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Heavy-fermion, Kondo, spin glass-like and antiferromagnetic behavior in (Ce, Gd)Al3
Journal of Magnetism and Magnetic Materials 63 & 64 (1987) 335-337 North-Holland, Amsterdam
335
HEAVY-FERMION, KONDO, SPIN GLASS-LIKE A N D ANTIFERROMAGNETIC BEHAVIOR IN (Ce,Gd)AI3 A S EDELSTEIN?, R L HOLTZ*?, D J GILLESPIEt, and Norman E PHILLIPS§
R A FISHER§
~fNaval Research Laboratory, Washington, DC 20375, USA §MMRD, Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA
We have found that Ce~_xGdxA13 with x = 0 23 IS a heavy-fermlon system which undergoes a spin glass transition The amphtude of the peak in the susceptlbIhty that characterizes the spin glass transition passes through a sharp maximum at x = 0 5 For x = 0 635 and 0 77 the system undergoes a spin glass-hke transltmn at T ~ 100 K and an antfferromagnetIc transition at T ~ 20 K T h e d e f i n m o n [1] of a h e a v y - f e r m i o n system as o n e for w h i c h 3' >~ 4 0 0 m J / m o l K 2 ( w h e r e 3/is the T---> 0 r a t i o of the low t e m p e r a t u r e specific h e a t C to the t e m p e r a t u r e T) is f r e q u e n t l y a d o p ted Some heavy-fermIon systems undergo a s u p e r c o n d u c t i n g t r a n s i t i o n [ 2 - 4 ] o r o r d e r antif e r r o m a g n e t l c a l l y [5,6] H e r e , we p r e s e n t s u s c e p t l b i h t y d a t a [7] w h i c h d e m o n s t r a t e that C e t _ ~ G d x A l 3 with x = 0 2 3 u n d e r g o e s a spin glass t r a n s i t i o n with a f r e e z i n g t e m p e r a t u r e , Tin, of 43 K a n d specific h e a t d a t a w h i c h shows that it has a l a r g e e n o u g h low t e m p e r a t u r e 3/ v a l u e to be c h a r a c t e r i z e d as a h e a v y - f e r m l o n system W e find this t r a n s l t m n persists to h i g h c o n c e n t r a t i o n s T h e C e a t o m s , w h i c h are r e s p o n s i b l e for the h e a v y - f e r m i o n c h a r a c t e r of the system, are also n e c e s s a r y for the l a r g e v a l u e of Tm T h e d a t a i n d i c a t e s that the two p h e n o m e n a affect o n e a n o t h e r A n o t h e r h i g h l y u n u s u a l result t h a t we h a v e o b t a i n e d IS that, for x = 0 635 a n d 0 77 t h e r e is b o t h a spin g l a s s - l i k e t r a n s i t i o n n e a r 120 K a n d an a n t l f e r r o m a g n e t i c t r a n s i t i o n n e a r 20 K F o r 0 635 ~< x ~< 0 9 a n d T < 20 K the spin g l a s s - l i k e and antlferromagnetic states probably coexist T h o u g h m o s t s t u d i e s on spin glasses h a v e b e e n c o n f i n e d to n o n m a g n e t i c hosts, we h a v e e m p l o y e d the h e a v y - f e r m l o n c o m p o u n d [ 8 - 1 1 ] C e A I ~ T h e n a t u r e of the host is c l e a r l y imp o r t a n t , since we did not o b s e r v e a spin glass t r a n s i t i o n in La~_xGdxAl~ for x = 0 2 a n d 0 4, 5K~
