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Resistivity and magnetic measurements on the CePt ferromagnet
~
S o l i d State C o m n u n i c a t i o n s , Printed in Great Britain.
Vo1.45,No.9,
pp.779-781,
0038-1098/83/090779-03503.00/0
1983.
Pergamon Press Ltd.
RESISTIVITY AND MAGNETIC MEASUREMENTS ON THE CePt FERROMAGNET D. G£ gnoux Laboratoire Louis N~el, C.N.R.S.,
166X, 38042 Grenoble-c~dex, France
and J.C. Gomez-Sal Departamento de Optica y Estructura de la Hateria, Fac. de Ciencia~, Santander, Spain Received 21 December 1982 by E.F. Bertaut
I n t h i s p a p e r we p r e s e n t r e s i s t i v i t y m e a s u r e m o n t s p e r f o r m e d on t h e CePt f e r r o m a g n e t (T c = 5 . 8 K) a n d on L a P t , t o g e t h e r w i t h m a g n e t i z a t i o n m e a s u r e m e n t s p e r f o r m e d on CePt a t low t e m p e r a t u r e s . The m a g n e t i c r e s i s t i v i t y o f CePt d e c r e a s e s as l n ( T ) when t e m p e r a t u r e i s i n c r e a s e d a b o v e 220 K. T h i s r e s u l t u s o o i a t e d w i t h a r e d u c t i o n o f t h e Ce m a g n e t i c moment a n d o f t h e m a g n e t i c e n t r o p y a t T c s u g g e s t s t h a t CePt c o u l d be a Kondo system.
Introduction
shown on t a b l e I. The c p a r a m e t e r d o e s n o t v a r y m o n o t o n o u s l y a s a f u n c t i o n o£ t h e a t o m i c n u m b e r b u t t h e v o l u m e o f t h e u n i t c e l l o f CePt i s i n accordance with that of the other compounds in which the rare earth ions are in the trivalent state. Magnetizatic- v e ' ~ u s the ~ p l i o d field. measured on CePt at 1.5, 4.2, 6 and I0 K, are drawn on figure I. In agreement with previous w o r k s 4 , 5 , t h i s compound i s p a r a m a g n e t i c a t 6 a n d l0 K. At 6 K we a r e v e r y c l o s e t o T c ( 5 . 8 K) and the initial susceptibility is still very high. At 4.2 K and 1.5 K a spontaneous magnetization a p p e a r s ; h o w e v e r , i n f i e l d s h i g h e r t h a n 2 kOe a s t r o n g f i e l d d e p e n d e n c e o f t h e m a g n e t i zation is obsorved. For that reason, the determination of the spontaneous magnetization leads to d i f f i c u l t i e s . The e s t i m a t e d v a l u e s a t 1 . 5 K and 4 . 2 K a r e 0 . 7 3 ± 0 . 0 2 ~B and 0 . 6 1 ~ 0 . 0 2 UB per cerium atom respectively.
In 1965 Dwight et al I showed that LaPt and CePt crystallize in the CrB-type orthorhombic structure (space group Cmcm) while the other equiatomic rare earth-platinum compounds crystallize in t h e F o B - t y p e orthorhombic s t r u c t u r e . H o w e v e r , i n 1978 Le Roy e t a l 2 h a v e s h o w n t h a t s a m p l e s of P r P t and NdPt, quenched from high temperature, n a v e t h e CrB-type s t r u c t u r e . These two l a s t compounds are f e r r o m a g n e t i c with C u r i e t e m p e r a t u r e s o f 15 K and 23 K r e s p e c t l v e l y 3 . Their magnetic structures d e t e r m i n e d by n e u C r o n diffraction are collinear3. I n 1978 H u b e r e t al 4 pointed out that CePt i s f e r r o m a g n e t i c with a Curie temperature of 5.8 K which was confirmed later by speciiic heat measurements 5. Such a ferromagnetic behaviour is quite unusual with cerium. In the p r e s c ~ paper we present resistivity measurements performed on CePt and LaPt together with magnetization measurements performed on CePt at low t e m p e r a t u r e .
Table
Experimental
I
C o m p a r i s o n of the cell p a r a m e t e r s LaPt, CePt, P r P t and N d P t (CrB-type structure).
