- Email: [email protected]
NMR evidence for localized moments in CoAl
Volume 30A, n u m b e r 2
PHYSICS
LETTERS
C l h e r e d e n o t e s t h e s h i f t of t h e K ~ . l i n e w i t h c o m p l e t e r e m o v a l of o n e v a l e n c e e ~ e c t r o n of l - type. In t a b l e 1 w e c o m p a r e t h e e x p e r i m e n t a l c o n stants Cl with those calculated theoretically$ [ 6 - 9 ] w i t h i n t h e f r a m e w o r k of s e l f - c o n s i s t e n t m o d e l s (of t h e H a r t r e e - F o c k t y p e ) . T h e a g r e e ment observed is particularly good in the case of 4 f - e l e c t r o n s . T h e d e p e n d e n c e of t h e e f f e c t u p o n t h e d i s t r i b u t i o n of m l e n a b l e s u s to u s e i t a s a n e x p e r i mental method for determing valence configurat i o n s of c r y s t a l c h e m i c a l b o n d s [2].
References 1. o . I . Sumbaev, A . F . Mezentsev, V.I. Marushenko, E.V. P e t r o v i c h and A.S. Rylnikov, Zh. Eksp. i T e o r . Fiz., 50 (1966) 861; Soy. Phys. - J E T P 23 (1966) 572 2. O.I. Sumbaev. E . V . P e t r o v i c h , Yu. P. Smirnov, A.I. Egorov, V . S . Z y k o v and A . I . G l u s h k o , Zh. Eksp. i Teor. Fiz., 53 (1967) 796; Sov. Phys. - J E T P 26 (1968) 489; 26 (1968) 891; 53 (1967) 1545; 55 (1968) 745; 56 (1969) 536. I n t e r n . Symp. on X - R a y s p e c t r a and e l e c t r o n i c s t r u c t u r e of substance, Kiev, 1968, T h e s e s of R e p o r t s , p. 67. 3. B. G. Gokhale, R . B . C h e s l e r and F. Boehm, Phys. Rev. L e t t e r s 18 (1967) 957. 4. L. Puling, The n a t u r e of the c h e m i c a l bond, ( C o r n e l l 1942 .) 5. S . S . B a t s a n o v , E l e c t r o n e g a t i v i t y of the E l e m e n t s and the Chemical Bond, (in R u s s i a n ) , Siberian Sec. AN SSSR (1962) p. 65. 6. J. T. Waber, p r i v a t e communication; D. L i b e r m a n , J . T . W a b e r and D . T . G r o m e r , Phys. Rev. 137A (1965) 27. 7. E . C l e m e n t i , J . C h e m . P h y s . 41 (1964) 295, 303. 8. C . J . R o o t h a a n and M. Synek, P h y s . Rev. 133A (1964) 1263. 9. Ch. F. F i s c h e r , p r i v a t e communication and Canad. J. Phys. 41 (1963) 1895.
T h e a u t h o r s i s g r a t e f u l t o P r o f . J. T. W a b e r w h o k i n d l y s e n t u s t h e r e s u l t s of c a l c u l a t i o n s f o r r a r e e a r t h e l e m e n t s a n d t o P r o f . Ch. F. Fischer for carrying out the calculations for molybdenum.
$Cl = (els-~ - e2p~)/-ion - (els-~ - £2p~)atom •
NMR
EVIDENCE
FOR
22 September 1969
LOCALIZED
MOMENTS
IN
CoAl*
D. O. V A N O S T E N B U R G , J . J . SPOKAS, C . H . S O W E R S Argonne National Laboratory, Argonne, Illinois 60115 and H. G. H O E V E Northern Illinois University, DeKalb, Illinois 60115 Received 1 July 1969
The t e m p e r a t u r e dependence of the nuclear T 1 of 27A1 in CoAl shows a deviation f r o m the K o r r i n g a r e l a tion. According to the work of Giovannini and Heeger this is suggestive of the p r e s e n c e of localized m a g netic m o m e n t s .
