MAGNETIC PROPERTIES OF BaFe4Mn2O11 R-TYPE HEXAGONAL FERRITE

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MAGNETIC PROPERTIES OF BaFe4Mn2O11 R-TYPE HEXAGONAL FERRITE

X. Obradors, A. Isalgué, A. Collomb, M. Pernet, J. Tejada, J. Joubert

To cite this version:

X. Obradors, A. Isalgué, A. Collomb, M. Pernet, J. Tejada, et al.. MAGNETIC PROPERTIES OF BaFe4Mn2O11 R-TYPE HEXAGONAL FERRITE. Journal de Physique Colloques, 1985, 46 (C6), pp.C6-339-C6-343. �10.1051/jphyscol:1985662�. �jpa-00224917�

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Colloque C 6 , suppl6ment a u n09, Tome 46, septembre 1985 page C6-339

MAGNETIC PROPERTIES OF BaFe4Mn2Ol1 R-TYPE HEXAGONAL FERRITE

++ ++

X . O b r a d o r s , A. 1 s a l g u 6 + , A. Collomb , M. P e r n e t , J . T e j a d a and J.C. J o u b e r t + +

FacuZtat de Fisica, U n i v e r s i t a t de BarceZona, Diagonal 645, 08028 .BarceZona, Spain

+Department de Fisica, ETSAB Universitat PoZytdcnica de CataZunya, DiagonaZ 649, 08028 Barce Zona, Spain

++Laboratoire de CristaZZographie, C.N.R.S., associe' d Z'Universitd S c i e n t i f i q u e e t Me'dicaZe de GrenobZe, 166 X , 38042 Grenoble Cedex, France RBsum6 - Les p r o p r i P t 6 s m a g n e t i q u e s e t l a d i s t r i b u t i o n c a t i o n i q u e d ' u n nou- veau f e r r i t e hexagonal f e r r i m a g n g t i q u e , d e t y p e R o n t 4 t k k t u d i e e s k p a r t i r d e mesures d ' a i m a n t a t i o n e t d e s p e c t r o s c o p i e Mossbauer. Le compos6 BaFe Mn 0 4 2 . 1 1 p r e s e n t e une s t r u c t u r e magngtique c o l i n e a i r e e t une f o r t e a n i s o t r o p i e u n l - a x i a l e . A l ' a i d e d e l a l o i d ' a p p r o c h e 5 l a s a t u r a t i o n e t d e l a t e c h n i q u e d e l a d e t e c t i o n d e p o i n t s i n g u l i e r d a n s l e s c o u r b e s d ' a i m a n t a t i o n , un champ d ' a n i s o - t r o p i e a t t e i g n a n t 50 kOe a e t e d e t e r m i n e 5 T = 4 , 2 K. On montre que l e s i o n s Mn o c c u p e n t d e p r e f e r e n c e l e s s i t e s o c t a k d r i q u e s ( 6 g ) c e c i p o u r d e s r a i s o n s c r i s t a l l o c h i m i q u e s .

A b s t r a c t - The m a g n e t i c p r o p e r t i e s and c a t i o n d i s t r i b u t i o n o f a new f e r r i m a g - n e t i c R-type hexagonal f e r r i t e have been s t u d i e d from i s o t h e r m a l m a g n e t i z a t i o n and Mossbauer s p e c t r o s c o p y measurements. The compound BaFe Mn 0 p o s s e s s a c o l l i n e a r s p i n s t r u c t u r e and a s t r o n g u n i a x i a l m a g n e t i c an?sogrAby. By means of t h e law of a p p r o a c h t o s a t u r a t i o n and t h e s i n g u l a r p o i n t d e t e c t i o n t e c h r d q u e m a g n e t i c a n i s o t r o p y f i e l d up t o 50 kOe i s o b t a i n e d a t T = 4 . 2 K. I t i s shown t h a t Mn i o n s have a f u l l p r e f e r e n c e f o r t h e o c t a h e d r a l 6g s u b l a t t i c e and c r y s - t a l l o c h e m i c a l r e a s o n s f o r t h i s f a c t a r e g i v e n .

