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ELECTRONIC PROPERTIES OF Er3+ IN ErRh4B4
B. Dunlap, G. Shenoy, F. Fradin, C. Kimball
To cite this version:
JOURNAL DE PHYSIQUE
Cofioque
C6,
suppfPment au
no
8 , Tome 39, aolif
1978,
page
C6-379
ELECTRONIC PROPERTIES OF E r 3 +
I N
ErRh4B4B.D. Dunlap, G.K. Shenoy, F.Y. Fradin and C.W. Kimballt
Argonne National Laboratory, Argonne, I l l i n o i s
60439,USA
t Nothern I l l i n o i s University, De Kalb, I l l i n o i s , USA
R6sumd.- La c o n f i g u r a t i o n d l e c t r o n i q u e de ~ rdans l e supraconducteur t e r n a i r e ErRh4BL, ~ + a Q t d Q t u d i 6 e p a r s p e c t r o s c o p i e Mijssbauer d e 1 6 6 ~ r . Le niveau fondamental d e champ c r i s - t a l l i n e s t un d o u b l e t comportant une c o n t r i b u t i o n d ' e n v i r o n 90 % d e l a c o n f i g u r a t i o n
= k 15/2>.Le moment d e l ' i o n Er e s t 8 , 1 +_ 0 , 3
vB.
Les e f f e t s d e r e l a x a t i o n paramagnc- t l q u e observ6s aux b a s s e s temp6ratures s u g g s r e n t l a pr6sence d ' a u t r e s niveaux d e champ c r i s t a l l i n 5 d e s Q n e r g i e s f a i b l e s .A b s t r a c t . - The e l e c t r o n i c s t a t e of ~ r i n t h e t e r n a r y superconductor ErRhqB4has been in- ~ + v e s t i g a t e d u s i n g t h e Mossbauer e f f e c t i n 1 6 6 d r . The c r y s t a l f i e l d ground s t a t e i s found t o be a d o u b l e t , about 90 % of which i s t h e s t a t e
I J
=*
15/2>. The moment on t h e E r i o n i s 8.1 f 0 . 3uB.
E f f e c t s of t h e paramagnetic r e l a x a f i o n a t low temperatures i n d i c a t e t h e presence of o t h e r low l y i n g c r y s t a l - f i e l d l e v e l s .Much i n t e r e s t has r e c e n t l y been shown i n t h e compound ErRh4B4. This m a t e r i a l shows a super- conducting t r a n s i t i o n a t T = 8 . 5 K i n s p i t e o f t h e l a r g e c o n c e n t r a t i o n of paramagnetic ~ ri o n s ~ + p r e s e n t i n t h e m a t e r i a l . However, due t o t h e s e i o n s a magnetic t r a n s i t i o n i s reached a t Tm = 0.95 K , a t which p o i n t t h e s u p e r c o n d u c t i v i t y i s quenched / I / . Recent neutron d i f f r a c t i o n measurements have i n d i c a t e d t h a t t h e compound i s i n a f e r r o m a g n e t i c s t a t e below T w i t h t h e E r ions having moments
m'
only about 60 % of t h e free-ion v a l u e , and l y i n g ~ e r p e n d i c u l a r t o t h e t e t r a g o n a l c a x i s / 2 / . Here we r e p o r t microscopic measurements o b t a i n e d by t h e Mossbauer e f f e c t of 1 6 6 ~r i n ErRhqB4 which f u l l y i d e n t i f y t h e e l e c t r o n i c s t a t e of t h e E r i o n and c l a r i f y t h e r o l e of c r y s t a l f i e l d s and exchange i n t e r a c t i o n s i n determining t h e ground s t a t e .
The c r y s t a l s t r u c t u r e f o r t h e r a r e - e a r t h rhodium b o r i d e s has been obtained by x-ray d i f f r a c - t i o n techniques /3/ and v e r i f i e d by n e u t r o n d i f f r a c - t i o n / 2 / . I n t h e s e m a t e r i a l s t h e r a r e - e a r t h i o n s form a body centered t e t r a g o n a l l a t t i c e , w i t h each i o n being surrounded by elongated t e t r a h e d r a of Rh and B i o n s . The p o i n t symmetry f o r t h e r a r e - e a r t h
-
s i t e i s 42 m. This s t r u c t u r e w i l l t h e r e f o r e have a c r y s t a l f i e l d Hamiltonian
H~~ = B ~ o ~ + B ~ o ~ + B ~ o ~ + B (1) ~ o ~ + B ~ o ~
where t h e Onm a r e Stevens o p e r a t o r s . An elementary point-charge c a l c u l a t i o n u s i n g t h e known s t r u c t u r e i n d i c a t e s t h a t t h e c r y s t a l f i e l d l e v e l s a r e s t r o n g - l y governed by t h e B; term. I f t h i s i s t h e s o l e c o n t r i b u t i o n , t h e 4 ~ 1 5/2ground s t a t e of an ~ ri o n ~ +
w i l l s p l i t i n t o d o u b l e t s which we l a b e l
I