The rare earth elements and the platinum used were 99.9 % and 99.99 % pure respectively. Polycrystalline sar-Jples ~ e r e induction malted in a cold c r u c i b l e . M a g n e t i c measurements were p e r f o r m e d a t t e m p e r a t u r e s down t o 1.5 K a n d i n f i e l d s up t o 60 kOe p r o d u c e d by a s u p e r c o n d u c t i n g coil. The rcsistivities were measured between 1.5 K and 300 K by an alternatingcurrent four probe method. The absolute precision is estimated to be about 4 %. If we compare the cell parameters given previously for LaFt and CePt on one hand I and for PrPt and NdPt on the other hand2t the unit cell volume found for CePt i s less t h a n t h a t e x p e c t e d f r o m t h e o t h e r c o m p o u n d s . We h a v e remeasured these parameters and the result is
bC j c X) vcX3
ref.
LaPt
3.974 3.972
II.037 11.023
4.558 4.548
199.92 199.13
i t h i s work
CePt
3.921 3.918
10.920 10.908
4.524 4.527
193.70 193.47
this
I work
3.891 3.884
10.899 10.724
4.569 4.572
193.76 190.43
this
2 work
3.846 3.858
10.769 10.692
4.542 4.551
188.12 187.73
this
2 work
PrPt NdPt
779
of
RESISTIVITY AND MAGNETIC MEASUREMENTS ON THE CePt FERROMAGNET
780
Vol. 4'5, No.
05 0.8
07
CePt
~
E u.
o..1
C
o;
to
'o
o~
,'0
~
~
e
W
b
APPLIED MAGNETIC FIELD (kOe)
Figure I
:
1C
¢
Field dependence o f the magnetization of CePt at low t e m p e r a t u r e s .
The t h e r m a l d e p e n d e n c e s of t h e r e a i s t i v i t i e s o f CePt and LaPt a r e shown i n f i g u r e 2. the r e s i s t i v i t y of LaPt i n c r e a s e s l i n e a r l y above 70 K i n agreement with the s c a t t e r i n g o f the e l e c t r o n s by phonon~. At low temperature t h e resistivity t e n d s r e g u l a r l y t o w a r d s Or when temperature is decreased. A strong discontinuity o f t h e r e s i s t i v y o f CePt i s o b s e r v e d a t t h e C u r i e t e m p e r a t u r e (T c = 5 . 8 K). The magnetic resistivity 0m of CePt was obtained by s u b t r a c t i n g t h e resistivity o f LaPt (dashed line in f i g u r e 2). Because of the s m a l l v a l u e of the o r d e r i n g t e m p e r a t u r e o f CePt t h e r e i s an u n c e r t a i n t y of the r e s i d u a l r e s i s t i v i t y determinat i o n and h e n c e t h e z e r o o f Pm i s n o t w e l l d e f i n e d . Pm i s a l m e s t c o n s t a n t i n a s m a l l t e m p e r a t u r e r a n g e j u s t above t h e C u r i e t e m p e r a t u r e and t h e n i n c r e a s e s s t r o n g l y . A b r o a d maximum o f 19.3 ± 0 . 2 ~QcJn i s o b s e r v e d a r o u n d 220 K. At higher t e m p e r a t u r e s t h e m a g n e t i c r e s i s t i v i t y d e c r e a s e s s l o w l y and r e a c h e s 17.9 ~ 0 . 2 ~Gcm a t 300 K. Within the experimental accuracy this d e c r e a s e i s l i n e a r a s a f u n c t i o n o f I ~ : T ) as shown i n f i g u r e 3. However t h i s r e s u l t m u s t be r e g a r d e d w i t h c a u t i o n b e c a u s e we h a v e a s s u m e d t h a t t h e phonon contribution t o t h e r e s i s t i v i t y of CePt is the same as in LaPt.
CePt C
%
LaPt
~00
~0
~00
~o
TEMPERATURE i g )
Fisure 2 : Thermal dependences of the resistivities of CePt and LaPt. Dashed line is the magnetic resistivity Om o f CePt.