r e p o r t e d h e r e p r o v i d e s e v i d e n c e t h a t the C e a t o m s assist the i n t e r a c t i o n b e t w e e n the G d atoms O t h e r u n u s u a l f e a t u r e s we h a v e o b s e r v e d m Ce~_xGdxAl3 are 1) T h e a m p l i t u d e of the a n o m a l y in the s u s c e p t i b i l i t y at the t r a n s i t i o n p e a k s s h a r p l y at x = 0 50 2) T h e v a l u e s of Tm, w h i c h are g r e a t e r t h a n 1 0 0 K for x > 0 5, a r e s u r p r i s i n g l y high c o n s i d e r i n g that t h e m a g n e t i c o r d e r i n g t e m p e r a t u r e s of all the r a r e e a r t h trla l u m l m d e s [12] a r e less t h a n 25 C O u r l n v e s t t g a t l o n s w e r e p e r f o r m e d o n arc melted, polycrystalhne samples which were a n n e a l e d for two w e e k s at 9 8 0 ° C X - r a y dtffractlon analysis i n d i c a t e s t h a t the latttce p a r a m e t e r s of the D 0 1 9 s t r u c t u r e v a r i e d s m o o t h l y as a f u n c tion of G d c o n c e n t r a t i o n Fig 1 is a p l o t of low field s u s c e p t l b l h t y X for x = 0 08 O n e sees that, similar to w h a t is o b s e r v e d in a spin glass f r e e z i n g t r a n s i t i o n , t h e r e is a c u s p in the s u s c e p t i b i l i t y if the s a m p l e is c o o l e d in z e r o field, but the cusp is no l o n g e r p r e s e n t if the s a m p l e is c o o l e d In a field of 10 O e M o s t m e a s u r e m e n t s w e r e p e r f o r m e d with i n c r e a s i n g t e m p e r a t u r e a f t e r the s a m p l e had b e e n c o o l e d in " z e r o " field M e a s u r e m e n t s m a d e on s a m p l e s with x = 0 15, 0 18, 0 20, 0 34, 0 4(J a n d 0 50 show similar spin g l a s s - l i k e cusps T h e a m p h t u d e of the a n o m a l y i n c r e a s e s as a f u n c t i o n of x up to x = 0 5 0 For x>05 t h e r e Is still a h i g h - t e m p e r a t u r e a n o m a l y in the s u s c e p t i b i l i t y , b u t the a m p l i t u d e of the s u s c e p t i b i l i t y m a x i m u m d e c r e a s e s a p p r o x i m a t e l y two o r d e r s of m a g n i t u d e in g o i n g f r o m x = 0 5 to 0 635 W e h a v e
0 3 0 4 - 8 8 5 3 / 8 7 / $ 0 3 50 © E l s e v i e r S c i e n c e P u b h s h e r s B V ( N o r t h - H o l l a n d Physics P u b h s h m g D i v i s i o n )
A S Edelstem et al / Heavy-[ermton, spin glass, ant~[erromagnenc ( Ce,Gd)Al~
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observed th~s h~gh-temperature anomaly for x = 0 635, 0 77 and 0 90 The temperatures at which the h~gh-temperature anomaly occurs for x > 0 5 falls on a smooth curve which ~s an extenMon of the curve for x ~ 0 5 In addmon for x > 0 5 there is a second maximum in the suscept~bdlty which occurs at T--~20 Because GdAI~ is an antfferromagnet w~th a Ndel temperature of 17 K and the specific heat data presented below, we ~dentffy the low temperature transmon as an antfferromagnetlc transmon Specific heat data on CeoTvGdo23A13 and Cco e~Gd. 77Ala are compared with data [10] on CeAI~ in fig 2 From the T = 0 intercept, one
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Fig 3 Reslsnwty of Ceo ~6sGdo6~sAl~ versus T sees that Ceo77Gd(~2~Ala (T = 4 1 0 m J / m o l K 2) is
a heavy-fermlon system The general shape of the curves is similar for CeAI~ and CeoyvGdoz3Al~ except that the peak In C / T occurs at a higher temperature m the case ot Ceo 77Gdo z~Al~ Since the spin glass transition is absent tn (La, Gd)AI~, the Ce atoms must be playing a role in the spin glass transition in CeovTGdo2~Al~
The
insert s h o w s the anttfer-
romagnetlc transition m Ceo2~Gdo77Al~ For 0 32 K <~ T ~<1(1 K these data can be fitted to C = 20 5 T + 98 T ~
( m J / m o l K)
The
T~
