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61Ni MÖSSBAUER EFFECT STUDIES OF THE FERRIMAGNETIC SPINEL NiCr2O4 IN AN
APPLIED MAGNETIC FIELD
P. Gütlich, H. Rummel, H. Spiering
To cite this version:
P. Gütlich, H. Rummel, H. Spiering. 61Ni MÖSSBAUER EFFECT STUDIES OF THE FERRIMAG-
NETIC SPINEL NiCr2O4 IN AN APPLIED MAGNETIC FIELD. Journal de Physique Colloques,
1980, 41 (C1), pp.C1-185-C1-186. �10.1051/jphyscol:1980154�. �jpa-00219723�
JOURNAL DE PHYSIQUE Colloque C1, suppldment au n
O1 , Tome 41, junvier 1980, page C1-185
61 Ni &SBAUEF! EFFECT STUDIES OF THE FERRIMAGNETIC SPINEL NiCr204 IN AN APPLIED MAGNETIC FIEU)
P. Giitlich, H. Rummel and H. S p i e r i n g
Institzit fur Anorganische und AnaZytische Chemie, Johmznes Gutenberg Universititt, Ll-6500 Maina, W. Gemany.
1 . I n t r o d u c t i o n . - Very l a r g e magnetic h y p e r f i n e f i e l d s (440-630 kG) have been measured a t t h e nu- c l e i of t e t r a h e d r a l l y c o o r d i n a t e d N i 2 + i o n s i n f e r - r i m a g n e t i c a l l y ordered s p i n e l s /1,2/. A s t h e Fermi- c o n t a c t f i e l d c a l c u l a t e d by Freeman and Watson /3/
i s small and n e g a t i v e ( H C o -300 kG) r a t h e r l a r g e o r b i t a l c o n t r i b u t i o n s H = 2&<r-3><~> a r e needed
L
i n o r d e r t o account f o r t h e observed v a l u e s o f t h e n e t f i e l d Sf=HL+HC ( n e g l e c t i n g t h o small spin-di- p o l a r c o n t r i b u t i o n ) . For NiCr204 t h e s e o r b i t a l con- t r i b u t i o n s a r e (+740,-140)kG depending on t h e s i g n of t h e n e t f i e l d . Consequently i f t h e 3 ~ 1 - ground s t a t e i s a p u r e d - s t a t e (no d-p mixing by h y b r i d i - 8 z a t i o n ) implying t h a t t h e o r b i t a l c o n t r i b u t i o n i s p o s i t i v e , t h e n l a r g e d e v i a t i o n s o f t h e e l e c t r o n i c g-value from t h e spin-only v a l u e o f 2.0023 o f about
t i c f i e l d o f 48.8 kG was p e r p e n d i c u l a r t o t h e y- d i r e c t i o n . The 50 mCi Nio.85CrO+15 s i n g l e - l i n e
s o u r c e was a c t i v a t e d i n a n e l e c t r o n l i n e a r a c c e l e - r a t o r , making use of t h e 6 2 ~ i ( ~ , p ) ~ l ~ o r e a c t i o n . The a b s o r b e r had an e f f e c t i v e t h i c k n e s s of t=0.5 s o t h a t no c o r r e c t i o n s of t h e l i n e - w i d t h due t o absor- ber t h i c k n e s s were necessary.
3.Results.- The powder spectrum measured a t 48.8 kG (Fig. 1 ) shows r e l a t i v e l y s h a r p l i n e s . It h a s been s u c c e s s f u l l y f i t t e d a s a s i n g l e c r y s t a l spectrum with t h e f o l l o w i n g f r e e parameters: t h e line-widths F l , T2, .... r12, t h e i n t e r n a l f i e l d H e f f l t h e i s o - mer s h i f t , t h e average c o s 8 ( 6 2 i s t h e a n g l e between
4
Heff and y ) , which d e t e r m i n e s t h e i n t e n s i t y r a t i o of t h e h = f l t o h = O l i n e s and t h e product o f t h e -
f i e l d g r a d i e n t VZZ and t h e average Y (6) = ( 3 c o s % - ~ / 2 . 2 - 1 /4/ a t e n e c e s s a r y t o e x p l a i n i n t e r n a l f i e l d s
using ae nuclear moments and vzzfrom 15/ the best-
l a r g e r t h a n 300 kG. - 2
f i t v a l u e s a r e : Y(?jt)=-0.45f0.10, c o s 8=0.35+0.05, To confixm t h e p r e d i c t e d p o s i t i v e s i g n of t h e n e t
H e f f = 47853 kG. The ri v a l u e s a r e broadened by 1 0 f i e l d we measured t h e Massbauer s p e c t r a of t h e f e r -
to 70% respect to the natural line-width.