Much information on the electronic structure of e q u i a t o m i c g r o u p VIII t r a n s i t i o n m e t a l a l u m i n u m c o m p o u n d s (CsC1 s t r u c t u r e ) c a n b e o b t a i n e d through nuclear magnetic resonance techniques. Previous investigations on the Knight shift, linewidth, line intensity and magnetic susceptibility i n s e v e r a l of t h e s e m a t e r i a l s h a v e b e e n r e p o r t e d [1]. T h e c u r r e n t w o r k h a s e x t e n d e d t h e s e m e a surements to nuclear T 1 studies and in addition
to N M R measurements of the other compounds in this series [2]. The compound which is exceptional in its T I behavior is CoAl. The Al TI'S were determined at four different tern-" peratures from 4.2°K to 300OK and are listed in table 1. These results can be represented by
*Work p e r f o r m e d under the auspices of the U.S. Atomic Energy Commission.
w h e r e A = 0.001 ( s e c ° K ) - I a n d B = 0.05 s e c -1. This expression is similar to that recently de-
130
1 / T 1 T =A + B I T
(1)
PHYSICS LETTERS
Volume 30A, number 2
Table 1. T1 T of 27AI in CoAl at various temperatures. T1 T(see°IQ 8.6 :e 1 60 ±2 105 :e 5 140 :e 5
T(OK) 4.2 27 77 300
r i v e d by Giovannini et al. [3] for r e l a x a t i o n in the p r e s e n c e of a l o c a l i z e d m a g n e t i c moment. The i m p u r i t y c o n t r i b u t i o n to the r e l a x a t i o n r a t e is given by (1/T1)im p
6 J ~BH~Sz)CZ (1/T1)Korring a = -~ff
(2)
w h e r e c is the i m p u r i t y c o n c e n t r a t i o n , z the n u m b e r of e l e c t r o n s p e r a t o m , Ef the F e r m i e n e r g y of the host, g the e l e c t r o n i c splitting factor, ~ B the Bohr m a g n e t o n , H the e x t e r n a l ly applied field, J the s - d exchange i n t e g r a l , and (Sz) the a v e r a g e i m p u r i t y e l e c t r o n spin. The c o n t r i b u t i o n to T~ ~ i s s e e n to be independent of field and t e m p e r a t u r e if (S z) cc H/T. Caskey et aL [4] r e p o r t r e s i s t i v i t y m i n i m a a r o u n d 30°K in CoAl a l l o y s c o n t a i n i n g a slight e x c e s s of Co. T h e i r r e s u l t s i n d i c a t e that the alloy can be r e g a r d e d a s the o r d e r e d m e t a l l i c compound CoAl in which the e x c e s s Co i s a s u b s t i t u t i o n a l i m p u r i t y ( s u b s t i t u t i n g for A1). Mtyatani et al. [5] m e a s u r e d the m a g n e t i c s u s c e p t i b i l i t i e s of CoAl a l l o y s and r e p o r t for s a m p l e s c o n t a i n i n g Co in s l i g h t e x c e s s of 50 at. %, a s u s c e p t i b i l ity which v a r i e s with t e m p e r a t u r e and c o n c e n t r a t i o n . The s a m p l e u s e d i n this i n v e s t i g a t i o n cont a i n s a p p r o x i m a t e l y 50.5 at. % Co. It was p r e p a r e d i n a n a r c f u r n a c e with A1 and Co of at l e a s t 5N and 3N p u r i t y , r e s p e c t i v e l y . The s a m ple was then h o m o g e n i z e d in a n a r g o n a t m o s p h e r e at 1000°C for 4 days and p u l v e r i z e d with a tungs t e n m o r t a r and pestle. F i n a l l y , the powder was a n n e a l e d at 900°C for 24 h o u r s in o r d e r to remove strains. In the work of Miyatani, the compound which m o s t c l o s e l y r e s e m b l e s the one u s e d i n this
22 September 1969
