grow a l m o s t p a r a l l e l t o t h e p u l l i n g d i r e c t i o n ) c o u l d r e s u l t i n a s i n g l e c r y s t a l s p h e r e . To r e d u c e t h e t y p i c a l l y con- cave i n t e r f a c e ( t o w a r d s t h e m e l t ) t h e d i a m e t e r of t h e s p h e r e was h e l d below 8 mm. Diameter c o n t r o l was e x t r e m e l y d i f f i c u l t b e c a u s e of a f l a r i n g tendency. Good c e n t e r i n g of t h e s e e d i n t h e t h e r m a l c e n t r e of t h e c r u c i b l e was found t o be i m p o r t a n t , because t h e m e l t meniscus was a s h i g h a s 4 mm. A f t e r growth t h e c r y s t a l was c o o l e d down from a p p r o x i m a t e l y 7000C t o room t e m p e r a t u r e d u r i n g two days. (010) "growth p l a n e s " were observed on t h e s u r f a c e o f t h e s p h e r e , c l e a v a g e p l a n e s o c c u r r e d f o r t h e same d i r e c t i o n . The c r y s t a l l i n i t y was checked w i t h x-ray photographs (Laue-back r e f l e c t i o n ) a t v a r i o u s p a r t s of t h e s p h e r e and under d i f f e r e n t low i n d e x d i r e c t i o n s . Neutron d i f f r a c t i o n r o c k i n g c u r v e s sliowed a m o s a i c i t y of a few minutes of a r c . Chemical a n a l y s i s of t h e c o b a l t ( T i t r a t i o n w i t h TITRIPLEX, Merck) confirmed t h e s t o i c h i o m e t r i c c o b a l t c o n t e n t ( t h e o r . : 43.45%, found: 43.90%). The l a t t i c e c o n s t a n t s i n t h e orthorhombic s p a c e group Cmca were r e f i n e d from n e u t r o n powder d a t a a t 25 K : a = 6 . 0 3 1 ( 1 ) , b

11.5011(1) , c = 8.3045 (1

8 , ^Z

4 . Room t e m p e r a t u r e v a l u e s a r e given i n p r e v i o u s

p a p e r s / 3 , 4 / . The g r a v i m e t r i c d e n s i t y of f e x

(4.688 + 0.002) g ~ m - ~ compares t o t h e c a l c u l a t e d d e n s i t y of gcal ^{= 4.693} (293 K ) . O r i e n t e d samples w i t H dimensions 5.2 x 2.2 x 2.2 p a r a l l e l t o a , b and c was c u t f o r t h e p o l a r i z e d n e u t r o n e x p e r i m e n t s . One specimen was tempered a t 900°C f o r two d a y s and c o o l e d down t o room t e m p e r a t u r e w i t h i n t h r e e d a y s t o r e d u c e s t r a i n . Powdered m a t e r i a l was o b t a i n e d from t h e same p r e p a r a t i o n .

Neutron powder p a t t e r n s were r e c o r d e d on t h e D f a c i l i t y of t h e ILL a t 25K and 4.2K w i t h a wavelength of .k

1.91 M a g n e t i z a t i o n measure ments were c a r r i e d out a t t h e L a b o r a t o i r e d e s Champs I n t e n s e s , G r e n o b l e . The p o l a r i z e d n e u t r o n s t u d y was performed on t h e D 3 - f a c i l i t y of t h e ILL w i t h a wavelength of A = 0.9 2 and a magnetic f i e l d o f 1.7 T e s l a p a r a l l e l t o t h e 0 0 I l d i r e c t i o n . About 350 r e f l e c t i o n s were r e c o r d e d up t o sine/A

0.9 8- a t 4.2K i n t h e n k l (n

0.1 , 2 , 3 ) p l a n e s . Data w e r e c o r r e c t e d for e x t i n c t i o n i n t h e f o l l o w i n g way. The e x t i n c t i o n

p a r a m e t e r s were r e f i n e d on a d a t a s e t c o l l e c t e d w i t h u n p o l a r i z e d neu- t r o n s on t h e same c r y s t a l /7/ a t 40K a c c o r d i n g t o t h e Becker-Coppens formalism /8/. The r e f i n e d v a l u e s were t h e n a p p l i e d f o r t h e c o r r e c t i o n o f t h e p o l a r i z e d n e u t r o n d a t a a s d e s c r i b e d by T a s s e t / 9 / .