°oJ
2'0
s'o
260
TEMPERATURE (K) Figure 3 : Magnetic resistivity o~ versus In(T) for CePt. Discussion From t h e v a l u e s o f t h e s p o n t a n e o u s magnet i z a t i o n m e a s u r e d a t 1.5 K and 4 . 2 K ( 0 . 7 3 z 0 . 0 2 UB and 0.61 +- 0 . 0 2 U8 r e s p e c t i v e l y ) it is possible using different extrapolation laws t o deduce a s p o n t a n e o u s ~ a g n e t i z a t i o o a t 0 K o f 0 . 7 4 z 0 . 0 2 ~B/Ce. T h i s v a l u e i s much s m a l l e r t h a n t h e f r e e i o n v a l u e f o r Ce3+ (gJJ~B = 2 . 1 4 UB)" T h i s r e d u c t i o n e s s e n t i a l l y r e s u l t s from t h e f a c t t h a t t h e m e a s u r e m e n t s were p e r f o r m e d on a p o l y c r y s t a l l l n e sample in w h i c h c r y s t a l f i e l d (CF) e f f e c t s l e a d t o a s t r o n g m a g n e t o c r y s t a l l i n e a n i s o t r o p y and t o a moment r e d u c t i o n . T h i s h a s a l r e a d y b e e n shown i n P r P t and NdPt 3. A s s u m i n g a s t r o n g a n i s o t r o p y and a c o l l l n e a r m a g n e t l c s t r u c t u r e i n w h i c h t h e Ce m a g n e t i c moments a r e , as i n P r P t , p a r a l l e l t o one o f t h e t h r e e main sywmetry a x e s ~ , ~ o r ~ , t h e m a g n e t i c moment on Ce m u s t be t w i c e t h a t d e d u c e d from m a g n e t i z a t i o n m e a s u r e m e n t s on a polycrystalline s a m p l e . Then t h e Ce m a g n e t i c moment i s o f t h e o r d e r o f 1.46 UB. The d i f f e r e n c e b e t w e e n t h i s v a l u e and t h e f r e e i o n one i s due t o t h e Ce moment r e d u c t i o n by CF and e v e n t u a l l y f o r a s m a l l p a r t t o a Kondo e f f e c t a n d / o r to a p o l a r i z a t i o n o f t h e c o n d u c t i o n electrons. Although the point charge model fails frequently to give quantitatively account for the CF effects in rare earth intermetaltic compounds, we have used it to have an indicaticm of these effects in CePt. With a 3+ charge on c e r i u m and no c h a r g e on p l a t i n u m CF e f f e c t s l e a d t o t h r e e d o u b l e t s at - 8 1 . 0 K, 37.5 K and 77.2 K w h i c h l e a d t o an o v e r a l l s p l i t t i n g o f 158.2 K. As w e l l , t h e e a s y m a g n e t i z a t i o n would be ~ and t h e n~agnetic moment i n t h e e s t i m a t e d m o l e c u l a r f i e l d (184 kOe) would be 1.97 ~B/Ce.
Vol. 45, No. 9
RESISTIVITY AND MAGNETIC MEASUREMENTS ON THE CePt FERROMACNET
The s t r o n g increase o f the magnetic resistivity of CePt above the Curie temperature r e s u l t s from CF e f f e c t s . A l t h o u g h i t i s d i f f i c u l t to d e t e r m i n e w i t h a g r e a t a c c u r a c y t h e magnetic resistivity a t high t e m p e r a t u r e , especially because t h e phonon c o n t r i b u t i o n is assumed t o be t h e same as i n L a P t , the o c c u r r e n c e of a maximum in this magnetic resistivity can be attributed to a Kondo effect on cerium. Generally, such a maximum is observed at a temperature of the order of the overall CF splitr i n g 6 , 7. Knowing t h a t g e n e r a l l y t h e p o i n t c h a r g e model g i v e s s m a l l e r v a l u e s f o r t h e CF p a r a m e t e r s t h a n t h e o b s e r v e d o n e s , t h e c a l c u l a t e d CF s p l i t t i n g (158.2 K) i s c o n s i s t e n t w i t h the t e m p e r a t u r e o f the maximum o f 0m (= 220 K). The ln(T) dependence o f t h e d e c r e a s e o f 0m b e t w e e n 220 and 300 K i s c o n s i s t e n t with the Kondo c o n t r i b u t i o n t o the electrical resistivity in cerium diluted alloys calculated by Cornut and Coqblin 7 in presence of CF. The existence of a Kondo effect in CePt is supported by the heat capacity measurements. Indeed, the entropy at Tc, deduced from the measurements of Holt et al 5, is only O.73xR Ln(2), the full entropy of the doublet being reached at about 15 K. From this experi-
mental value of the entropy at T c we can estimate a Kondo ten~perature T K of about 8 K. In fact this small value of the magnetic entropy at T c could be also due to the persistence of short range order above T c. Recently, Yashima et al8 have foumd a ferromagnetic ordering at low temperature in CeSi I ~ and CeSi I 7- From the observation of a reductzon of the ~ g n e t i c moment of Ce atoms and of the magnetic entropy at T c they propose a dense Kondo behaviour in these compounds. CePt could be another example of ferromagnetic dense Kondo behaviour. In conclusion, in the CePt ferromagnet, resistivity measurements as well as magnetization and heat capacity measurements seem to indicate a gondo effect due to a negative coupling between cerium and conduction electron spins. In order to confirm such an interpretation polarized neutron experiments on e single crystal are in progress. Acknowledgements We are pleased to acknowledge B. Barbara, C. Lacroix and J. Pierre for stimulating discussions.
