Pressure dependence of magnetism in C60TDAE

~

Solid State Communications,Vol. 82, No. 10, pp. 779-782, 1992. Printed in Great Britain.

PRESSURE

DEPENDENCE

OF M A G N E T I S M

0038-1098/9255.00+.00 Pergamon Press Ltd

IN C 6 0 T D A E

G. S p a r n ~ J. D. T h o m p s o n Los A l a m o s N a t i o n a l L a b o r a t o r y , Los A l a m o s , N M 87545

Institute

for

Polymers

P.-M. A l l e m a n d , Q. Li, F. W u d l a n d O r g a n i c Solids, U n i v e r s i t y of C a l i f o r n i a , S a n t a B a r b a r a , CA 93106

Santa

Barbara,

K. H o l c z e r C.N.R.S., Orsay FRANCE

Department

and P. W. S t e p h e n s of P h y s i c s , S t a t e U n i v e r s i t y S t o n y Brook, NY 11794

(Received We have

determined

2 April

the pressure

1992

of N e w Y o r k

by A. A. M a r a d u d i n )

response

of m a g n e t i s m

in C 6 0 T D A E , w h e r e

TDAE is tetrakis (dimethylamino) ethylene, and its temperature dependent susceptibility above and below the magnetic ordering temperature Tc=16.1K. The magnetism is d e p r e s s e d very rapidly with applied pressure. We consider several possible interpretations magnetic transition a n d its p r e s s u r e d e p e n d e n c e and conclude itinerant-ferromagnet d e s c r i p t i o n is m o s t likely.

The r e l a t i v e l y s h o r t i n t e r m o l e c u l a r s e p a r a t i o n i n C60TDAE s u g g e s t e d t h a t t h e r e c o u l d be s u f f i c i e n t w a v e f u n c t i o n o v e r l a p t o form a p a r t i a l l y filled band and consequently metallic-like conductivity. Further, the separation anisotropy would imply a very anlsotropic bandstructure, which, combined with the observation of a s m a l l s a t u r a t e d moment, would f a v o r an t t t n e r ant-ferromagnet interpretation. To i n v e s t i g a t e i n g r e a t e r d e t a i l t h e magnetism i n C60TDAE, we have measured t h e r e s p o n s e o f C60TDAE t o a p p l i e d h y d r o s t a t i c pressure. The sample u s e d i n t h i s s t u d y was p r e p a r e d as described earlier.7, a However, because of the extreme sensitivity of C60TDAE to air and of the possibility that the sample might be exposed briefly to air in the process of loading it into the pressure cell, it was deslrable to minimize the surface-to-volume ratio of the sample by pelletizing the powder in a glove box containing an inert atmosphere, dc magnetization measurements were performed to confirm that pelletizing had no effect on the magnetism. Results of these measurements are shown in Fig. I where clear evidence for a ferromagnetic-llke transition is seen at 16.1K, with a temperature dependence and magnitude below T c that is similar to reported 7 results on a powder sample. The pressure dependence of magnetism in C60TDAE was studied by monitoring the inductive response of the sample, at a frequency of 457Hz, to applied pressure produced by a beryllium-copper self-clamplng cell. ° To minimize exposure of the sample to air, it was loaded into the cell in a glove bag containing an

Until recently, the occurrence of ferromagnetism i n p u r e l y o r g a n i c compounds h a s b e e n restricted to temperatures below approximately 1K (Refs. 1-6). However, this has changed slgnificantly by the report 7 of a soft ferromagnetic state below 16.1K in the fullerene compound C60TDAE, where TDAE is tetrakls (dimethylamino) ethylene, C~N4(CH3)$. Magnetization studies revealed a saturated moment Ms > 0.1~B/C60

at 5Kwhich

decreased

for t h e that an

monotonically

to zero at Tc-16.1K and the absence of coercivity and remanence within experimental resolution of ± 20e. Together with a relatively high room temperature conductivity v o - 10 -2 S/cm, these observations suggested a very soft, possibly itinerant, ferromagnetic state. Unlike crystalline C60 or superconductors K3C60 and Rb3Ce0 which have cubic crystal s t r u c t u r e s , C60'rDAE has a c - c e n t e r e d m o n o c l i n i c u n i t c e l l s w i t h two f o r m u l a u n i t s p e r c e l l and with dimensions a-15.849A, b-12.987A, c-9.965A and ~-93.31". Rietveld analysis of the x-ray spectrum gave a best fit to the data if the long axis of the TDAE molecule was assumed to be along the monoclinlc c-axls. Interestingly, at room temperature the intermolecular separation is only 9.96A along the c-axls and 10.24A within the a-b plane. These separations are less than or comparable to those in metallic alkali-doped AsC60 superconductors which have intermolecular distances of 10.07 to 10.15A. Present address: Technlsche Hochschule Darmstadt, Institut fur Festkorperphyslk, Darmstadt, Germany. 779