coefficient ~s like one expects for antfferromagnenc spin waves The reslstwlty resembles [13] that of (Ce, M)A13 for M = La, Y and Th Fig 3 shows a plot of the resistivity for x = 0 635 as a function of both increasing and decreasing temperature The difference is due to nonldeal sample contacts The resistivity for T < 100 K Is similar to the resistivity of other concentrated Kondo systems H o w e v e r , for T = 125 K there Is an add m o n a l , unusual a n o m a l y w h i c h was not observ e d m the measurements [13] on (Ce, M)AIa T h i s a d d m o n a l reMsnvlty a n o m a l y o c c u r s at the s a m e t e m p e r a t u r e as the h i g h - t e m p e r a t u r e susc e p t l b d l t y a n o m a l y T h i s a n o m a l y is absent for
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50
10
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Fig 2 The specific heat C d~v~ded by the temperature T for CeAla, Ce~ 77Gd. z~AI~ and Ceo ~Gdo 77A13
x~(15 Using our data we have constructed the tentative phase dmgram shown m fig 4 We have taken the temperature at the heavy-fermlon phase boundary as the highest temperature for
A S Edelstem et al I Heavy-fermlon, spin glass, anaferromagneac (Ce,Gd)Al~
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W e wish to t h a n k Prof C h i a - L l n g C h i e n for helpful discussions T h e work at B e r k e l e y was s u p p o r t e d by the D i r e c t o r , Office o n E n e r g y R e s e a r c h , Office of Basic E n e r g y Sciences, Materials Sctences D i v i s i o n of the U S D e p a r t m e n t of E n e r g y u n d e r C o n t r a c t no D E - A C 0 3 76SF00098
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References
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Fig 4 Tentative phase diagram The symbols AF, HF, P and SGL represent the antlferromagnetlc, heavy-fermlon, paramagnetlc and spin glass-hke phases w h i c h C / T ~>400 m J / m o l K 2 T h e v a l u e o n the a n t l f e r r o m a g n e t i c b o u n d a r y for G d A l 3 was t a k e n from ref [12] E x c e p t for these cases, the values p l o t t e d are the t e m p e r a t u r e s of susceptlbfltty m a x i m a O n e sees there is a c o n c e n t r a t i o n r a n g e , x ~< 0 23, w h e r e there is a spin glass t r a n sition a n d the system is still a h e a v y - f e r m l o n system F o r 0 635 ~< x ~< 0 90 there IS a spln glasslike t r a n s i t i o n followed by a n a n t l f e r r o m a g n e t l c t r a n s i t i o n at lower t e m p e r a t u r e s B e c a u s e of the high t e m p e r a t u r e s of the spin glass-like t r a n sitions, it is likely that the a h g n m e n t of the spms i n v o l v e d is n o t affected v e r y m u c h by the anttf e r r o m a g n e t i c t r a n s i t i o n T h u s we suggest that these two phases m a y coexist W o n g et al h a v e r e p o r t e d [14] the c o e x i s t e n c e of the spin glass a n d a n t l f e r r o m a g n e t i c states in Fe0 55Mgo45C12 T w o facts s u g g e s t that the C e a t o m s play a v e r y i m p o r t a n t role First, the v a l u e s of Tm for x > 0 3 are so m u c h larger t h a n the o r d e r i n g t e m p e r a t u r e s of the rare e a r t h t r t a l u m m i d e s S e c o n d l y , o u r m e a s u r e m e n t s o n the isostructural series (La, Gd)Al3 fall to show a spin glass t r a n sltton Since a p p a r e n t l y b o t h the C e a n d G d a t o m s play a role in the transition, we s u p p o s e that local r e g i o n s c o n t a i n i n g b o t h C e a n d G d a t o m s are I n v o l v e d T h e s e m a y be r e g i o n s In