study is a s a m p l e containing 50.4 at. % Co. F o r this compound X = C/TwithC ~ 2 × 1 0 -3 e m u °K/g. A C u r i e - t y p e b e h a v i o r for × i s r e q u i r e d i n o r d e r that the i m p u r i t y c o n t r i b u t i o n to the r e l a x a t i o n r a t e behave as r e p r e s e n t e d by eq. (1). C o n s e quently, as suggested by Caskey et al. [4] CoAl with s m a l l e x c e s s a m o u n t s of Co b e h a v e s as an i n t e r m e t a l l i c compound with the p r e s e n c e of localized m a g n e t i c m o m e n t s . F o r this compound, p r e c i s e v a l u e s for E f and J a r e not known, howe v e r , taking Ef ~ 10 eV, g = 2. c = 0.005 and z = 1 gives J ~ 8 eV. 'rne T 1 data follow eq. (2) both above and below the t e m p e r a t u r e of the r e sistivity minimum. The r e l a x a t i o n r a t e for 59Co in CoAl is m o r e than 300 t i m e s that of 27A1 and i s p r o b a b l y due to an o r b i t a l c o n t r i b u t i o n at the Co s i t e s . I t ' s t e m p e r a t u r e dependence is of the K o r r i n g a type except at t e m p e r a t u r e s below 25°K. If T 1 for Co a l s o would have a t e r m independent of t e m p e r a t u r e f r o m 4.2°K to 300°K, it is p r o b a b l y masked. The o b s e r v a t i o n of the 59Co s i g n a l i n dicates that the l o c a l i z e d m o m e n t s do not e x i s t at all Co sites. Below ~ 77°K, the 27A1 r e l a x a t i o n t i m e for the r e m a i n i n g compounds of this s e r i e s , that is, FeAI, NIA1, RuA1, Rb_A1, OsA1 and IrAl, a l s o deviates f r o m the K o r r i n g a r e l a t i o n . T h i s d i s p a r i t y is s i m i l a r to that found i n CoAl but is much l e s s pronounced. A m o r e detailed study of this p h e n o m e n o n i s now i n p r o g r e s s .
References 1. G.W. West, Phil. Mag. 9 (1964)979; Phil. Mag. 15 (1967) 855; J.A. Seitchik and R.H. Walmsley, Phys. Rev. 131 (1953) 1473; Phys. Rev. 137 (1965) A143; L . E . D r a i n , AERE-PR/SSP 10 (1966) 10; M. H~hl, Ann. der Physik 19 (1967) 15. 2. To be published. 3. B. Giovannini and A.J. Heeger, Solid State Comm. 7 (1969) 287. 4. G.R. Caskey and D, J. Sellmyer. J. Appl. Phys. 40 (1969) 1476. 5. K. Miyatani and S.Iida. J. Phys. Soc. Japan 25 (1968) 1008.
* * * * *
131
PHYSICS
LETTERS
C l h e r e d e n o t e s t h e s h i f t of t h e K ~ . l i n e w i t h c o m p l e t e r e m o v a l of o n e v a l e n c e e ~ e c t r o n of l - type. In t a b l e 1 w e c o m p a r e t h e e x p e r i m e n t a l c o n stants Cl with those calculated theoretically$ [ 6 - 9 ] w i t h i n t h e f r a m e w o r k of s e l f - c o n s i s t e n t m o d e l s (of t h e H a r t r e e - F o c k t y p e ) . T h e a g r e e ment observed is particularly good in the case of 4 f - e l e c t r o n s . T h e d e p e n d e n c e of t h e e f f e c t u p o n t h e d i s t r i b u t i o n of m l e n a b l e s u s to u s e i t a s a n e x p e r i mental method for determing valence configurat i o n s of c r y s t a l c h e m i c a l b o n d s [2].
References 1. o . I . Sumbaev, A . F . Mezentsev, V.I. Marushenko, E.V. P e t r o v i c h and A.S. Rylnikov, Zh. Eksp. i T e o r . Fiz., 50 (1966) 861; Soy. Phys. - J E T P 23 (1966) 572 2. O.I. Sumbaev. E . V . P e t r o v i c h , Yu. P. Smirnov, A.I. Egorov, V . S . Z y k o v and A . I . G l u s h k o , Zh. Eksp. i Teor. Fiz., 53 (1967) 796; Sov. Phys. - J E T P 26 (1968) 489; 26 (1968) 891; 53 (1967) 1545; 55 (1968) 745; 56 (1969) 536. I n t e r n . Symp. on X - R a y s p e c t r a and e l e c t r o n i c s t r u c t u r e of substance, Kiev, 1968, T h e s e s of R e p o r t s , p. 67. 3. B. G. Gokhale, R . B . C h e s l e r and F. Boehm, Phys. Rev. L e t t e r s 18 (1967) 957. 4. L. Puling, The n a t u r e of the c h e m i c a l bond, ( C o r n e l l 1942 .) 5. S . S . B a t s a n o v , E l e c t r o n e g a t i v i t y of the E l e m e n t s and the Chemical Bond, (in R u s s i a n ) , Siberian Sec. AN SSSR (1962) p. 65. 6. J. T. Waber, p r i v a t e communication; D. L i b e r m a n , J . T . W a b e r and D . T . G r o m e r , Phys. Rev. 137A (1965) 27. 7. E . C l e m e n t i , J . C h e m . P h y s . 41 (1964) 295, 303. 8. C . J . R o o t h a a n and M. Synek, P h y s . Rev. 133A (1964) 1263. 9. Ch. F. F i s c h e r , p r i v a t e communication and Canad. J. Phys. 41 (1963) 1895.