780

Vol. 82, No. 10

MAGNETISM IN C60 TDAE

F

20

CsoTDAE A OI

15

=1

•

°

H=lOe

5.0

C6oTDAE

E q) 10

ambient

o

:Z

"0

5

"o -40

0

40

~-4.5

Field (kOe)

1'0 1'5 Temperature (K)

Fig.

20

1. dc magnetization as a function of t e m p e r a t u r e f o r a p e l l e t o f Ce0TDAE i n an a p p l i e d f i e l d of = 1 0 e . The o v e r a l l shape and magnitude of the magnetization is similar to that reported ~ on a powder sample of C#0TDAE. The inset gives the isothermal maEnetization at 5K.

D J=

4.0

3.5 a t m o s p h e r e o f h e l i u m g a s . Within t h e c e l l , t h e s a m p l e was p l a c e d i n a s m a l l t e f l o n cup a r o u n d w h i c h was wound a s e c o n d a r y ac p i c k - u p c o i l a n d t h e e n t i r e a s s e m b l y was i m m e r s e d i n F l o u r i n e r t FC-75 w h i c h was u s e d a s t h e h y d r o s t a t i c press u r e medium. I n d e p e n d e n t e x p e r i m e n t s a t ambie n t p r e s s u r e showed t h a t C60TDAE d o e s n o t r e a c t w i t h FC-75. P r e s s u r e w i t h i n t h e c e l l was d e t e mined at low temperature by the inductively-measured shift in the superconducting transition temperature of a piece of htgh purity lead. The r e l a t i v e accuracy of the pressure determinations was e s t i m a t e d t o be ± 0.1kbar. Results of the pressure measurements are shown i n F i g . 2 where t h e ac s u s c e p t i b i l i t y of C#0TDAE a t s e l e c t e d p r e s s u r e s i s p l o t t e d as a function of temperature. The c u r v e a t a m b i e n t p r e s s u r e i s v e r y s i m i l a r t o t h a t shown i n F i g . 1. The o r i g i n o f t h e p e a k n e a r 10K i s unknown b u t , a s shown i n R e f . 7, i s s t r o n g l y f i e l d dependent, e v o l v i n g i n t o o n l y a weak c h a n g e i n s l o p e when t h e dc m a g n e t i z a t i o n i s m e a s u r e d i n a 100-Oe f i e l d . The more p r o n o u n c e d p e a k i n F i g . 2 i s t h e n p r o b a b l y due t o t h e s m a l l e r field u s e d i n t h e ac m e a s u r e m e n t compared t o t h e a p p r o x i m a t e l y 1-Oe dc f i e l d u s e d t o a c q u i r e t h e d a t a i n F i g . 1. At 0 . 1 k b a r , t h e l a r g e ac response found at ambient pressure is suppressed substantially a n d Tc a p p e a r s t o be shifted t o l o w e r t e m p e r a t u r e by 1-2K. By 1 . 6 k b a r t h e ac r e s p o n s e shows no e v i d e n c e f o r a phase transition above 2K. (Steps in these c u r v e s n e a r 7K a r e c a u s e d b y t h e s u p e r c o n d u c t ing transition of t h e l e a d manometer which c a u s e s a s m a l l f l u x c h a n g e t o be s e n s e d by t h e pick-up coil c e n t e r e d on t h e C#0TDAE s a m p l e . ) A d d i t i o n a l m e a s u r e m e n t s a t 0 . 5 k b a r , n o t shown in Fig. 2, r e v e a l a v e r y weak a n o m a l y i n t h e response at -llK t h a t c o u l d be a s s o c i a t e d w i t h a magnetic transition. Upon releasing the pressure from its highest value, the ambientpressure curve in Fig. 2 was reproduced. Therefore, these data indicate a very strong, reversible pressure-lnduced suppression of the