[1] G R Stewart, Rev Mod Phys 56 (1984)755 [2] F Steghch, J Aarts, C D Bredl, W Lleke, D Meschede, W Franz and H Schafer, Phys Rev Lett 43 (1979) 1892 [3] G R Stewart, Z Fisk, J O Wdhs and J L Smith, Phys Rev Lett 52 (1984) 679 [4] H R Ott, H Rudlgler, Z Fisk and J L Smith, Phys Rev Lett 50 (1983)1595 [5] H R Ott, H Rudlgler, P Delsmg and Z Flsk, Phys Rev Lett 52 (1984)1551 H R Ott, H Rudlgler, Z Fisk and J L Smith, J Appl Phys 57 (1985) 3044 [6] Z Flsk, G R Stewart, J O Wdhs, H R Ott and F Hulhger, Phys Rev B30 (1984) 6360 [7] Earher measurements (A S Edelstem and N C Koon, J Appl Phys 55 (1984) 1984) showed the existence of a magnetic transition at T = 34 K for x = 0 2 but did not estabhsh the other properties reported here [8] K Andres, J E Graebner and H R Ott, Phys Rev Lett 35 (1975) 1779 [9] A S Edelstem, G E Brodale, R A Fisher, C M Llsse and N E Phdhps, Sohd State Commun 56 (1985) 271 [10] G E Brodale, R A Fisher, C M Llsse, N E Phllhps and A S Edeistem, J Magn Magn Mat 54-57 (1986)416 G E Brodale, R A Fisher, N E Phflhps, J Flouquet and C Marcenat, J Magn Magn Mat 54-57 (1986)419 G E Brodale, R A Fisher, N E Phdhps and J FIouquet, Phys Rev Lett 56 (1986)390 [11] A S Edelstem, C J Tranchlta, O D McMasters and K A Gschneldner, Jr, Sohd State Commun 15 (1974) 81 [12] K H J Buschow and J F Fast, Z Phys Chem (Frankfurt) 50 (1966) 1 [13] K H J Buschow, H J van Daal, F E Maranzana and P B van Aken, Phys Rev B3 (1971) 1662 [14] Po-zen Wong, S von Molnar, T T M Palstra, J A Mydosh, H Yoshmawa, S M Shapiro and A Ito, Phys Rev Lett 55 (1985) 2043
335
HEAVY-FERMION, KONDO, SPIN GLASS-LIKE A N D ANTIFERROMAGNETIC BEHAVIOR IN (Ce,Gd)AI3 A S EDELSTEIN?, R L HOLTZ*?, D J GILLESPIEt, and Norman E PHILLIPS§
R A FISHER§
~fNaval Research Laboratory, Washington, DC 20375, USA §MMRD, Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA
We have found that Ce~_xGdxA13 with x = 0 23 IS a heavy-fermlon system which undergoes a spin glass transition The amphtude of the peak in the susceptlbIhty that characterizes the spin glass transition passes through a sharp maximum at x = 0 5 For x = 0 635 and 0 77 the system undergoes a spin glass-hke transltmn at T ~ 100 K and an antfferromagnetIc transition at T ~ 20 K T h e d e f i n m o n [1] of a h e a v y - f e r m i o n system as o n e for w h i c h 3' >~ 4 0 0 m J / m o l K 2 ( w h e r e 3/is the T---> 0 r a t i o of the low t e m p e r a t u r e specific h e a t C to the t e m p e r a t u r e T) is f r e q u e n t l y a d o p ted Some heavy-fermIon systems undergo a s u p e r c o n d u c t i n g t r a n s i t i o n [ 2 - 4 ] o r o r d e r antif e r r o m a g n e t l c a l l y [5,6] H e r e , we p r e s e n t s u s c e p t l b i h t y d a t a [7] w h i c h d e m o n s t r a t e that C e t _ ~ G d x A l 3 with x = 0 2 3 u n d e r g o e s a spin glass t r a n s i t i o n with a f r e e z i n g t e m p e r a t u r e , Tin, of 43 K a n d specific h e a t d a t a w h i c h shows that it has a l a r g e e n o u g h low t e m p e r a t u r e 3/ v a l u e to be c h a r a c t e r i z e d as a h e a v y - f e r m l o n system W e find this t r a n s l t m n persists to h i g h c o n c e n t r a t i o n s T h e C e a t o m s , w h i c h