R e s u l t s . - The m a g n e t i z a t i o n measurement ( F i g . 1 ) showed a pronounced a n i s o t r o p y f o r b o t h t h e p a r a m a g n e t i c and t h e f e r r o m a g n e t i c r e g i o n s . The s a t u r a t i o n i s n o t r e a c h e d i n f i e l d s up t o 15 T e s l a . A t o t a l moment of 8 . 5 2 ~ ~ i s o b t a i n e d p e r formula u n i t .

P r o f i l e r e f i n e m e n t / l o / of t h e n e u t r o n powder p a t t e r n r e s u l t e d i n d i f -

f e r e n t moments o f t h e two c r y s t a l l o g r a p h i c a l l y n o n - e q u i v a l e n t c o b a l t

i o n s . A v a l u e of

1.87 ( 6 ) p ~ was found f o r CoI i n t h e f o u r f o l d 4a

p o s i t i o n , w h i l e p

3 . 5 4 ( 3 ) r e s u l t e d f o r CoII i n t h e p o s i t i o n 8 e , de-

s p i t e t h e f a c t t h a t b o t h i o n s a r e i n an o c t a h e d r a l environment.

p r e l i m i n a r y r e f i n e m e n t of t h e

4.2 K p o l a r i z e d n e u t r o n d a t a r e s u l t e d i n v a l u e s of p

1.97 ( 7 )pB and p

3.27 ( 6 ) p B r e s p e c t i v e l y . The

/.-

t o t a l moment of 8 . 5 1 ~ i s i n good agreement w i t h tae magneti- z a t i o n measurements.

The r e f i n e m e n t i s based on t h e t h e o r y of one-electron-wavefunc- t i o n s f o r an o c t a h e d r a l e n v i r o n - ment . The f i v e f o l d degeneracy o f t h e d - l e v e l s i s r e d u c e d t o a

F ( t 2 9 ) =

s (:jo)-~(~,y)~j4>)+1(<j0>+<j2>)

5

w i t h ~ ( 8 , y ) =

0 10 15 [Teslal

1 ( 3 5 c 0 ~ 4 e - 3 0 ~ 0 ~ 2 e + 3 ) $sin4ecos4Y.

8

*

0 5 10 15 ITeslal

t w o f o l d e g - l e v e l and a t h r e e - f o l d t z g - l e v e l . The c a l c u l a t i o n

A

of t h e o r b i t a l c o n t r i b u t i o n i s

F i g . 1 : Magnetic moments ( p e r i o n ) a s a f u n c t i o n o f f i e l d i n Co3V208

3.

The ( j k ) a r e t h e r a d i a l i n t e g r a l s o f t h e f o r m f a c t o r s . Numerical v a l u e s were t a k e n from Freeman and Watson /12/, S i s t h e s p i n v a l u e and 1 i s t h e o r b i t a l moment v a l u e . A n i s o t r o p i c c o n t r i b u t i o n y i e l d s o c c u p a t i o n of 40% f o r eg and 60% The r e s u l t s of t h e r e f i n e m e n t i n d i c a t e a pronounced anisotropio;o:2%oth c o b a l t i o n s ( T a b l e 1 ) .

25 K based on a d i p o l e approximation

Table 1 : Refined moment and e l e c t r o n d i s t r i b u t i o n f o r Co3V208 /11/. The magnetic s t r u c t u r e f a c t o r s a r e e x p r e s s e d a s

2

-

F ( e g ) '

s (<jo>+~(e,~)<j4>)+l(~jo>+<j2))

p [ p B ~ O r b i t e

9 t

29

The a n i s o t r o p y i s r e f l e c t e d i n t h e form f a c t o r c u r v e ( F i g . 2 ) .

JOURNAL DE PHYSIQUE

The evaluation of our data revealed that all low angle data (sine/;l<

0.3 2-I

suffer considerably from extinction. Large discrepancies be- tween observed and calculated structure factors are therefore present in the data. Further polarized neutron experiments are planned for the near future.

3.18-

IJf [PBI 2 63-

2.09

1.5L

099-

0.L5.

- 0.1 0

The support of this work by the Bundesministerium fur Forschung und Technologic is gratefully acknowledged.

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fcalc

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4, +

.

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_* . ' %

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* A

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References

0 0 0.20 0.40 0.60 0.80 1.00

sin 0th 1 2 1

Fig. 2 : Magnetic form factors for Co V 0 3 2 8

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(19791, 863.

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(1978), 1

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Sci. E A , (19771, 455

C. Brisi, Ann-chim. (Rome) 47, (1957), 815

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J.Schweizer, in: Electron and Magnetization Densities in Molecules arid Crystals (Ed. P.Becker), Plenum Press, New York, 1980,p. 479.

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