Fig. 1. Structure of Ge 46 Clathrates endohedrally encapsulating Ba and magnetic d-electron Mn. GERMANIUM CLATHRATE WITH d -TRANSITION ELEMENT New aspects in magnetism have recently arisen with the advent of nano-materials. Nano-cage materials directed to magnetism have become one of the most important issues in materials science, as seen in endohedral-fullerenes [1] and in a series of rare-earth boron compounds [2,3]. Giant magneto- resistance in manganese copper-oxides has also gained intense interests from the scientific and technological fields which concentrates on the unique interactions between magnetic- and conduction-electrons [4,5]. In this system, the long-distant magnetic d -electrons can interact with each other through nano-scale spacing in an isotropic three-dimensionality, leading to the occurrence of a unique spontaneous spin- ordering at 10 K. The clathrate Ba 8 Mn 2 Ge44 , was made by simply melting the stoichiometric amounts of the elements using an RF-induction furnace under an argon atmosphere. It should be noted that the germanium atoms are melt followed by self-assembling into the clathrate structure during the process of natural We have taken a new approach in designing a novel magnetic material on a model of germanium nano-cluster crystals with a polyhedral cage structure [6], where d - block element Mn resides on the position connecting Ge 20 dodecahedra cluster cages and alkaline-earth metals Ba are encapsulated inside the cluster cages as shown in Fig. 1 [7]. The both elements thus introduced act as independent sources of magnetic- and conduction- electrons. cooling. The product obtained was then analyzed by X-ray diffraction measurements at room temperature using synchrotron radiation at beamline BL02B2 as shown in Fig. 2. Preliminary Rietveld refinement using a Cerius [2] program has been performed, assuming that two Mn atoms reside on the crystallographic 6c positions and eight Ba atoms are spherically encapsulated both inside the dodecahedral Ge 20 0 5 10 15 20 25 30 35 40 2 θ angle Intensity (arbitrary unit) ( Pm 3n, a = 10.68 Å) R p = 3.0%, R wp = 4.7% Ba 8 Mn 2 Ge 44 Fig. 2. High resolution X-ray diffraction spectrum of Ba 8 Mn 2 Ge 44 . and tetrakaidecahedral Ge 24 cages (at the 2a and 6 b p o s i t i o n s ) . T h i s r e s u l t e d i n a r e a s o n a b l e R wp fa ct or o f 4. 7% . T he l at ti ce p ar am et er o f Ba 8 Mn 2 Ge 44 at room temperature is 10.68 Å with a space group o f P m - 3 n . T h i s i s t h e f i r s t c o m p o u n d t h a t ac co mm od at es d -e le ct ro n el em en t Mn i n ge rm an iu m t y p e I c l a t h r a t e s . S i n c e B a i s e n c a p s u l a t e d , t h e c o m p o u n d i s a i r - s t a b l e l i k e e n d o h e d r a l l y - d o p e d C 82 , being compared to the fact that C 60 fullerides are generally air-sensitive. E x p e c t i n g n e w m a g n e t i c p r o p e r t i e s i n B a 8 M n 2 G e 4 4 , w e m e a s u r e d t h e m a g n e t i z a t i o n under a low magnetic field. When the temperature was decreased from 20 K to 1.8 K under 10 G, a s p o n t a n e o u s m a g n e t i z a t i o n w a s o b s e r v e d w i t h a steep increase in intensity at 10 K, as seen in Fig. 3 . A h y s t e r e s i s w a s a l s o o b s e r v e d w h e n t h e magnetic field was scanned in a loop of 100 G at 1 . 