T h e a u t h o r s i s g r a t e f u l t o P r o f . J. T. W a b e r w h o k i n d l y s e n t u s t h e r e s u l t s of c a l c u l a t i o n s f o r r a r e e a r t h e l e m e n t s a n d t o P r o f . Ch. F. Fischer for carrying out the calculations for molybdenum.
$Cl = (els-~ - e2p~)/-ion - (els-~ - £2p~)atom •
NMR
EVIDENCE
FOR
22 September 1969
LOCALIZED
MOMENTS
IN
CoAl*
D. O. V A N O S T E N B U R G , J . J . SPOKAS, C . H . S O W E R S Argonne National Laboratory, Argonne, Illinois 60115 and H. G. H O E V E Northern Illinois University, DeKalb, Illinois 60115 Received 1 July 1969
The t e m p e r a t u r e dependence of the nuclear T 1 of 27A1 in CoAl shows a deviation f r o m the K o r r i n g a r e l a tion. According to the work of Giovannini and Heeger this is suggestive of the p r e s e n c e of localized m a g netic m o m e n t s .
Much information on the electronic structure of e q u i a t o m i c g r o u p VIII t r a n s i t i o n m e t a l a l u m i n u m c o m p o u n d s (CsC1 s t r u c t u r e ) c a n b e o b t a i n e d through nuclear magnetic resonance techniques. Previous investigations on the Knight shift, linewidth, line intensity and magnetic susceptibility i n s e v e r a l of t h e s e m a t e r i a l s h a v e b e e n r e p o r t e d [1]. T h e c u r r e n t w o r k h a s e x t e n d e d t h e s e m e a surements to nuclear T 1 studies and in addition
to N M R measurements of the other compounds in this series [2]. The compound which is exceptional in its T I behavior is CoAl. The Al TI'S were determined at four different tern-" peratures from 4.2°K to 300OK and are listed in table 1. These results can be represented by
*Work p e r f o r m e d under the auspices of the U.S. Atomic Energy Commission.
w h e r e A = 0.001 ( s e c ° K ) - I a n d B = 0.05 s e c -1. This expression is similar to that recently de-
130
1 / T 1 T =A + B I T
(1)
PHYSICS LETTERS
Volume 30A, number 2
Table 1. T1 T of 27AI in CoAl at various temperatures. T1 T(see°IQ 8.6 :e 1 60 ±2 105 :e 5 140 :e 5
T(OK) 4.2 27 77 300
r i v e d by Giovannini et al. [3] for r e l a x a t i o n in the p r e s e n c e of a l o c a l i z e d m a g n e t i c moment. The i m p u r i t y c o n t r i b u t i o n to the r e l a x a t i o n r a t e is given by (1/T1)im p
6 J ~BH~Sz)CZ (1/T1)Korring a = -~ff
(2)
w h e r e c is the i m p u r i t y c o n c e n t r a t i o n , z the n u m b e r of e l e c t r o n s p e r a t o m , Ef the F e r m i e n e r g y of the host, g the e l e c t r o n i c splitting factor, ~ B the Bohr m a g n e t o n , H the e x t e r n a l ly applied field, J the s - d exchange i n t e g r a l , and (Sz) the a v e r a g e i m p u r i t y e l e c t r o n spin. The c o n t r i b u t i o n to T~ ~ i s s e e n to be independent of field and t e m p e r a t u r e if (S z) cc H/T. Caskey et aL [4] r e p o r t r e s i s t i v i t y m i n i m a a r o u n d 30°K in CoAl a l l o y s c o n t a i n i n g a slight e x c e s s of Co. T h e i r r e s u l t s i n d i c a t e that the alloy can be r e g a r d e d a s the o r d e r e d m e t a l l i c compound CoAl in which the e x c e s s Co i s a s u b s t i t u t i o n a l i m p u r i t y ( s u b s t i t u t i n g for