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MÖSSBAUER STUDY OF Fe2+ IONS IN THE LOW TEMPERATURE PHASE OF MAGNETITE
S. Umemura, S. Iida
To cite this version:
S. Umemura, S. Iida. MÖSSBAUER STUDY OF Fe2+ IONS IN THE LOW TEMPERATURE PHASE OF MAGNETITE. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-307-C2-309.
�10.1051/jphyscol:19792108�. �jpa-00218477�
Co//oque C2,
suppl6ment au n o 3, Tome
40,mars 1979, page C2-307
MOSSBAUEX STUDY OF ~ e IONS ~ +
I N
THE L3W TEM?ERATURE PHASE 3F MAGNETITES. Umemura and S. I i d a
Department of Physics, F a c u l t y of S c i e n c e , The l l n i v e r s i t y of Tokyo, Bunkyo-ku, Tokyo, Japmz
R6sumC.- La phase b a s s e t e m p d r a t u r e d e l a m a g n d t i t e (Fe,O,) a Q t d C t u d i e e p a r e f f e t ~ z s s b a u e r s u r d e s L c h a n t i l l o n s m o n o c r i s t a l l i n s , p l a c d s B 85 K d a n s un champ magngtique d e 5 , 5 kOe a p p l i q u e l e long d e s d i v e r s e s d i r e c t i o n s d e s plan? ( 1 1 0 ) ; p e n d a n t l e r e f r o i d i s s e m e n t , l e champ m a g n e t i q u e Citait a p p l i q u e
l e l o n g de l a d i r e c t i o n [l 121. I1 a p p a r a Z t que l e s r a i e s c o r r e s p o n d a n t 1 F e 2 + d e p e n d e n t de l a d i r e c - t i o n du champ a ~ p l i q u d ce q u i f a c i l i t e une a n a l y s e d b t a i l l e e . Les p a r t i e s i s 0 t r 0 p e e t a n i s o t r o p e du champ h y p e r f i n d e s F ~ Z ' - I e t I1 o n t a i n s i pu S t r e sCparCes. Les d i r e c t i o n s de l ' a x e p r i n c i p a l Z du g r a d i e n t de champ g l e c t r i q u e o n t d t e t r o u v d e s l e l o n g de [001] pour Fe +-11 e t l e l o n g de [100] ou [OIO] pour Fe2+-I.
A b s t r a c t . - Mgssbauer s p e c t r a o f t h e low t e m p e r a t u r e phase o f s i n g l e c r y s t a l m a g n e t i t e (Fe,O,) were measured a t 85 K i n t h e m a g n e t i c f i e l d o f 5.5 kOe a l o n g v a r i o u s d i r e c t i o n s i n ( 1 l O ) p l a n e a f t e r c o o l i n g
t h e specimen w i t h t h e m a g n e t i c f i e l d a l o n g [112]. The l i n e s due t o F e 2 + were found t o be s e n s i t i v e t o t h e d i r e c t i o n of t h e a p p l i e d f i e l d , a l l o w i n g t o p e r f o r m d e t a i l e d a n a l y s i s . I s o t r o p i c and a n i s o t r o p i c p a r t s o f t h e h y p e r f i n e f i e l d o f Fe2+-1 and I1 were s e p a r a t e d . We c o n c l u d e d t h a t d i r e c t i o n o f t h e p r i n -
c i p a l Z a x i s of EFG of Fe2+-11 i s [001j, w h e r e a s t h a t of F ~ ~ + - I i s e i t h e r [loo] o r [ O I O ~ .
1. I n t r o d u c t i o n . - M a g n e t i t e , Fe,O,, e x h i b i t s a sharp m a g n e t i c f i e l d d u r i n g measurements were a l w a y s i n p h a s e t r a n s i t i o n a t a b o u t 120 K. Verwey h a s proposed b-c p l a n e .
t h a t below t h e t r a n s i t i o n t e m p e r a t u r e Fe2+ and F e 3 + ~ z s s b a u e r d a t a were o b t a i n e d a t 85 K f o r t h e fol- l a y e r s l a m i n a t e a l t e r n a t e l y a l o n g t h e c - a x i s / I / . lowing c a s e s , t h a t i s , OH=OO, 3S0, 40°, 45". 55', However, r e c e n t e x p e r i m e n t s s u g g e s t e d t h e p r e s e n c e 60' and 70" where OH i s a n a n g l e between [OOI] and o f a more c o m p l i c a t e d s t r u c t u r e / 2 , 3 / . Mizoguchi d i r e c t i o n o f t h e a p p l i e d f i e l d . I n t h e s e c o n d i t i o n s measured t h e NMR s p e c t r u m and e s t i m a t e d t h e d i r e c - d i r e c t i o n s o f t h e m a g n e t i z a t i o n a r e c a l c u l a t e d from t i o n s o f t h e l o c a l symmetry a x e s o f Fe3+ i o n s on B- t h e 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 181 a s O =0°, 25'.