0

I

I

I

5

10

15

Temperature

Fig. 2. Inductive

response

20

(K)

of

C#0TDAE versus of the applied pressure• Note the absence of a magnetic transition at P-l.6 khar. Steps in the curves near 7K are due to the superconducting transition of the lead manometer. The ambient-pressure curve was reproducible upon releasing the pressure from Its largest value•

temperature at selected values

m a g n e t i s m i n C#oTDAE. I n s p e c t i o n o f t h e d a t a s u g g e s t s t h a t i t i s t h e m a g n i t u d e o f t h e ac r e s p o n s e , i . e . t h e m a g n e t i c moment i t s e l f , that is most strongly s u p p r e s s e d by p r e s s u r e a n d that the transition temperature, though decreasing rapidly w i t h p r e s s u r e , does n o t dec r e a s e a s r a p i d l y a s t h e moment. There are at a minimum four distinct a p p r o a c h e s to producing a small saturated moment: (I) superparamagnetism (2) ferrimagnetism, (3) fleld-lnduced weak ferromagnetism and (4) itinerant ferromagnetism. The first possihillty was ruled out in Ref. 7. Mechanisms 2 and 3 assume localized moments that are coupled dominantly by antiferromagnetic exchange. In fact, measurements of the magnetic susceptibility above 50K indicate Curle-Welss behavior with a negative paramagnetlc temperature 8p-22.5K and an effective moment ~f 1.72BB/C60, a value close to that expected for a simple spin-l/2 system. See Fig. 3. Favoring a ferrimagnetlc interpretation is the presence of a maximum in the magnetization near 10K (as shown in Figs. 1 and 2) that could arise from a difference in magnitude of two different sublattlce magnetizations. Generally, though, ferrlmaEnetlsm requires two ins-

MAGNETISM IN C60 TDAE

Vol. 82, No. 10 1 .oxlo e

I

I

I

I

I

I 150

I 200

I 250

Temperature

(K)

Cso TDAE 0.8

E

0.6

0.4 't,,-

0.2

510

0"00

I 100

300

Fig. 3. Inverse susceptibility of C60TDAE versus temperature measured in an applied fleld of 50 kOe. Corrections for core diamagnetism have been estimated from EPR measurements and subtracted from the measured susceptibility. Above 50K, X - P ~2 f /(T-O) with

pdf

- 1.72PB/C60 and 0 - - 2 2 . 5 K .

quivalent magnetic sites. Best refinement of the crystal structure of C#0TDAE suggests that all C60 sites are crystallographically equivalent, and furthermore the strong suppression of magnetism with pressure would require that the local moments be unstable with respect to modestly decreasing volume, an unlikely posslbillty. On the other hand, the lack of inversion symmetry in the crystal structure of C60TDAE could allow fleld-lnduced weak ferromagnetism through anlsotropic exchange, as discussed by Dzyaloshinskl I° and Moriya 11 . The strong pressure dependence observed in Fig. 2 then could be explained by a large, anlsotroplc compressibility of C60TDAE, a distinct possibility. However, the Dzyaloshlnski-Morlya interaction leads to a relationship between the saturated magnetization Ms and the Lande g-factor: M~/&UBS = Ag/g. Taking S - 1/2, as inferred from the high-temperature susceptibility 12, and g 2.0008, measured by EPR, T gives Ag/g - 4 x 10 .4 which is much smaller than M s / ~ B S = 0.i. Further, there is no evidence from isothermal M(H) curves for a field-induced transition, unless the field dependence of the magnetization near IOK were interpreted this way. This leaves the possibility of itinerant ferromagnetlsm, as suggested originally 7 for C#0TDAE. In this case Coulomb correlations among electrons in a relatlvely narrow, partlally-filled band produces a ferromagnetic ground state among the conduction electrons. Theoretical argumentsls, 14 suggest that this might be posslble in C60TDAE. The strong pressure dependence of magnetism in this picture would arise from a strong volume dependence of