REFERENCES 1. 2. 3. 4. 5.
6. 7. 8.
IY~ight, A . E . , Conner, R.A. and Downey, J r . J . W . , Acta C r y s t . 18, 835 (1965). Le Roy, J . , e,o r e a u , d . . ~ . , P a c c a r d , D. and P a r t h ~ , E . , Acre C~-~t. 834, 9 (1978). C a s t e r s , A., Gignoux, D., Gomez-Sal, d.C. and Roudaut, E . , S o l . S t a t . C o , t i n . 44, 1329 (1982). Huber, J.G. and kuengo, C.A., d. Physique C6, 781 (1978). Holt, B.J., Ramsden, J.D., Sample, H.H. and--Huber, J.G., Physics I07.___B,255 (1981). P i e r r e , 2 . , Murani, A.P. and G a l e r a , R . H . , J . Phys. F I1, 679 (1981). C o r n u t , B. and C o q b l i n , B., Phys. Rev. B5, 4541 (1972). Yashima, I1., H o r i , H., S a t o h , T. and K o ~ , K., $ o l . S t a r . Commun. 4 3 , 193 (1982).
781
S o l i d State C o m n u n i c a t i o n s , Printed in Great Britain.
Vo1.45,No.9,
pp.779-781,
0038-1098/83/090779-03503.00/0
1983.
Pergamon Press Ltd.
RESISTIVITY AND MAGNETIC MEASUREMENTS ON THE CePt FERROMAGNET D. G£ gnoux Laboratoire Louis N~el, C.N.R.S.,
166X, 38042 Grenoble-c~dex, France
and J.C. Gomez-Sal Departamento de Optica y Estructura de la Hateria, Fac. de Ciencia~, Santander, Spain Received 21 December 1982 by E.F. Bertaut
I n t h i s p a p e r we p r e s e n t r e s i s t i v i t y m e a s u r e m o n t s p e r f o r m e d on t h e CePt f e r r o m a g n e t (T c = 5 . 8 K) a n d on L a P t , t o g e t h e r w i t h m a g n e t i z a t i o n m e a s u r e m e n t s p e r f o r m e d on CePt a t low t e m p e r a t u r e s . The m a g n e t i c r e s i s t i v i t y o f CePt d e c r e a s e s as l n ( T ) when t e m p e r a t u r e i s i n c r e a s e d a b o v e 220 K. T h i s r e s u l t u s o o i a t e d w i t h a r e d u c t i o n o f t h e Ce m a g n e t i c moment a n d o f t h e m a g n e t i c e n t r o p y a t T c s u g g e s t s t h a t CePt c o u l d be a Kondo system.