M
s i t e s / 4 , 5 / . For F e 2 + i o n s , however, t h e NMR s i g n a l s 30°, 35', 4 0 ° , 45' and 55' r e s p e c t i v e l y , where O i s M due t o ~ e were s o weak t h a t s u f f i c i e n t informa- ~ + a n a n g l e between [OOI] and t h e d i r e c t i o n o f t h e ma- t i o n s c o u l d n o t be o b t a i n e d . g n e t i z a t i o n .
Mada e t a l . measured t h e Nzssbauer s p e c t r a Measured s p e c t r a a r e shown i n f i g u r e 1 . The s p e c w i t h c h a n g i n g t h e d i r e c t i o n o f t h e a p p l i e d m a g n e t i c trum o f O
=oD
i s s i m i l a r t o t h a t by Hargrove e t a l .?l
f i e l d and e s t i m a t e d t h e o r i e n t a t i o n of e l e c t r i c d e t e c t i n g t w e l v e p e a k s a s i n d i c a t e d i n t h e u p p e m s t f i e l d g r a d i e n t (EFG) o f Fe2+-11 / 6 / . But t h e i r spec- s p e c t r u m . Changes of t h e s p e c t r a w i t h t h e d i r e c t i o n t r a were s o b r o a d t h a t t h e y c o u l d n o t d e t e c t t h e of t h e m a g n e t i z a t i o n can b e u n d e r s t o o d a s s h i f t s of change o f t h e s p e c t r a w i t h t h e d i r e c t i o n o f t h e ap- t h e s e p e a k s . L o c a t i o n s of t h e c u s p s of p e a k s 1 , 3,
p l i e d f i e l d . 6 , 7 , 9 and 12 a r e i n d e p e n d e n t of OM w i t h i n t h e ex-
I n t h e p r e s e n t work Mijssbauer s p e c t r a were m a - p e r i m e n t a l e r r o r . These a r e a s s i g n e d t o b e due t o s u r e d a t 85 K i n t h e m a g n e t i c f i e l d a l o n g v a r i o u s F e 3 + i o n s on A - s i t e s . These p e a k s i n c l u d e t h e p e a k s d i r e c t i o n s i n (170) p l a n e a f t e r c o o l i n g t h e specimen due t o F e 3 + i o n s o n B - s i t e s . O t h e r p e a k s depend on w i t h t h e m a g n e t i c f i e l d a l o n g [l 121. O more o r l e s s , e s p e c i a l l y p . 8 s h i f t s r e m a r k a b l y t o
?l
2. E x p e r i m e n t a l . - A p l a t e specimen w i t h ( l T 0 ) p l a n e t h e r i g h t . Another n o t i c e a b l e change i s t h a t , compa was c u t from a s i n g l e c r y s t a l r o d o b t a i n e d w i t h t h e r e d t h e s p e c t r u m o f 25' o r 30' w i t h t h a t of
o O ,
a Bridgemann method and was p o l i s h e d t o t h e t h i c k n e s s r e m a r k a b l e t a i l i s o b s e r v e d on t h e l e f t s i d e of p . 3 o f 20L30y. A m a g n e t i c f i e l d of 5 . 5 kOe was a p p l i e d a n d , w i t h t h e i n c r e a s e of OM, t h i s t a i l s e p a r a t e s a l o n g t h e specimen p l a n e d u r i n g c o o l i n g and i n t h e from p . 3 , a p p e a r i n g a d o u b l e t l o c a t e d a t a b o u t measurement. D i r e c t i o n o f t h e f i e l d d u r i n g c o o l i n g - 6 m / s i n t h e c a s e o f OM=4S0. I n t h e c a s e o f O =Ss0M was always [l 121
,
s o t h a t [lTO] ,
[I 101
and [OO l] t h i s d o u b l e t i s a p p e a r e d t o be a broad peak n e a r d i r e c t i o n s s h o u l d b e m a g n e t i c a , b and c a x e s r e s - -6.5mmfs. I n t h e f o l l o w i n g s t h i s peak i s named p.13.p e c t i v e l y / 7 / . A c c o r d i n g l y , t h e d i r e c t i o n s of t h e
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792108
JOURNAL DE PHYSlQllE
c I I m I I @ % * * . I
-8 - 4 0 4 8
VELOCITY (mm/s)