are r e s p o n s i b l e for the h e a v y - f e r m i o n c h a r a c t e r of the system, are also n e c e s s a r y for the l a r g e v a l u e of Tm T h e d a t a i n d i c a t e s that the two p h e n o m e n a affect o n e a n o t h e r A n o t h e r h i g h l y u n u s u a l result t h a t we h a v e o b t a i n e d IS that, for x = 0 635 a n d 0 77 t h e r e is b o t h a spin g l a s s - l i k e t r a n s i t i o n n e a r 120 K a n d an a n t l f e r r o m a g n e t i c t r a n s i t i o n n e a r 20 K F o r 0 635 ~< x ~< 0 9 a n d T < 20 K the spin g l a s s - l i k e and antlferromagnetic states probably coexist T h o u g h m o s t s t u d i e s on spin glasses h a v e b e e n c o n f i n e d to n o n m a g n e t i c hosts, we h a v e e m p l o y e d the h e a v y - f e r m l o n c o m p o u n d [ 8 - 1 1 ] C e A I ~ T h e n a t u r e of the host is c l e a r l y imp o r t a n t , since we did not o b s e r v e a spin glass t r a n s i t i o n in La~_xGdxAl~ for x = 0 2 a n d 0 4, 5K~
r e p o r t e d h e r e p r o v i d e s e v i d e n c e t h a t the C e a t o m s assist the i n t e r a c t i o n b e t w e e n the G d atoms O t h e r u n u s u a l f e a t u r e s we h a v e o b s e r v e d m Ce~_xGdxAl3 are 1) T h e a m p l i t u d e of the a n o m a l y in the s u s c e p t i b i l i t y at the t r a n s i t i o n p e a k s s h a r p l y at x = 0 50 2) T h e v a l u e s of Tm, w h i c h are g r e a t e r t h a n 1 0 0 K for x > 0 5, a r e s u r p r i s i n g l y high c o n s i d e r i n g that t h e m a g n e t i c o r d e r i n g t e m p e r a t u r e s of all the r a r e e a r t h trla l u m l m d e s [12] a r e less t h a n 25 C O u r l n v e s t t g a t l o n s w e r e p e r f o r m e d o n arc melted, polycrystalhne samples which were a n n e a l e d for two w e e k s at 9 8 0 ° C X - r a y dtffractlon analysis i n d i c a t e s t h a t the latttce p a r a m e t e r s of the D 0 1 9 s t r u c t u r e v a r i e d s m o o t h l y as a f u n c tion of G d c o n c e n t r a t i o n Fig 1 is a p l o t of low field s u s c e p t l b l h t y X for x = 0 08 O n e sees that, similar to w h a t is o b s e r v e d in a spin glass f r e e z i n g t r a n s i t i o n , t h e r e is a c u s p in the s u s c e p t i b i l i t y if the s a m p l e is c o o l e d in z e r o field, but the cusp is no l o n g e r p r e s e n t if the s a m p l e is c o o l e d In a field of 10 O e M o s t m e a s u r e m e n t s w e r e p e r f o r m e d with i n c r e a s i n g t e m p e r a t u r e a f t e r the s a m p l e had b e e n c o o l e d in " z e r o " field M e a s u r e m e n t s m a d e on s a m p l e s with x = 0 15, 0 18, 0 20, 0 34, 0 4(J a n d 0 50 show similar spin g l a s s - l i k e cusps T h e a m p h t u d e of the a n o m a l y i n c r e a s e s as a f u n c t i o n of x up to x = 0 5 0 For x>05 t h e r e Is still a h i g h - t e m p e r a t u r e a n o m a l y in the s u s c e p t i b i l i t y , b u t the a m p l i t u d e of the s u s c e p t i b i l i t y m a x i m u m d e c r e a s e s a p p r o x i m a t e l y two o r d e r s of m a g n i t u d e in g o i n g f r o m x = 0 5 to 0 635 W e h a v e
0 3 0 4 - 8 8 5 3 / 8 7 / $ 0 3 50 © E l s e v i e r S c i e n c e P u b h s h e r s B V ( N o r t h - H o l l a n d Physics P u b h s h m g D i v i s i o n )