A1). Mtyatani et al. [5] m e a s u r e d the m a g n e t i c s u s c e p t i b i l i t i e s of CoAl a l l o y s and r e p o r t for s a m p l e s c o n t a i n i n g Co in s l i g h t e x c e s s of 50 at. %, a s u s c e p t i b i l ity which v a r i e s with t e m p e r a t u r e and c o n c e n t r a t i o n . The s a m p l e u s e d i n this i n v e s t i g a t i o n cont a i n s a p p r o x i m a t e l y 50.5 at. % Co. It was p r e p a r e d i n a n a r c f u r n a c e with A1 and Co of at l e a s t 5N and 3N p u r i t y , r e s p e c t i v e l y . The s a m ple was then h o m o g e n i z e d in a n a r g o n a t m o s p h e r e at 1000°C for 4 days and p u l v e r i z e d with a tungs t e n m o r t a r and pestle. F i n a l l y , the powder was a n n e a l e d at 900°C for 24 h o u r s in o r d e r to remove strains. In the work of Miyatani, the compound which m o s t c l o s e l y r e s e m b l e s the one u s e d i n this
22 September 1969
study is a s a m p l e containing 50.4 at. % Co. F o r this compound X = C/TwithC ~ 2 × 1 0 -3 e m u °K/g. A C u r i e - t y p e b e h a v i o r for × i s r e q u i r e d i n o r d e r that the i m p u r i t y c o n t r i b u t i o n to the r e l a x a t i o n r a t e behave as r e p r e s e n t e d by eq. (1). C o n s e quently, as suggested by Caskey et al. [4] CoAl with s m a l l e x c e s s a m o u n t s of Co b e h a v e s as an i n t e r m e t a l l i c compound with the p r e s e n c e of localized m a g n e t i c m o m e n t s . F o r this compound, p r e c i s e v a l u e s for E f and J a r e not known, howe v e r , taking Ef ~ 10 eV, g = 2. c = 0.005 and z = 1 gives J ~ 8 eV. 'rne T 1 data follow eq. (2) both above and below the t e m p e r a t u r e of the r e sistivity minimum. The r e l a x a t i o n r a t e for 59Co in CoAl is m o r e than 300 t i m e s that of 27A1 and i s p r o b a b l y due to an o r b i t a l c o n t r i b u t i o n at the Co s i t e s . I t ' s t e m p e r a t u r e dependence is of the K o r r i n g a type except at t e m p e r a t u r e s below 25°K. If T 1 for Co a l s o would have a t e r m independent of t e m p e r a t u r e f r o m 4.2°K to 300°K, it is p r o b a b l y masked. The o b s e r v a t i o n of the 59Co s i g n a l i n dicates that the l o c a l i z e d m o m e n t s do not e x i s t at all Co sites. Below ~ 77°K, the 27A1 r e l a x a t i o n t i m e for the r e m a i n i n g compounds of this s e r i e s , that is, FeAI, NIA1, RuA1, Rb_A1, OsA1 and IrAl, a l s o deviates f r o m the K o r r i n g a r e l a t i o n . T h i s d i s p a r i t y is s i m i l a r to that found i n CoAl but is much l e s s pronounced. A m o r e detailed study of this p h e n o m e n o n i s now i n p r o g r e s s .
References 1. G.W. West, Phil. Mag. 9 (1964)979; Phil. Mag. 15 (1967) 855; J.A. Seitchik and R.H. Walmsley, Phys. Rev. 131 (1953) 1473; Phys. Rev. 137 (1965) A143; L . E . D r a i n , AERE-PR/SSP 10 (1966) 10; M. H~hl, Ann. der Physik 19 (1967) 15. 2. To be published. 3. B. Giovannini and A.J. Heeger, Solid State Comm. 7 (1969) 287. 4. G.R. Caskey and D, J. Sellmyer. J. Appl. Phys. 40 (1969) 1476. 5. K. Miyatani and S.Iida. J. Phys. Soc. Japan 25 (1968) 1008.
* * * * *
131