3. A n a l y s i s and d i s c u s s i o n . - A c c o r d i n g t o p r e v i o u s s t u d i e s on Fe
'+
i o n s , t h e m a g n i t u d e s o f e2qQ/2 and t h e isomer s h i f t 6 o f Fe3+ i n s p i n e 1 t y p e o x i d e a r e i n t h e r a n g e s o f l e Z q Q / 2 ( < 0.5mm/s and O.Zmm/s 6< 0 . 6 m / s r e s p e c t i v e l y . Making u s e o f t h e r e s u l t s o f
NMR / 4 / t o g e t h e r w i t h t h e above l i m i t a t i o n s , we con- c l u d e t h a t F e 2 + i o n s on B s i t e s c o n t r i b u t e t o t h e f o l l o w i n g s e v e n p e a k s , i . e . , p . 2 , p . 4 , p . 5 , p . 8 , p.10, p.11 and p.13. A c c o r d i n g t o t h e NMR measure- ments / 4 / t h e r e e x i s t s F ~ Z + whose h y p e r f i n e f i e l d i s
c o n s i d e r a b l y s m a l l e r t h a n t h o s e of o r h e r Fe i o n s i n t h e c a s e o f O =O0 and t h i s F e 2 + i s named Fe2+-11 by M Hargrove e t a l . / 9 / . Because o f t h e s m a l l v a l u e o f h y p e r f i n e f i e l d , Fe2+-11 s h o u l d c o n t r i b u t e t o p . 4 , p . 5 , p . 8 , p.11 and p . 1 3 , where p.4 s h o u l d s h i f t r e - markably t o t h e l e f t and a p p e a r a s p . 1 3 w i t h t h e i n - c r e a s e o f OM. Another F e 2 + i o n named Fe2+-I i s con- s i d e r e d t o c o n t r i b u t e t o p . 2 , p . 5 and p.10. I n t h e above a s s i g n m e n t s i t i s assumed t h a t t h e isomer s h i f t i s i n d e p e n d e n t of t h e d i r e c t i o n o f t h e m a g n e t i z a t i o n
The change of t h e s p e c t r a w i t h t h e d i r e c t i o n o f t h e a p p l i e d m a g n e t i c f i e l d i s c o n s i d e r e d t o be due t o t h e a n i s o t r o p y of h y p e r f i n e f i e l d and t h e change of t h e a n g l e between t h e d i r e c t i o n o f h y p e r f i n e f i e l d and t h a t o f EEG p r i n c i p a l a x e s . T h e r e f o r e , t h e orien- t a t i o n of EFG p r i n c i p a l a x e s , t h e a n i s o t r o p y of t h e hyperf i n e f i e l d and o t h e r hyperf i n e p a r a m e t e r s of Fe2+-I and I1 can be e s t i m a t e d from t h e changes of t h e s p e c t r a . From t h e a c c u r a c y , we c o n c e n t r a t e t h e o u t e r l i n e s of t h e s p e c t r u m i n t h e a n a l y s i s . D u r i n g t h e a n a l y s i s of h y p e r f i n e p a r a m e t e r s o f F e 2 + , f o l - lowing a s s u m p t i o n s were made.
I ) Magnitude o f e 2 q ~ / 2 and t h e o r i e n t a t i o n of EFG p r i n c i p a l a x e s a r e i n d e p e n d e n t of t h e d i r e c t i o n o f t h e m a g n e t i z a t i o n .
2) P r i n c i p a l a x e s of h y p e r f i n e f i e l d t e n s o r c o i n c i d e w i t h t h o s e of EEG t e n s o r .
As a r e s u l t of r a t h e r c o m p l i c a t e d a n a l y s e s w i t h u s i n g c o m p u t e r s , we g e t t h e f o l l o w i n g r e s u l t s :
1) Fe2+-I
6 = ( 1 . 0
+
O . I ) m / s ( r e l a t i v e t o a-Fe) e2qQ/2 = ( 2 . 0 'L 2.6)mm/sr l = 0 . 3 5 % 0 . 5
Z / / [l 001 o r [OI
01
and X / / [00 l]H. = (-450 Q -470) kOe
HX = (-30 -35)kOe
Hy, H Z : u n d e t e r m i n e d
F i g . l : ~ E s s b a u e r s p e c t r a o f ( 1 10) p l a n e o f Fe,O,
-
6 = ( 1 . 0+
0 . I ) m / s ( r e l a t i v e t o a-Fe) a t 85 K.$
i s a n a n g l e between t h e d i r e c t i o n ofe 2 q ~ / 2 = ( 2 . 0
+
0 . I)mm/s t h e m a g n e t ~ z a t i o n and [OOI].
q : undetermined
2.