A S Edelstem et al / Heavy-[ermton, spin glass, ant~[erromagnenc ( Ce,Gd)Al~
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observed th~s h~gh-temperature anomaly for x = 0 635, 0 77 and 0 90 The temperatures at which the h~gh-temperature anomaly occurs for x > 0 5 falls on a smooth curve which ~s an extenMon of the curve for x ~ 0 5 In addmon for x > 0 5 there is a second maximum in the suscept~bdlty which occurs at T--~20 Because GdAI~ is an antfferromagnet w~th a Ndel temperature of 17 K and the specific heat data presented below, we ~dentffy the low temperature transmon as an antfferromagnetlc transmon Specific heat data on CeoTvGdo23A13 and Cco e~Gd. 77Ala are compared with data [10] on CeAI~ in fig 2 From the T = 0 intercept, one
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Fig 3 Reslsnwty of Ceo ~6sGdo6~sAl~ versus T sees that Ceo77Gd(~2~Ala (T = 4 1 0 m J / m o l K 2) is
a heavy-fermlon system The general shape of the curves is similar for CeAI~ and CeoyvGdoz3Al~ except that the peak In C / T occurs at a higher temperature m the case ot Ceo 77Gdo z~Al~ Since the spin glass transition is absent tn (La, Gd)AI~, the Ce atoms must be playing a role in the spin glass transition in CeovTGdo2~Al~
The
insert s h o w s the anttfer-
romagnetlc transition m Ceo2~Gdo77Al~ For 0 32 K <~ T ~<1(1 K these data can be fitted to C = 20 5 T + 98 T ~
( m J / m o l K)
The
T~
coefficient ~s like one expects for antfferromagnenc spin waves The reslstwlty resembles [13] that of (Ce, M)A13 for M = La, Y and Th Fig 3 shows a plot of the resistivity for x = 0 635 as a function of both increasing and decreasing temperature The difference is due to nonldeal sample contacts The resistivity for T < 100 K Is similar to the resistivity of other concentrated Kondo systems H o w e v e r , for T = 125 K there Is an add m o n a l , unusual a n o m a l y w h i c h was not observ e d m the measurements [13] on (Ce, M)AIa T h i s a d d m o n a l reMsnvlty a n o m a l y o c c u r s at the s a m e t e m p e r a t u r e as the h i g h - t e m p e r a t u r e susc e p t l b d l t y a n o m a l y T h i s a n o m a l y is absent for
i
o 4
2
_
(K)
Ftg 1 Suscept~bdlty of Ceo 92Gdo 08Al-~ versus increasing temperature after the sample was first cooled m zero field and m decreasing temperature m a field of 10 Oe
-A
_
0
50
10
T(K)
Fig 2 The specific heat C d~v~ded by the temperature T for CeAla, Ce~ 77Gd. z~AI~ and Ceo ~Gdo 77A13
x~(15 Using our data we have constructed the tentative phase dmgram shown m fig 4 We have taken the temperature at the heavy-fermlon phase boundary as the highest temperature for
A S Edelstem et al I Heavy-fermlon, spin glass, anaferromagneac (Ce,Gd)Al~
1,0I
/
+:1 --~"
J
(Ce,Gd)AI 3
/+
W e wish to t h a n k Prof C h i a - L l n g C h i e n for helpful discussions T h e work at B e r k e l e y was s u p p o r t e d by the D i r e c t o r , Office o n E n e r g y R e s e a r c h , Office of Basic E n e r g y Sciences, Materials Sctences D i v i s i o n of the U S D e p a r t m e n t of E n e r g y u n d e r C o n t r a c t no D E - A C 0 3 76SF00098
80 SGL
g,o 140
"FS\"2,+Pk \ \ /" \ ~/-A
\
\
\
~"""~
AF+SGL
0 0
w h i c h the G d a t o m s p r i m a r i l y h a v e C e n e a r e s t neighbors and Gd second nearest neighbors
/
1 O0
2O
337
02
06
04
08
References
X