781

the anlsotroplc band structure and of the effective electron-electron interaction. Indeed, Wohlfarth I$ has pointed out that, among weak itinerant ferromagnets based on Fe, dTc/dP = -=/Tc, where o~(2000~_100)K~/kbar. More generally, if the moment Itself changes with pressure, we expect le dTc/dP -2 kxoTc(P-O), where Xo is the high field susceptibility at T - 0 and k is a constant. Therefore, a very large negative pressure coefficient of Tc might be expected if C#0TDAE were an itinerant ferromagnet. Further, the observation of a Curle-Welss susceptibility above T c is not necessarily at odds with the existence of itinerant ferromagnetism, as shown 17 recently from a Fermi-llquld analysis of itlnerant-electron magnetism in low-tattler density systems. To summarize, we have found that pressure suppresses the magnetism in C60TDAE extremely rapidly and have considered several possible interpretations for the origin of both the magnetism and its pressure dependence. None of the mechanisms suggested provide a completely adequate description of all observed properties of C60TDAE. In part this may arise from a strong anisotropy of these properties that has not been revealed from studies on polycrystalline samples but that is expected on the basis of the quasi-i dimensional crystal structure of C00TDAE. In the absence of single crystals of C60TDAE, perhaps the most important experiment is to determine accurately the temperature dependence and magnitude of the conductivity of C#0TDAE, a measurement that would help to decide between localized and itinerant descriptions. Certainly, single crystals will be essential for a complete understanding of magnetism in C60TDAE. Acknowledgement Work at Los Alamos was performed under the auspices of the U.S. Department of Energy. F. W. was supported in part by National Science Foundation grants DMR-88-20933 and CHE 89-08323 and P. W. S. by NSF grant DMR-8922066.

References

i.

2.

3.

4. 5. 6.

K. Awaga, T. Inabe, U. Nagashlma and Y. Maruyama, J. Chem. Soc. Chem. Commun. 1989, 1616 (1989). K. Awaga and Y. Maruyama, J. Chem. Phys. 91, 2743 (1989); Chem. Phys. Lett. 158, 556 (1989). K. Awaga, T. Inabe, U. Nagashlma and Y. Maruyama, J. Chem. Soc. Chem. Commun. 1990, 520 (1990). K. Awaga, T. Sugano and M. Klnoshlta, J. Chem. Phys. 85, 2211 (1986). P.-M. Allemand, G. Srdanov and F. Wudl, J. Amer. Chem. Soc. 112, 9392 (1990). M. Takahashi, P. Turek, Y. Nakazawa, M. Tamura, K. Nozawa, D. Shlomi, M. Ishlkawa and M. Kinoshlta, Phys. Rev. Lett. 67, 746

(1991). 7.

P.-M. Al!emand, K. C. Khemanl, A. Koch, F. Wudl, K. Holczer, S. Donovan, G. Gruner and J. D. Thompson, Science 253, 301 (1991).

782 8.

9.

10. 11. 12.

M A G N E T I S M IN C60 T D A E P. W. Stephens, D. Cox, J. W. Lauher, L. Mihaly, J. B. Wiley, P.-M. Allemand, A. Hirsch, K. Holczer, Q. Li, J. D. Thompson and F. Wudl, Nature 355, 331 (1992). J. D. Thompson, Rev. Scl. Instrum. 55, 231 (1984). I . D z y a l o s h t n s k i , J . Phys. Chem. S o l i d s 4, 241 ( 1 9 5 8 ) . T. Moriya, Phys. Rev. 120, 91 ( 1 9 6 0 ) . J . D. Thompson, G. S p a r n , F. D t e d e r i c h , G. G r u n e r , K. H o l c z e r , R. B. Kaner, R. L. W h e t t e n , P.-M. Allemand, Q. Li and F. Wudl, P r o c . M a t e r . Res. Soc. ( i n p r e s s ) .

13. 14. 15.

16.

17.

Vol. 82, No. 10

S. C h a k r a v a r t y , M. P. G e l f a n d and S. K i v e l s o n , S c i e n c e 254, 970 ( 1 9 9 1 ) . C.M. Varma, Z. Zaanen and K. R a g h a v a c h a r i , S c i e n c e 254, 989 ( 1 9 9 1 ) . E. P. W o h l f a r t h , in Itinerant-Electron Ma2netism, ed. R. D. Lowde and E. P. Wohlfarth ( N o r t h - H o l l a n d , Amsterdam, 1977) p. 305. M. White and T.H. Geballe, Order in Solids, (Academic Press, Boston, 1979) p. 143. S. Misawa, T. Tanaka and K. Tsuru, Europhys. Left. 14, 377 (1992).