Introduction
shown on t a b l e I. The c p a r a m e t e r d o e s n o t v a r y m o n o t o n o u s l y a s a f u n c t i o n o£ t h e a t o m i c n u m b e r b u t t h e v o l u m e o f t h e u n i t c e l l o f CePt i s i n accordance with that of the other compounds in which the rare earth ions are in the trivalent state. Magnetizatic- v e ' ~ u s the ~ p l i o d field. measured on CePt at 1.5, 4.2, 6 and I0 K, are drawn on figure I. In agreement with previous w o r k s 4 , 5 , t h i s compound i s p a r a m a g n e t i c a t 6 a n d l0 K. At 6 K we a r e v e r y c l o s e t o T c ( 5 . 8 K) and the initial susceptibility is still very high. At 4.2 K and 1.5 K a spontaneous magnetization a p p e a r s ; h o w e v e r , i n f i e l d s h i g h e r t h a n 2 kOe a s t r o n g f i e l d d e p e n d e n c e o f t h e m a g n e t i zation is obsorved. For that reason, the determination of the spontaneous magnetization leads to d i f f i c u l t i e s . The e s t i m a t e d v a l u e s a t 1 . 5 K and 4 . 2 K a r e 0 . 7 3 ± 0 . 0 2 ~B and 0 . 6 1 ~ 0 . 0 2 UB per cerium atom respectively.
In 1965 Dwight et al I showed that LaPt and CePt crystallize in the CrB-type orthorhombic structure (space group Cmcm) while the other equiatomic rare earth-platinum compounds crystallize in t h e F o B - t y p e orthorhombic s t r u c t u r e . H o w e v e r , i n 1978 Le Roy e t a l 2 h a v e s h o w n t h a t s a m p l e s of P r P t and NdPt, quenched from high temperature, n a v e t h e CrB-type s t r u c t u r e . These two l a s t compounds are f e r r o m a g n e t i c with C u r i e t e m p e r a t u r e s o f 15 K and 23 K r e s p e c t l v e l y 3 . Their magnetic structures d e t e r m i n e d by n e u C r o n diffraction are collinear3. I n 1978 H u b e r e t al 4 pointed out that CePt i s f e r r o m a g n e t i c with a Curie temperature of 5.8 K which was confirmed later by speciiic heat measurements 5. Such a ferromagnetic behaviour is quite unusual with cerium. In the p r e s c ~ paper we present resistivity measurements performed on CePt and LaPt together with magnetization measurements performed on CePt at low t e m p e r a t u r e .
Table
Experimental
I
C o m p a r i s o n of the cell p a r a m e t e r s LaPt, CePt, P r P t and N d P t (CrB-type structure).
The rare earth elements and the platinum used were 99.9 % and 99.99 % pure respectively. Polycrystalline sar-Jples ~ e r e induction malted in a cold c r u c i b l e . M a g n e t i c measurements were p e r f o r m e d a t t e m p e r a t u r e s down t o 1.5 K a n d i n f i e l d s up t o 60 kOe p r o d u c e d by a s u p e r c o n d u c t i n g coil. The rcsistivities were measured between 1.5 K and 300 K by an alternatingcurrent four probe method. The absolute precision is estimated to be about 4 %. If we compare the cell parameters given previously for LaFt and CePt on one hand I and for PrPt and NdPt on the other hand2t the unit cell volume found for CePt i s less t h a n t h a t e x p e c t e d f r o m t h e o t h e r c o m p o u n d s . We h a v e remeasured these parameters and the result is
bC j c X) vcX3
ref.
LaPt
3.974 3.972
II.037 11.023
4.558 4.548
199.92 199.13
i t h i s work
CePt
3.921 3.918
10.920 10.908
4.524 4.527
193.70 193.47
this
I work
3.891 3.884
10.899 10.724
4.569 4.572
193.76 190.43
this
2 work
3.846 3.858
10.769 10.692
4.542 4.551
188.12 187.73
this
2 work
PrPt NdPt
779
of
RESISTIVITY AND MAGNETIC MEASUREMENTS ON THE CePt FERROMAGNET
780
Vol. 4'5, No.
05 0.8
07
CePt
~
E u.
o..1
C
o;
to
'o
o~
,'0
~
~
e
W
b
APPLIED MAGNETIC FIELD (kOe)
Figure I
:
1C
¢
Field dependence o f the magnetization of CePt at low t e m p e r a t u r e s .