I - INTRODUCTION

The i n t e r e s t on R-type hexagonal f e r r i t e s h a s c o n t i n u o u s l y grown s i n c e Haberey and Velicescu s y n t h e s i z e d a compound of t h i s f a m i l y /I/. The R s t r u c t u r e (S.G. Pb3/rnmc) may be s y m b o l i c a l l y d e s c r i b e d a s RXR, where R r e p r e s e n t s a b l o c k o f t h r e e hexagonal c l o s e packed oxygen l a y e r s w i t h a Ba c a t i o n s u b s t i t u i n g a n oxygen i n t h e i n n e r l a y e r , and where * symbol d e n o t e s t h a t t h e c o r r e s p o n d i n g b l o c k h a s been 180° t u r n e d around t h e hexagonal c - a x i s . The u n i t f o r m u l a of t h e R s t r u c t u r e - t y p e compounds may b e w r i t - t e n a s BaFe M 0 , where d e n o t e s a t e t g a v a l e n t t r a n s i t i o n m e t a l , and t h e l a t t i c e p a r a m e t e r s

$2

A f u l l s t u d y of t h e s t r u c t u r a l and m a g n e t i c p r o p e r t i e s of t h e R-type hexagonal f e r - r i t e s i s o b v i o u s l y n e c e s s a r y f o r g e t t i n g a d e e p e r i n s i g h t i n t o t h e p r o p e r t i e s of more complex and t e c h n o l o g i c a l l y r e l e v a n t f e r r i m a g n e t i c o x i d e s .

Among t h e R-type hexagonal f e r r i t e s s t u d i e d up t o now, no f e r r i m a g n e t i c compounds have been found. Only a s p i n - g l a s s l i k e b e h a v i o u r was found i n T i and Sn compounds.

However, v e r y r e c e n t l y , a new f e r r i m a g n e t i c compound w i t h BaFe4Mn20Al c o m p o s i t i o n was s u c c e s s f u l l y s y n t h e s i z e d / 3 , 4 / . I n t h i s work, we r e p o r t on t h e s y n t e s i s , c a t i o n d i s t r i b u t i o n and m a g n e t i c p r o p e r t i e s of t h i s compound.

I1 - SYNTHESIS AND X-RAY CHARACTERIZATION

The main d i f f i c u l t y i n t h e s y n t h e s i s of B ~ F ~ ~ + M ~ ~ + O ~ ~ compound is t o avoid t h e reduc- t i o n o f t e t r a v a l e n t Mn i o n s . We have o n l y s u c c e e z by u s i n g a h i g h t e m p e r a t u r e - h i g h

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985662

C6-340 JOURNAL DE PHYSIQUE

p r e s s u r e b e l t - t y p e a p p a r a t u s . A m i x t u r e o f Ba02, a-Fe 0 and Mn02 was s u b j e c t e d t o a p r e s s i o n o f 80 kb a t T = 1173 K d u r i n g 1 h. The ~ - t 8 a ~ e ~ ~Mn2011 o b t a i n e d compound z always c o n t a i n s a few amount of a-Fe 0 which c o u l d b e e a s i ; t y e l r m i n a t e d by a magne- t i c s e p a r a t i o n p r o c e d u r e . F u r t h e r ~ & s $ a u e r s p e c t r a gave a r e s i d u a l amount of a b o u t 2 % o f a-Fe203, which c o u l d n o t be d e t e c t e d i n l o n g e x p o s u r e X-ray p a t t e r n s .

0

The l a t t i c e papameters o b t a i n e d from a G u i n i e r p a t t e r n were : a = 5 . 7 8 8 ( 2 ) A and c = 13.445(9) A, i n agreement w i t h t h o s e r e p o r t e d by Cad6e / 3 / .

111 - MAGNETIC MEASUREMENTS

I s o t h e r m a l m a g n e t i z a t i o n c u r v e s of BaFe Mn2011 were p l o t t e d i n t h e t e m p e r a t u r e r a n g e 4.2 K < T < 300 K i n f i e l d s up t o 150 k b e , by u s l n g t h e a x i a l e x t r a c t i o n t e c h n i q u e of t h e S e r v i c e N a t i o n a l d e s Champs I n t e n s e s i n Grenoble. The s a t u r a t i o n m a g n e t i z a t i o n above room t e m p e r a t u r e and t h e C u r i e t e m p e r a t u r e was deduced from measurements p e r - formed on a f i e l d o r i e n t e d s a m p l e , by u s i n g a F a r a d a y b a l a n c e . I n t r i n s i c m a g n e t i c p a r a m e t e r s such a s s a t u r a t i o n m a g n e t i z a t i o n Mo, h i g h f i e l d d i f f e r e n t i a l s u s c e p t i b i - l i t y x and a n i s o t r o p y f i e l d Ha a t t e m p e r a t u r e s below 300 K were deduced from a nume- r i c a l a n a l y s i s of t h e l a w of a p p r o a c h t o s a t u r a t i o n . Moreover, H a a t T = 4.2 K was a l s o deduced by means of t h e n u m e r i c a l d e t e r m i n a t i o n of t h e s i n g u l a r p o i n t i n t h e m a g n e t i z a t i o n c u r v e ( F i g . 1 ) .

F i g . 1 - S i n g u l a r p o i n t d e t e r m i n a t i o n of Ha i n BaFe4Mn2011 ( a t T = 4.2 K).