/ l
[OOI] and X / /[loo]
o r [OIO] ( i f rl # 0 ) H. = (-440 f 5)kOeHZ = (85 ? 5)kOe
+, $
: undeterminedwhere X , Y and Z a x e s i n d i c a t e t h e p r i n c i p a l a x e s o f EFG d e f i n e d i n t h e u s u a l way and Ho, HX,
%
and HZ a r e d e f i n e d a s f o l l o w s . A n i s o t r o p i c h y p e r f i n e f i e l d i s e x p r e s s e d by a t e n s o r a s f o l l o w s :where
y(
+ Hy + HZ = 0and + p i s t h e m a g n e t i c moment of t h e i o n .
On t h e o t h e r hand t h e s p l i t t i n g of p.13 o b s e r - ved i n t h e s p e c t r u m of OM=45' s u g g e s t s t h e e x i s t e n c e of two k i n d s of FeZ+-11 i o n s , whose o r i e n t a t i o n s of t h e p r i n c i p a l a x e s of EFG a r e s l i g h t l y ( a b o u t 10') d i f f e r e n t w i t h e a c h o t h e r .
P r i n c i p a l p a r t o f h y p e r f i n e f i e l d a t t h e Fe2+
n u c l e i c o n s i s t s of Fermi c o n t a c t t e r m H,, o r b i t a l c u r r e n t He and i n t r a - a t o m i c s p i n d i p o l a r i n t e r a c t i o n Hd, i n which He and Hd a r e a n i s o t r o p i c . Assuming t h a t e Z q ~ / 2 comes e n t i r e l y from an a d d i t i o n a l 3d e l e c t r o n , t h e v a l u e of Hd c a l c u l a t e d from t h e obser- ved v a l u e of e Z q ~ / 2 a f t e r t h e method o f O k i j i e t al.
/ 1 0 / , a g r e e s w i t h t h e o b s e r v e d v a l u e of t h e a n i s o - t r o p i c p a r t of t h e h y p e r f i n e f i e l d f o r Fe2+-I and 11. T h e r e f o r e , t h e o r b i t a l c u r r e n t c o n t r i b u t i o n i s c o n s i d e r e d t o b e s u f f i c i e n t l y s m a l l and t h e o r b i t a l moment w i l l b e quenched f o r F e 2 + i n t h e low tempera- t u r e p h a s e of Fe,O,. A c c o r d i n g l y t h e o b s e r v e d v a l u e of H 0 i s c o n s i d e r e d t o b e a l m o s t due t o t h e Fermi c o n t a c t i n t e r a c t i o n .
The l o c a l symmetry a t B - s i t e of s p i n e 1 s t r u c - t u r e i s g e n e r a l l y t h o u g h t t o b e t r i g o n a l a l o n g one o f < I l l > . However, t h e p r e s e n t r e s u l t s of t h e o r i e n - t a t i o n of EFG p r i n c i p a l a x e s show t h a t t h e l o c a l symmetry a t t h e p o s i t i o n of Fe2+ on B s i t e s a r e r a - t h e r n e a r l y t e t r a g o n a l w i t h <100> d i r e c t i o n s a s t h e i r a x e s . T h i s r e s u l t s u g g e s t s t h a t t e t r a g o n a l d:splacements of oxygen i o n s a r e l a r g e .
R e f e r e n c e s
/ l / Verwey, E.J.W., Haaymann, P.W. and Romeijin, F.C., J . Chem. Phys.
2
(1947) 181./2/ S h i r a n e , G. e t a l . , J. Phys. Soc. J a p a n
2
(1975) 947.
1 3 1 I i d a , S. e t a l . , J. P h y s i q u e C o l l o q .
2
(1977) Cl-73./ 4 / Mizoguchi, X., J. Phys. Soc. J a p a n
5
(1978) 1501./ 5 / Mizoguchi. M., J . Phys. Soc. J a p a n
g
(1978) 1512.1 6 1 Mada, J . and I i d a , S . , J . Phys. Soc. J a p a n
2
(1977) 1184.
/ 7 / Calhoun, B . A . , Phys. Rev. (1954) 1577.
1 8 1 Abe, K., Hiyamoto, Y. and Chikazumi, S., J.
Phys. Soc. J a p a n
41
(1976) 1894./ g / Hargrove, R.S. and Kiindig, W . , S o l i d S t a t e Corn mun.
8
(1970) 303./ 1 0 / O k i j i , A. and Kanamori, J . , J. Phys. Soc. Japar 19 (1964) 908.