Fig 4 Tentative phase diagram The symbols AF, HF, P and SGL represent the antlferromagnetlc, heavy-fermlon, paramagnetlc and spin glass-hke phases w h i c h C / T ~>400 m J / m o l K 2 T h e v a l u e o n the a n t l f e r r o m a g n e t i c b o u n d a r y for G d A l 3 was t a k e n from ref [12] E x c e p t for these cases, the values p l o t t e d are the t e m p e r a t u r e s of susceptlbfltty m a x i m a O n e sees there is a c o n c e n t r a t i o n r a n g e , x ~< 0 23, w h e r e there is a spin glass t r a n sition a n d the system is still a h e a v y - f e r m l o n system F o r 0 635 ~< x ~< 0 90 there IS a spln glasslike t r a n s i t i o n followed by a n a n t l f e r r o m a g n e t l c t r a n s i t i o n at lower t e m p e r a t u r e s B e c a u s e of the high t e m p e r a t u r e s of the spin glass-like t r a n sitions, it is likely that the a h g n m e n t of the spms i n v o l v e d is n o t affected v e r y m u c h by the anttf e r r o m a g n e t i c t r a n s i t i o n T h u s we suggest that these two phases m a y coexist W o n g et al h a v e r e p o r t e d [14] the c o e x i s t e n c e of the spin glass a n d a n t l f e r r o m a g n e t i c states in Fe0 55Mgo45C12 T w o facts s u g g e s t that the C e a t o m s play a v e r y i m p o r t a n t role First, the v a l u e s of Tm for x > 0 3 are so m u c h larger t h a n the o r d e r i n g t e m p e r a t u r e s of the rare e a r t h t r t a l u m m i d e s S e c o n d l y , o u r m e a s u r e m e n t s o n the isostructural series (La, Gd)Al3 fall to show a spin glass t r a n sltton Since a p p a r e n t l y b o t h the C e a n d G d a t o m s play a role in the transition, we s u p p o s e that local r e g i o n s c o n t a i n i n g b o t h C e a n d G d a t o m s are I n v o l v e d T h e s e m a y be r e g i o n s In
[1] G R Stewart, Rev Mod Phys 56 (1984)755 [2] F Steghch, J Aarts, C D Bredl, W Lleke, D Meschede, W Franz and H Schafer, Phys Rev Lett 43 (1979) 1892 [3] G R Stewart, Z Fisk, J O Wdhs and J L Smith, Phys Rev Lett 52 (1984) 679 [4] H R Ott, H Rudlgler, Z Fisk and J L Smith, Phys Rev Lett 50 (1983)1595 [5] H R Ott, H Rudlgler, P Delsmg and Z Flsk, Phys Rev Lett 52 (1984)1551 H R Ott, H Rudlgler, Z Fisk and J L Smith, J Appl Phys 57 (1985) 3044 [6] Z Flsk, G R Stewart, J O Wdhs, H R Ott and F Hulhger, Phys Rev B30 (1984) 6360 [7] Earher measurements (A S Edelstem and N C Koon, J Appl Phys 55 (1984) 1984) showed the existence of a magnetic transition at T = 34 K for x = 0 2 but did not estabhsh the other properties reported here [8] K Andres, J E Graebner and H R Ott, Phys Rev Lett 35 (1975) 1779 [9] A S Edelstem, G E Brodale, R A Fisher, C M Llsse and N E Phdhps, Sohd State Commun 56 (1985) 271 [10] G E Brodale, R A Fisher, C M Llsse, N E Phllhps and A S Edeistem, J Magn Magn Mat 54-57 (1986)416 G E Brodale, R A Fisher, N E Phflhps, J Flouquet and C Marcenat, J Magn Magn Mat 54-57 (1986)419 G E Brodale, R A Fisher, N E Phdhps and J FIouquet, Phys Rev Lett 56 (1986)390 [11] A S Edelstem, C J Tranchlta, O D McMasters and K A Gschneldner, Jr, Sohd State Commun 15 (1974) 81 [12] K H J Buschow and J F Fast, Z Phys Chem (Frankfurt) 50 (1966) 1 [13] K H J Buschow, H J van Daal, F E Maranzana and P B van Aken, Phys Rev B3 (1971) 1662 [14] Po-zen Wong, S von Molnar, T T M Palstra, J A Mydosh, H Yoshmawa, S M Shapiro and A Ito, Phys Rev Lett 55 (1985) 2043