From t h e t e m p e r a t u r e dependence of t h e s a t u r a t i o n m a g n e t i z a t i o n ( F i g . 2 ) , Mo a t T = 0 K is e s t i m a t e d a s 3 7 . 5 ( 7 ) emu/g o r 4 . 7 ( 1 ) p B / f - u .

The a b s o l u t e s a t u r a t i o n m a g n e t i z a t i o n s u g g e s t s a c o l l i n e a r o r n e a r l y c o l l i n e a r G o r t e r - t y p e m a g n e t i c s t r u c t u r e w i t h ~ n i o n s occupying t h e 6g o c t a h e d r a l s u b l a t t i c e ~ + The c a l c u l a t e d v a l u e by assuming p ( ~ e ~ + ) = 5 p ~ and p ( ~ n ~ + ) = 2.6 p ~ /5/ is

Mo = 5.2 p 8 / f e u which i s r o u g h l y i n agreement w i t h t h e measured v a l u e .

The m a g n e t i c a n i s o t r o p y f i e l d Ha s u g g e s t s t h a t BaFeuMn2011 owns a n u n i a x i a l a n i s o t r o - PY t h e w h i c h is d e s c r i b e d u s i n g t h e o n l y f i r s t o r d e r c o n s t a n t K 1 : Ea =

y:ff&

kl=4.2 106 erg/cm3, w h i l e a t room t e m p e r a t u r e Ha = 40 Koe and K 1 = 1 . 3 l o 6 ^erg/cm 3

.

These m a g n e t i c a n i s o t r o p y c o n s t a n t s a r e q u i t e s i m i l a r t o t h o s e of BaFel201g. The o r i g i n of t h e h i g h m a g n e t i c a n i s o t r o p y i n some hexagonal f e r r i t e s h a s b e e n c o n t r o v e r - s e d , b u t as it is shown i n a n accompanying p a p e r /6/ it a r i s e s m o s t l y from m a g n e t i c

d i p o l a r energy.

F i n a l l y t h e C u r i e t e m p e r a t u r e o b t a i n e d from o u r high t e m p e r a t u r e measurements i s Tc = 480(5) K. T h i s v a l u e is s l i g t h l y lower than t h a t p r e v i o u s l y r e p o r t e d by Cadge /3/. T h i s low v a l u e of T c , compared t o t h a t of BaFel2019, c l e a r l y d e m o n s t r a t e s t h e s o f t e n i n g of exchange i n t e r a c t i o n s induced by t h e ~ n i o n s . ~ +

Fig. 2 - S a t u r a t i o n m a g n e t i z a t i o n of BaFe4MnqOll v e r s u s t e m p e r a t u r e . I V - MOSSBAUER SPECTROSCOPY STUDY

Two Mossbauer s p e c t r a w e r e e n r e g i s t r e d : a paramagnetic one ( F i g . 3 ) a n o t h e r a t room t e m p e r a t u r e ( F i g . 4 ) .

v e l o c i t y (mmlsl

F i g . 3 - Paramagnetic Mossbauer spectrum or BaFe4Mn2011.

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v e l o c i t y (mm/s!

F i g . 4 - Room t e m p e r a t u r e Mossbaeur spectrum of BaFe4Mn2011.

Table 1 summarizes t h e h y p e r f i n e p a r a m e t e r s deduced from t h e f i t t i n g of t h e s p e c t r a a s a sum of L o r e n t z i a n l i n e s . I n t h e m a g n e t i c a l l y s p l i t t e d spectrum an h y p e r f i n e f i e l d d i s t r i b u t i o n was i n c l u d e d i n o r d e r t o a c c o u n t f o r t h e inhomogeneous broadening of t h e l i n e s . The i d e n t i f i c a t i o n of t h e s u b s p e c t r a w a s a c h i e v e d on t h e grounds of t h e i r c h a r a c t e r i s t i c h y p e r f i n e p a r a m e t e r s /7/ and r e l a t i v e i n t e n s i t i e s .

( a ) r e s p e c t t o Co-Rh

( b ) h a l f width of t h e L o r e n t z i a n - l i k e h y p e r f i n e f i e l d d i s t r i b u t i o n . Table 1 - Hyperfine p a r a m e t e r s of Mossbauer s p e c t m of BaFe4Mn2011.

T = 533 K

Rorn t m p e r r r t m e

From t h e r e l a t i v e i n t e n s i t y of t h e two o c t a h e d r a l s u b s p e c t r a , it is found t h a t ~ n ~ + i o n s a r e o n l y occupying t h e 6g s u b l a t t i c e , t h u s f u l l y c o n f i r m i n t h e a s s e r t i o n s made from m a g n e t i z a t i o n measurements. T h i s s e l e c t i v e occupancy of Mnf+ i o n s i n BaFe4Mn2Oll seems t o be c o n s i s t e n t w i t h t h e tendancy found i n p r e v i o u s s t u d i e s on R-type hexago- n a l f e r r i t e s /3/. We would l i k e t o p o i n t o u t t h a t from t h e comparison of s t r u c t u r a l and Mijssbauer d a t a of T i , Sn / 7 , 8 / and Mn R-type hexagonal f e r r i t e s a f u l l c o r r e l a - t i o n among dynamical c h a r a c t e r i s t i c s of F e ( 4 f ) i o n s and l o c a l s t r u c t u r e does emerge.