r i m a g n e t i c powdered samples o f NiCr204 i n an app-
4.Discussion.- F i g . 2 shows t h e s p i n s t r u c t u r e o f l i e d magnetic f i e l d .
NiCr 0 determined by B e r t a u t e t a l . / 6 / . The e a s y 2.Experimental.- The Mossbauer s p e c t r a were r e c o r - 2 4
f e r r i m a g n e t i c d i r e c t i o n (x-axis i n Fig. 2) and t h e ded i n t r a n s m i s s i o n geometry with t h e s o u r c e and
d i r e c t i o n s of t h e s p i n s a t t h e N i s i t e s a r e shown t h e a b s o r b e r a t 4.2 K. The d i r e c t i o n of t h e magne- in Fig. 3 . me applied field has been drawn paralkl
F i g . 1 : 6 1 ~ i Mdssbauer-powder-spectrum o f NiCr 0 i n ,an a p p l i e d m@gnetic f i e l d of 48.8 kG normal t o t h e y - d i r e c t i o n a t 4.2 K. The s o l i d l i n e ii B e f i t t e d t h e o r e t i c a l c u r v e .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980154
c1-186 JOURNAL DE PHYSIQUE
F i g . 2: Spin s t r u c t u r e of NiCr204 from r e f e r e n c e /6/. N i s i t e s a r e marked by s q u a r e s , C r si- t e s by c i r c l e s . The c r o s s x shows t h e four- f o l d c r y s t a l l o g r a p h i c screw a x i s of t h e 14 /amd-space group. Heights a r e i n d i c a t e d i n e i g h t s o f t h e 1 c parameter
t o t h e r e s u l t a n t magnetic momentFf of t h e N i - and
The e s s e n t i a l r e s u l t i s t h a t t h e a p p l i e d f i e l d i n - c r e a s e s t h e f i e l d a t t h e n u c l e u s . I f we assume: ( i ) t h e exchange i n t e r a c t i o n t o be s u f f i c i e n t l y s t r o n g t o p r e s e r v e t h e r e l a t i v e o r i e n t a t i o n o f t h e s p i n s , and ( i i ) t h e m a g n e t i z a t i o n E - F f t o be p a r a l l e l t o t h e a p p l i e d f i e l d independent of t h e d i r e c t i o n of t h e f i e l d with r e s p e c t t o t h e c r y s t a l a x i s (imply- i n g t h a t z.gais l a r g e a s compared t o t h e a n i s o t r o p y energy) , t h e n t h e c a l c u l a t e d e f f e c t i v e f i e l d o f 482.423 kG ( u s i n g Hi=442+3 kG from /4/) i s i n agree- ment with t h e measured v a l u e of 478+3 kG. The s i g n of t h e f i e l d Hi i s n e g a t i v e F ( N i ) i s a n t i p a r a l l e l
t o q).
The e x p e c t a t i o n v a l u e o f -)=-0.45 i m p l i e s t h a t
- Heff i s almost normal t o t h e V - a x i s and c a n be zz
q u a l i t a t i v e l y understood by assuming a p l a n e of e a s y magnetization f o r F ( N i ) normal to t h e V zz -
a x i s . T h i s i s a n t i c i p a t e d from t h e o r b i t a l s i n g l e t g r o u n d s t a t e i n a n a x i a l l i g a n d f i e l d . Then t h e mo- ments F ( N ~ ) of b o t h N i - s i t e s (Fig. 2) o r i e n t them- s e l v e s a s c l o s e a s p o s s i b l e t o t h e easy p l a n e . Ta- k i n g t h e average&£ Y (I%) from '$= 0/4 t o T/2 c o r - responding t o I s i n ~ d l h s 7 1 % of a l l p o s s i b l e d i -
9714
