JOURNAL DE PHYSIQUE
Colloque
C1,suppldment au n
O 1 ,Tome
41,janvier 1980,page C1-197
MAGNETIC BEHAVIOUR OF
Y ( F ~ , M ~ ) ~A.M. van d e r Kraan, P.C.M. Gubbens and K . H . J . Buschow i
~ n t e r u n i v e r s i t a i r Reactor I n s t i t u u t , Delft, The Netherlands
i
Philips Research Laboratories, Eindhoven, The Netherlands.
During t h e l a s t y e a r s we have made a s y s t e m a t i c i n v e s t i g a t i o n of t h e ~ o s s i b l e w i g i n of t h e magnetic behaviour of t h e cubic Laves phase compounds of s t o i c h i o m e t r y RB2 formed between t h e r a r e e a r t h o r Yttrium (R) and t r a n s i t i o n metals (B). P r e v i o u s l y we have r e p o r t e d ~ G s s b a u e r s t u d i e s of t h e pseudo b i n a r y systems H ~ ( F e , c o ) ~ , Y ( F ~ , C O ) ~ , Y(Fe,A1)2 and
Y ( F e , I r ) 2 / 1 - 5 / . Now we have s t u d i e d t h e system Y (Fe,Mn)2. YMn2 i s a paramagnet j u s t a s YCo2, YA12
and Y I r 2 , w h i l e YFe2 o r d e r s f e r r o m a g n e t i c a l l y below 540 K .
The i n t e r m e t a l l i c compounds were prepared by a r c m e l t i n g i n an argon atmosphere. The samples were vacuum annealed f o r f i v e weeks a t 8 0 0 ' ~ i n a T a - f o i l w i t h i n a s i l i c a tube. A f t e r t h e a n n e a l i n g procedure
t h e c r y s t a l l i n e s t r u c t u r e i s examined by X-ray d i f - f r a c t i o n . The samples c o n s i s t of a m i x t u r e of t h e Laves phase and a small amount of m e t a l l i c Yttrium.
A s manganese e v a p o r a t e s d u r i n g t h e p r e p a r a t i o n of t h e p s e u d o b i n a r y compounds, t h e r e a l manganese c o n t e n t h a s been determined by n e u t r o n a c t i v a t i o n a n a l y s e s .
MEssbauer experiments have been performed on n i n e d i f f e r e n t samples of Y(Fel-xMnx)g w i t h x = 0.035, 0.07, 0.13, 0.19, 0.30, 0.38, 0.50, 0.59 and 0 . 7 1 , r e s p e c t i v e l y . From m a g n e t i z a t i o n measurements t h e mean magnetic moment p e r 3d-atom h a s been deduced f o r x = 0.035 (p = 1.34 ) x = 0.07 (11=1.31
u
) ,B B
x = 0.13 ( u = 1.22 y ) , x = 0.30 ( u = 0.90 p ) and
B B
x = 0.71 ( p = 0.04 IJ ) r e s p e c t i v e l y . The mean magne- 6
t i c moment p e r 3d-atom f o r x = 0.30 and t h e
v a n i s h i n g moment n e a r x = 0.71 a r e i n agreement w i t h t h e r e s u l t s of H i l s c h e r e t a1 161.
I n f i g u r e 1 t h e MGssbauer s p e c t r a of some Y (Fel,xMnx)2 samples measured a t T = 4.2 K a r e g i v e n . From t h e measured s p e c t r a i t f o l l o w s t h a t t h e mean e f f e c t i v e h y p e r f i n e f i e l d a t a Fe-nucleus d e c r e a s e s by i n c r e a s i n g Mn c o n c e n t r a t i o n s and becones n e a r l y z e r o f o r x = 0.71.
From t h e m a g n e t i z a t i o n measurements a s w e l l a s from t h e ~ E s s b a u e r s t u d i e s i t f o l l o w s t h a t a t r a n - s i t i o n t a k e s p l a c e from t h e f e r r o m a g n e t i c s t a t e t o t h e paramagnetic s t a t e n e a r x = 0.71. I n c o n t r a s t
more complex magnetic t r a n s i t i o n s h a s been observed i n Y(Fe,A1)2 /4/ and Y ( F e , I r I 2 151.
The spectrum of YFe2 c o n s i s t s of two s i x - l i n e h y p e r f i n e p a t t e r n s w i t h an i n t e n s i t y r a t i o of 1 : 3 r e s p e c t i v e l y , which i s c o n s i s t e n t w i t h an e a s y d i r e c t i o n of magnetization along one of t h e Cllll d i r e c t i o n s . I n f i g u r e 1 i t i s shown t h a t t h e asym- m e t r i c a b s o r p t i o n p a t t e r n becomes symmetrically f o r x = 0.30, i n d i c a t i n g t h a t t h e easy d i r e c t i o n of mag- n e t i z a t i o n g r a d u a l l y changes from a Cllll d i r e c t i o n t o a [0011 d i r e c t i o n i n t h e c o n c e n t r a t i o n range x=O up t o x = 0.30.
Furthermore t h e shape of t h e observed s p e c t r a i n d i c a t e s t h a t l o c a l i z e d Fe moments a r e p r e s e n t , t h e v a l u e of which s e n s i t i v e l y depends upon t h e number of and d i s t a n c e from t h e neighbouring Fe atoms. S i - m i l a r e f f e c t s have been observed i n t h e Y(Fe,Al)2 system p r e v i o u s l y 141, where t h e e f f e c t i v e h y p e r f i n e f i e l d a t a Fe s i t e , surrounded by n and m A 1 n e a r e s t and n e x t n e a r e s t neighbours r e s p e c t i v e l y i s v e r y w e l l d e s c r i b e d by
H(n,m) = H(0,O)-n AH(1,O)-m AH(0,I) w i t h
AH(1 ,0) = H(0,O)-H(1 ,0) and AH(0, I) = H(0,O)-H(0,I).