8 K . T h e s e r e s u l t s d e m o n s t r a t e t h a t a f e r r o m a g n e t i c t r a n s i t i o n o c c u r s i n t h i s c r y s t a l . I t should be emphasized that the observation of the f e r r o m a g n e t i c b e h a v i o r i s n o t d u e t o t h e c o n v e n t i o n a l m a g n e t i c d i r e c t - i n t e r a c t i o n s . I t i s u n a m b i g u o u s t h a t t h e f e r r o m a g n e t i c o r d e r i n g o c c u r s i n t h i s d i l u t e m a g n e t i c s y s t e m ( 9 w t % o f Mn ), since a hysteresis was seen as shown in the inset of Fig. 3 even though the value is small. We have also used another mode to investigate magnetic behavior. First, the temperature was s e t at 1.8 K under a zero-fiel d within the experimen tal errors of our SQUID apparatus, and magnetization w a s m o n i t o r e d u n d e r 1 0 G w i t h i n c r e a s i n g t e m p e r a t u r e to 16 K. In te re st in gl y, th e cu rv e ob se rv ed in th is z e r o - f i e l d c o o l i n g m o d e ( Z F C ) s h o w e d a s m a l l different temperature dependence from that in the field cooling mode ( FC ). This is quite unusual for conventional ferromagnetic interactions. Although it still needs the further investigation t o h a v e a s a t i s f a c t o r y e l u c i d a t i o n a b o u t t h e mechanism of the magnetism in this compound, it would at least be worthwhile to discuss here. One o f t h e m o s t i m p o r t a n t f a c t o r s r e s p o n s i b l e f o r t h e oc cu rre nc e of the ma gn eti c ord eri ng s is li ke ly the increase in the density of states at the Fermi level N( E F ) associated with conduction electrons from Ba as well as a little itineration of s,d -electrons on Mn. I n f a c t , w h e n M n i s r e p l a c e d b y a n o t h e r n o n m a g n e t i c el em en t li ke Au , no si gn if ic an t ma gn et iz at io n wa s Fig. 3. Spo nta neou s mag neti zat ion curv e of Ba 8 Mn 2 Ge 44 measured under 10 G. o b s e r v e d i n B a 8 A u 6 G e 40 , s u p p o r t i n g t h e idea that the d -electrons on Mn atoms are e s s e n t i a l l y i n v o l v e d i n t h e m a g n e t i c p h e n o m e n a o b s e r v e d a t l o w t e m p e r a t u r e s . Considering the large average interval of 8.3 Å between Mn atoms, it is plausible to suppose here that the spin ordering of the d - e l e c t r o n s c a n r e s u l t f r o m t h e i r R K K Y l i k e inte ract ions v i a t h e c o n d u c t i o n e l e c t r o n s spreading over the clathrate network. Since the new aspects in magnetism d e s c r i b e d i n t h e p r e s e n t p a p e r c a n b e r e a l i z e d w i t h n a n o - s c a l e c o n t r o l i n t h e p o s i t i o n o f t h e e l e m e n t s , c l a t h r a t e s w i t h m a g n e t i c e l e m e n t s w i l l p r o v i d e a g o o d s c i e n t i f i c s t a g e f o r s h e d d i n g a n e w l i g h t on magnetism in nano scale. References [1] T. Ogawa et al. , J. Am. Chem. Soc. 122 (2000) 3538. [2] T. Susaki et al. , Phys. Rev. Lett. 82 (1999) 992. [3] D.P. Young et al. , Nature 397 (1999) 412. [4] S. Jin et al. , Science 264 (1994) 413. [5] Y. Tokura et al. , J. Phys. Soc. Jpn. 63 (1994) 3931. [ 6 ] H . K a w a j i e t a l . , P h y s . R e v . L e t t . 7 4 ( 1 9 9 5 ) 1427. [7] R.F.W. Herrmann, K. Tanigaki, T. Kawaguchi, S. Kuroshima and O. Zhou, Phys. Rev. B 60 (1999) 1 3 2 4 5 ; T . K a w a g u c h i , K . T a n i g a k i a n d M . Y a s u k a w a , Appl. Phys. Lett. 77 (2000) 3438. K a t s u m i T a n i g a k i a , b , T e t s u j i K a w a g u c h i a a n d Masahiro Yasukawa c (a) Osaka City University (b) PRESTO, Japan Science and Technology Corp. (c) CREST, Japan Science and Technology Corp. E-mail: tanigaki @ sci.osaka-cu.ac.jp 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0 2 4 6 8 10 12 14 16 18 20 22 24 (emu/g) Temperature (K) B G = 1 0 emu/g field (G)
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