The t h e r m a l d e p e n d e n c e s of t h e r e a i s t i v i t i e s o f CePt and LaPt a r e shown i n f i g u r e 2. the r e s i s t i v i t y of LaPt i n c r e a s e s l i n e a r l y above 70 K i n agreement with the s c a t t e r i n g o f the e l e c t r o n s by phonon~. At low temperature t h e resistivity t e n d s r e g u l a r l y t o w a r d s Or when temperature is decreased. A strong discontinuity o f t h e r e s i s t i v y o f CePt i s o b s e r v e d a t t h e C u r i e t e m p e r a t u r e (T c = 5 . 8 K). The magnetic resistivity 0m of CePt was obtained by s u b t r a c t i n g t h e resistivity o f LaPt (dashed line in f i g u r e 2). Because of the s m a l l v a l u e of the o r d e r i n g t e m p e r a t u r e o f CePt t h e r e i s an u n c e r t a i n t y of the r e s i d u a l r e s i s t i v i t y determinat i o n and h e n c e t h e z e r o o f Pm i s n o t w e l l d e f i n e d . Pm i s a l m e s t c o n s t a n t i n a s m a l l t e m p e r a t u r e r a n g e j u s t above t h e C u r i e t e m p e r a t u r e and t h e n i n c r e a s e s s t r o n g l y . A b r o a d maximum o f 19.3 ± 0 . 2 ~QcJn i s o b s e r v e d a r o u n d 220 K. At higher t e m p e r a t u r e s t h e m a g n e t i c r e s i s t i v i t y d e c r e a s e s s l o w l y and r e a c h e s 17.9 ~ 0 . 2 ~Gcm a t 300 K. Within the experimental accuracy this d e c r e a s e i s l i n e a r a s a f u n c t i o n o f I ~ : T ) as shown i n f i g u r e 3. However t h i s r e s u l t m u s t be r e g a r d e d w i t h c a u t i o n b e c a u s e we h a v e a s s u m e d t h a t t h e phonon contribution t o t h e r e s i s t i v i t y of CePt is the same as in LaPt.
CePt C
%
LaPt
~00
~0
~00
~o
TEMPERATURE i g )
Fisure 2 : Thermal dependences of the resistivities of CePt and LaPt. Dashed line is the magnetic resistivity Om o f CePt.
°oJ
2'0
s'o
260
TEMPERATURE (K) Figure 3 : Magnetic resistivity o~ versus In(T) for CePt. Discussion From t h e v a l u e s o f t h e s p o n t a n e o u s magnet i z a t i o n m e a s u r e d a t 1.5 K and 4 . 2 K ( 0 . 7 3 z 0 . 0 2 UB and 0.61 +- 0 . 0 2 U8 r e s p e c t i v e l y ) it is possible using different extrapolation laws t o deduce a s p o n t a n e o u s ~ a g n e t i z a t i o o a t 0 K o f 0 . 7 4 z 0 . 0 2 ~B/Ce. T h i s v a l u e i s much s m a l l e r t h a n t h e f r e e i o n v a l u e f o r Ce3+ (gJJ~B = 2 . 1 4 UB)" T h i s r e d u c t i o n e s s e n t i a l l y r e s u l t s from t h e f a c t t h a t t h e m e a s u r e m e n t s were p e r f o r m e d on a p o l y c r y s t a l l l n e sample in w h i c h c r y s t a l f i e l d (CF) e f f e c t s l e a d t o a s t r o n g m a g n e t o c r y s t a l l i n e a n i s o t r o p y and t o a moment r e d u c t i o n . T h i s h a s a l r e a d y b e e n shown i n P r P t and NdPt 3. A s s u m i n g a s t r o n g a n i s o t r o p y and a c o l l l n e a r m a g n e t l c s t r u c t u r e i n w h i c h t h e Ce m a g n e t i c moments a r e , as i n P r P t , p a r a l l e l t o one o f t h e t h r e e main sywmetry a x e s ~ , ~ o r ~ , t h e m a g n e t i c moment on Ce m u s t be t w i c e t h a t d e d u c e d from m a g n e t i z a t i o n m e a s u r e m e n t s on a polycrystalline s a m p l e . Then t h e Ce m a g n e t i c moment i s o f t h e o r d e r o f 1.46 UB. The d i f f e r e n c e b e t w e e n t h i s v a l u e and t h e f r e e i o n one i s due t o t h e Ce moment r e d u c t i o n by CF and e v e n t u a l l y f o r a s m a l l p a r t t o a Kondo e f f e c t a n d / o r to a p o l a r i z a t i o n o f t h e c o n d u c t i o n electrons. Although the point charge model fails frequently to give quantitatively account for the CF effects in rare earth intermetaltic compounds, we have used it to have an indicaticm of these effects in CePt. With a 3+ charge on c e r i u m and no c h a r g e on p l a t i n u m CF e f f e c t s l e a d t o t h r e e d o u b l e t s at - 8 1 . 0 K, 37.5 K and 77.2 K w h i c h l e a d t o an o v e r a l l s p l i t t i n g o f 158.2 K. As w e l l , t h e e a s y m a g n e t i z a t i o n would be ~ and t h e n~agnetic moment i n t h e e s t i m a t e d m o l e c u l a r f i e l d (184 kOe) would be 1.97 ~B/Ce.