*Jz>. Mijssbauer s p e c t r a taken w i t h t h e 1 6 6 - ~ r resonance f o r any one of t h e s e d o u b l e t s w i l l be a t h r e e - l i n e asymmetric quadrupole p a t t e r n i n t h e l i m i t of f a s t e l e c t r o n i c r e l a x a t i o n , i . e . , when t h e e l e c t r o n i c r e l a x a t i o n frequency i s small compared t o t h e hy- p e r f i n e f r e q u e n c i e s . I n t h e l i m i t of slow r e l a x a - t i o n , a l l b u t one of t h e d o u b l e t s w i l l g i v e a f i v e l i n e spectrum w i t h e q u a l i n t e n s i t i e s i n each l i n e and w i t h a s p l i t t i n g determined b y t h e v a l u e of J Z . The e x c e p t i o n i s t h e case of 1i 1/2> where t h e f a c t t h a t t h e e l e c t r o n i c g v a l u e s a r e i s o t r o p i c , gx = gy = g Z , c a u s e s t h e spectrum t o be two l i n e s w i t h unequal i n t e n s i t i e s .S p e c t r a shown i n F i g u r e 1 , have been o b t a i - ned a t 4.2 K and 1.5 K. I n b o t h c a s e s , t h e s e tempe- r a t u r e s a r e below t h e superconducting t r a n s i t i o n b u t above t h e magnetic t r a n s i t i o n temperature. The f i v e - l i n e s p e c t r a observed i d e n t i f y t h e c r y s t a l - f i e l d ground s t a t e t o b e d o u b l e t w i t h
a
h i g h l y a n i s o t r o p i c g 'censor. From t h e observed l i n e p o s i - t i o n s we o b t a i n a magnetic h y p e r f i n e c o n s t a n t A = (3639+
80) MHz, and an e l e c t r i c quadrupole i n t e r a c t i o n e Z q 9 : (773*
15) MHz. Using known v a l u e s f o r t h e n u c l e a r moments of l 6 E r,
t h e 4f r a d i a l i n t e g r a l s <r-3> and assuming t h e l a t t i c e c o n t r i b u t i o n t o t h e e l e c t r i c f i e l d g r a d i e n t t o be s m a l l , b o t h of t h e s e h y p e r f i n e parameters a r e produced w i t h i n experimental e r r o r by a ground s t a t e wave f u n c t i o nY = 0.942 15/2>
+
0.335 It 712, (2)References
F i g
-60 -40 -20 0 20 40 60
VELOCITY (mrnls)
.
I : Ryperfine s p e c t r a obtained u s i n g t h e lS6 E r Mijssbauer resonances i n ErRh4B4 a t 4.21.5 K.
Here t h e predominent component 1+15/2> a r i s e s from
0 0
t h e B202 term i n Equation (1) when B; < 0 , and t h e
If 7 / 2 > may be considered t o be an admixture a r i - s i n g p r i m a r i l y from t h e h i g h e s t o r d e r o f f - d i a g o n a l
0 0
term, B409
.
The magnetic moment f o r ~ ri n t h e ~ + c r y s t a l f i e l d s t a t e of Equation (2) i s 8.1 pB, i n comparison w i t h a f r e e - i o n v a l u e of 9uB.
T h i s i s i n s h a r p c o n t r a s t w i t h t h e v a l u e of 5.6 p B o b t a i - ned by n e u t r o n - d i f f r a c t i o n i n t h e f e r r o m a g n e t i c s t a t e . D e v i a t i o n s from e q u a l l i n e i n t e n s i t i e s i n t h e s p e c t r a of F i g u r e 1 i n d i c a t e t h e p r e s e n c e of e f f e c t s due t o paramagnetic r e l a x a t i o n of t h e Er magnetic moments. For an e l e c t r o n i c s t a t e such a s o b t a i n e d h e r e , w i t h t h e g v a l u e ~ e r p e n d i c u l a r t o t h e c a x i s being z e r o , one can show t h a t r e l a x a t i o n e f f e c t s from s p i n - s p i n i n t e r a c t i o n s o r from Korringa coupling w i t h t h e conduction e l e c t r o n s a r e n o t e f - f e c t i v e . A t t h e s e t e m p e r a t u r e s , phonon induced t r a n s i t i o n s a r e not expected t o be l a r g e . The most l i k e l y mechanism f o r producing r e l a x a t i o n e f f e c t s i s then p r o c e s s e s going through e x c i t e d c r y s t a l f i e l d s t a t e s . The f a c t t h a t ~ u c h ' ~ h e n o m e n a a r e seen even a t 1.5 K i n d i c a t e s t h a t t h e f i r s t e x c i t e d c r y s t a l f i e l d l e v e l i s w i t h i n a few d e g r e e s of t h e ground s t a t e . A d d i t i o n a l e x p e r i m e n t a l work and cal- c u l a t i o n s a r e underway t o c l a r i f y t h i s p o i n t , a s w e l l a s t h e n a t u r e of t h e Er i o n i n t h e m a g n e t i c a l l y ordered s t a t e ./ I / F e r t i g , W.A., J o h n s t o n , D . C . , Delong, L . E . , McCallum, R.W., Maple, M.B. and M a t t h i a s , B.T. Phys. 9ev. L e t t .
2
(1977) 987./ 2 / Moncton, D . E . , McWhen, D.B., E c k e r t , J . , Shi- r a n e , G . and Thomlinson, W . , Phys. Rev. L e t t . 39 (1977) 1164.
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