For i n s t a n c e we may s t a t e t h a t h i g h e r t e t r a v a l e n t i o n s i n d u c e a major d i s p l a c e m e n t of t h e Fe i o n s towards t h e c e n t e r of t h e t e t r a h e d r a l forming t h e bipyramid, t h u s l e a d i n g t o lower quadrupole s p l i t t i n g /3/ (more symmetrical environment). F i n a l l y , some i n s i g h t on t h e m i c r o s c o p i c magnetic c h a r a c t e r i s t i c s of BaFeqMn2011 m y be g o t from t h e m a g n e t i c a l l y s p l i t t e d Mossbauer s p e c t r a . We n o t e f i r s t t h a t t h e e f f e c t i v e quadrupole s p l i t t i n g of t h e ~ e ( 4 k ) i o n s show t h a t t h e i r magnetic moments l i e a l o n g t h e hexagonal c - a x i s , t h u s c o n f i r m i n g o u r p r e v i o u s c o n c l u s i o n of a c o l l i n e a r magnetic s t r u c t u r e w i t h s t r o n g u n i a x i a l magnetic a n i s o t r o p y .

Area ( % )

} 8 9 ( 4 ) 1 1 ( 4 ) 3 1 ( 4 ) 5 9 ( 9 ) 1 0 ( 7 ) 6g

4e 4f 6g 4e 4f

Then, some comments may be done c o n c e r n i n g t h e e f f e c t of a chemical d i s o r d e r i n 6g

~ s o m e r ( ~ ) s h i f t (mm s - l )

-0.09(1) O.Ol(5) 0 . 3 6 ( 5 ) 0.27(1) 0.04(4)

Quadrupole s p l i t t i n g

(mm s - l ) 0.53(1) 2-00(10) 0.72(10) 0.13(2) 1 . 9 6 ( 8 )

Hyperf i n e f i e l d (H)

(kOe)

3 2 8 ( 6 ) 444(1) 3 7 6 ( 4 )

AH(^)

(kOe)

5 1 ( 8 ) 1 8 ( 2 ) 2 0 ( 7 )

s u b l a t t i c e , on t h e exchange i n t e r a c t i o n s between Fe i o n s . The s t a b i l i s a t i o n of a c o l l i n e a r G o r t e r - t y p e m a g n e t i c s t r u c t u r e means t h a t t h e Mn(6g)-Fe(4e) i n t e r a c t i o n is a n t i f e r r o m a g n e t i c , l i k e t h e c o r r e s p o n d i n g Fe(6g)-Fe(4e) one. However, it is c l e a r t h a t i t s s t r e n g t h i s l o w e r , a s it i s confirmed by t h e d e c r e a s e of t h e C u r i e tempera- t u r e of BaFeqMn2011 compared t o BaFel2019. T h e r e f o r e , a s t r o n g l o c a l magnetic unequi- v a l e n c y is o b t a i n e d f o r t h e neighbouring Fe i o n s . T h i s is t h e o r i g i n of t h e i m p o r t a n t h y p e r f i n e f i e l d d i s t r i b u t i o n observed i n t h e m a g n e t i c a l l y s p l i t t e d Mossbauer spectrum.

V - CONCLUSION

From i s o t h e r m a l high f i e l d m a g n e t i z a t i o n and Mossbauer s p e c t r o s c o p y measurements it h a s been shown t h a t BaFe4Mn2011 R-type hexagonal f e r r i t e owns a c o l l i n e a r G o r t e r - t y p e magnetic s t r u c t u r e , w i t h t h e ~ n 4 + i o n s occupying t h e 6g o c t a h e d r a l s u b l a t t i c e . A s compared t o BaFe12019, t h e s a t u r a t i o n m a g n e t i z a t i o n i s lower b u t t h e magnetic a n i s o - t r o p y energy is s i m i l a r . T h i s l e a d s t o a v e r y h i g h magnetic a n i s o t r o p y f i e l d , i . e . Ha = 50 kOe a t T = 4.2 K. The d e c r e a s e of t h e C u r i e t e m p e r a t u r e , a s compared t o BaFe12019 and t h e h y p e r f i n e f i e l d v a l u e s deduced from Mossbauer s p e c t r a , a l l o w us t o conclude t h a t t h e Mn i o n s s t r o n g l y d e c r e a s e t h e superexchange p a t h s .

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