Vol. 45, No. 9
RESISTIVITY AND MAGNETIC MEASUREMENTS ON THE CePt FERROMACNET
The s t r o n g increase o f the magnetic resistivity of CePt above the Curie temperature r e s u l t s from CF e f f e c t s . A l t h o u g h i t i s d i f f i c u l t to d e t e r m i n e w i t h a g r e a t a c c u r a c y t h e magnetic resistivity a t high t e m p e r a t u r e , especially because t h e phonon c o n t r i b u t i o n is assumed t o be t h e same as i n L a P t , the o c c u r r e n c e of a maximum in this magnetic resistivity can be attributed to a Kondo effect on cerium. Generally, such a maximum is observed at a temperature of the order of the overall CF splitr i n g 6 , 7. Knowing t h a t g e n e r a l l y t h e p o i n t c h a r g e model g i v e s s m a l l e r v a l u e s f o r t h e CF p a r a m e t e r s t h a n t h e o b s e r v e d o n e s , t h e c a l c u l a t e d CF s p l i t t i n g (158.2 K) i s c o n s i s t e n t w i t h the t e m p e r a t u r e o f the maximum o f 0m (= 220 K). The ln(T) dependence o f t h e d e c r e a s e o f 0m b e t w e e n 220 and 300 K i s c o n s i s t e n t with the Kondo c o n t r i b u t i o n t o the electrical resistivity in cerium diluted alloys calculated by Cornut and Coqblin 7 in presence of CF. The existence of a Kondo effect in CePt is supported by the heat capacity measurements. Indeed, the entropy at Tc, deduced from the measurements of Holt et al 5, is only O.73xR Ln(2), the full entropy of the doublet being reached at about 15 K. From this experi-
mental value of the entropy at T c we can estimate a Kondo ten~perature T K of about 8 K. In fact this small value of the magnetic entropy at T c could be also due to the persistence of short range order above T c. Recently, Yashima et al8 have foumd a ferromagnetic ordering at low temperature in CeSi I ~ and CeSi I 7- From the observation of a reductzon of the ~ g n e t i c moment of Ce atoms and of the magnetic entropy at T c they propose a dense Kondo behaviour in these compounds. CePt could be another example of ferromagnetic dense Kondo behaviour. In conclusion, in the CePt ferromagnet, resistivity measurements as well as magnetization and heat capacity measurements seem to indicate a gondo effect due to a negative coupling between cerium and conduction electron spins. In order to confirm such an interpretation polarized neutron experiments on e single crystal are in progress. Acknowledgements We are pleased to acknowledge B. Barbara, C. Lacroix and J. Pierre for stimulating discussions.
REFERENCES 1. 2. 3. 4. 5.
6. 7. 8.
IY~ight, A . E . , Conner, R.A. and Downey, J r . J . W . , Acta C r y s t . 18, 835 (1965). Le Roy, J . , e,o r e a u , d . . ~ . , P a c c a r d , D. and P a r t h ~ , E . , Acre C~-~t. 834, 9 (1978). C a s t e r s , A., Gignoux, D., Gomez-Sal, d.C. and Roudaut, E . , S o l . S t a t . C o , t i n . 44, 1329 (1982). Huber, J.G. and kuengo, C.A., d. Physique C6, 781 (1978). Holt, B.J., Ramsden, J.D., Sample, H.H. and--Huber, J.G., Physics I07.___B,255 (1981). P i e r r e , 2 . , Murani, A.P. and G a l e r a , R . H . , J . Phys. F I1, 679 (1981). C o r n u t , B. and C o q b l i n , B., Phys. Rev. B5, 4541 (1972). Yashima, I1., H o r i , H., S a t o h , T. and K o ~ , K., $ o l . S t a r . Commun. 4 3 , 193 (1982).
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