For Y(Fe,A1)2 i t ' i s found t h a t AH(0,I) = 113 AH(1,O) and p o s i t i v e . Kirchmayr /7/ e x p l a i n e d h i s r e s u l t s on Y(Fel-xMnx)2 compounds with a model i n which t h e n e a r e s t Fe neighbours and n e x t n e a r e s t Fe neighbours c o n t r i b u t e t o t h e magnetic h y p e r f i n e f i e l d a t a Fe nucleus 57.5 and -11.5 kOe r e s p e c t i v e l y . Computer s i m u l a t i o n of M k s b a u e r s p e c t r a on t h e base of t h i s model d e l i v e r s s p e c t r a which d e v i a t e very much from t h e observed ones. The observed s p e c t r a a r e i n much b e t t e r agreement w i t h t h e s i m u l a t e d s p e c t r a o b t a i n e d w i t h our model i n which
AH(n,rn)=l/3 &(n,o)
&LCB p o s i t i v e l i k e i n t h e Y(Fe,A1)2 system. However, t h e observed s p e c t r a cannot b e d e s c r i b e d by t h e simple l i n e a r model i n which AH(n,O) = nAH(1,O) a s i n t h e c a s e of t h e Y (Fe ,A1) sys tem. Furthermore AH( I ,O),
AH(2,O) a . s.0. i n c r e a s e w i t h i n c r e a s i n g Mn concen- t r a t i o n . Some of t h e s i m u l a t e d s p e c t r a a r e shown i n f i g u r e 2 , w h i l e t h e AH(n,O) v a l u e s used a r e given i n t a b l e I . The maximum v a l u e of AH(n,O) i s approxi- mately e q u a l t o t h e v a l u e AH(1,O) i n t h e Y (Fe,A1)2
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980160
(21-198 JOURNAL DE PHYSIQUE
I
I I 1-'-- 6 - 4 - 2 0 2 4 6
DOPPLER VELOCITY m m s-'
F i g . 1: ~ E s s b a u e r s p e c t r a o f some Y ( F e l - x ~ n x ) 2 com- pounds a t T = 4.2 K.
s y s t e m / 4 / which v a l u e i s e q u a l t o 116
*
H(0,O). As i n t h e c u b i c Laves p h a s e compounds a Fe atom h a s s i x n e i g h b o u r i n g atoms i n t h e f i r s t c o o r d i n a t i o n s h e l l , t h e v a l u e AH(1,O) i n t h e Y (Fe,A1)2 compounds c a n b e c o n s i d e r e d a s t h e l a r g e s t p o s s i b l e magnetic i n - f l u e n c e o f a s u b s t i t u t e d atom on t h e h y p e r f i n e ' f i e l d a t a Fe n u c l e u s . Although computer s i m u l a t i o n of~ E s s b a ~ e r s p e c t r a can n o t proof t h a t t h e u s e d model d e s c r i b e d t h e o b s e r v e d s p e c t r a i n t h e r i g h t way, we b e l i e v e t h a t i t s t i l l g i v e s i n s i g h t i n t h e main f e a t u r e s o f t h e i n v e s t i g a t e d compounds. I t i s ob- v i o u s t h a t t h e d i s c r e t e v a l u e s of AH(n,o) g i v e n i n t a b l e I , a r e a rough a p p r o x i m a t i o n of t h e r e a l i t y , however t h e y c l e a r l y show t h a t t h e Y (Fe ,Mn) s y s t e m t e n d s t o t h e Y (Fe,A1)2 system b y i n c r e a s i n g Mn con- c e n t r a t i o n .
As a Mn atom i n YMn2 h a s no magnetic moment we suppose t h a t t h e non l i n e a r composition dependence of t h e mean magnetic h y p e r f i n e f i e l d a t a Fe n u c l e u s is due t o an i n c r e a s i n g magnetic moment a t t h e Mn atom by an i n c r e a s i n g Fe c o n c e n t r a t i o n .
F i n a l l y , we h a v e found t h a t t h e s u b s t i t u t i o n of Fe by Mn i s not, f u l l y random. D e v i a t i o n s of t h e random d i s t r i b u t i o n h a * c l e a r l y b e e n found i n t h e
DOPPLER VELOCITY mm s-'
Fig.2: Computer s i m u l a t e d MEssbauer s p e c t r a o f some
Y (Fe compounds.
c o n c e n t r a t i o n r a n g e x = 0.035 up t o x = 0.30 i n s u c h a way t h a t t h e p r o b a b i l i t y P(0,O) i s approxi- m a t e l y twice a s much a s f o l l o w s from a random d i s - t r i b u t i o n of Mn atoms.
T a b l e I.
AH(l.0) AH(2,O) AH(3,O) aH(4,O) aH(5,O)
(kOe) (kOe) (kOe) (kOe) W e )
x P 0 + 0 . 4 0 11.5 17 23 34.5 34.5
R e f e r e n c e s
=
9J
/
1'1 Van d e r Kraan, A.M. and Gubbens,
.P. C.M.,
J .Phys.
C6(1974)469.
-
/ 2 / L u i j p e n , M.G., Gubbens, P.C.M., Van d e r Kraan, A.M. a& Buschow, K.H.J., P h y s i c a 86-88B(1977)
141.
/ 3 / Corson, M.R., Kolk, B., Hoy,G., Zimmerman,G.O.,
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Van d e r Kraan, A.M. and Gubbens,
P. C .M.,
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