PHASE SEPARATION AND INSULATOR-METAL BEHAVIOR OF CMR MANGANITES

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F i g . 1 . E l e c t r o n i c p h a s e d i a g r a m o f N d 0 . 5 5 ( S r 1 - y C a y ) 0 . 4 5 M n O 3 . C O I , F M a n d P S r e p r e s e n t t h e c h a r g e - o r d e r e d i n s u l a t i n g , f e r r o m a g n e t i c m e t a l l i c a n d p h a s e - s e p a r a t e d s t a t e s , r e s p e c t i v e l y . I n s e t s h o w s m a g n e t i z a t i o n curves at 10 K. PHASE SEPARATION AND INSULATOR-METAL BEHAVIOR OF CMR MANGANITES The doped manganites, R 1- x Ca x MnO 3 , where R i s a t r i v a l e n t r a r e - e a r t h m e t a l , h a v e a d i s t o r t e d - p e r o v s k i t e s t r u c t u r e w i t h t h r e e - d i m e n s i o n a l n e t w o r k s o f t h e M n O 6 o c t a h e d r a . T h e i r g e n e r i c b e h a v i o r o f p a r a m a g n e t i c - t o - ferromagnetic transition is understood within the f r a m e w o r k o f d o u b l e - e x c h a n g e t h e o r y . W e n e e d t o c o n s i d e r a d d i t i o n a l e f f e c t s i n o r d e r t o understand the insulator-metal behavior as well as th e co lo ss al ma gn et or es is ta nc e ( CM R ). At p r e s e n t , t h e m o s t p r o b a b l e c a n d i d a t e i s t h e p e r c o l a t i o n m o d e l [ 1 ] , i n w h i c h t h e i n s u l a t o r - m e t a l b e h a v i o r o c c u r s w h e n t h e p e r c o l a t i o n pass of the metallic (ferromagnetic metallic; FM) is connected from one end to the other regions i n t h e s e a o f t h e i n s u l a t i n g ( c h a r g e - o r d e r e d i n s u l a t i n g ; C O I ) s t a t e . T h e c h a r g e - o r d e r i n g t r a n s i t i o n o f d o p e d m a n g a n i t e s u s u a l l y a c c o m p a n i e s a n a n t i f e r r o m a g n e t i c t r a n s i t i o n with the CE-type structure, while the FM state is of half-metallic. Here, to judge the suitability of t h i s m o d e l , w e h a v e p e r f o r m e d a synchrotron radiation X-ray powder diffraction e x p e r i m e n t o n N d 0 . 5 5 ( S r 1 - y C a y ) 0 . 4 5 M n O 3 a t be am li ne BL 02 B2 wi th hi gh an gu la r re so lu ti on and counting statistics [2] . t h e c h a r g e - o r d e r i n g t e m p e r a t u r e T C O w e r e d e t e r m i n e d from the temperature-variation of the magnetization M a n d r e s i s t i v i t y ρ . T h e i n s u l a t o r - m e t a l b e h a v i o r w a s e n h a n c e d i n t h e p r o x i m i t y o f F M - C O I p h a s e boundary (hatched region of Fig. 1 ). Thus, we have c h o s e n N d 0 . 5 5 ( S r 0 . 1 7 C a 0 . 8 3 ) 0 . 4 5 M n O 3 f o r p r e s e n t in ve st ig at io n. Th e in se t sh ow s th e ma gn et iz at io n c u r v e s m e a s u r e d a t 1 0 K . T h e s u p p r e s s e d magnetization curve at 0.83 suggests a coexistence of the FM and antiferromagnetic COI phases [3] . 0.2 0.4 0.6 0.8 1.0 0 100 200 300 400 500 FM C O I PS Ca concentration y N d 0 . 5 5 ( S r 1 - y C a y ) 0 . 4 5 M n O 3 Temperature (K) T C T CO 1 2 0 1 2 3 4 μ μ 0 H (T) y =0.2 at 10 K y =0.7 y =0.83 y =1.0 y =0.87 M ( μ μ B ) To choose an appropriate chemical composition for the pre sen t stu dy, we fir st hav e syn the siz ed a se ri es of ce ra mi cs Nd 0. 55 ( Sr 1- y Ca y ) 0. 45 Mn O 3 , fi ne ly c o n t r o l l i n g t h e o n e - e l e c t r o n b a n d w i d t h t h r o u g h c h e m i c a l p r e s s u r e . P o w d e r X - r a y d i f f r a c t i o n m e a s u r e m e n t s a t r o o m t e m p e r a t u r e a l o n g w i t h R i e t v e l d a n a l y s i s i n d i c a t e t h a t t h e s a m p l e s w e r e single phase without detectable impurities. F i g u r e 1 s h o w s a n e l e c t r o n i c p h a s e d i a g r a m o f N d 0 . 5 5 ( Sr 1- y Ca y ) 0.45 MnO 3 . The Curie temperature T C and F i g . 2 . X - r a y p o w d e r d i f f r a c t i o n p a t t e r n s ( c r o s s e s ) o f Nd 0.55 ( Sr 0.17 Ca 0.83 ) 0.45 MnO 3 at (a) 265 K and (b) 110 K. The solid curve is the result of the Rietveld analysis with a model for (a) single distorted-perovskite ( Pbnm ; Z = 4) and (b ) two distorted-perovskites, respectively. In Fig. 2 , we demonstrate prototypical examples of the X-ray powder diffraction patterns of Nd 0.55 ( Sr 0.17 Ca 0.83 ) 0.45 MnO 3 at 265 K and 110 K. At 110 K we observed r e m a r k a b l e s p l i t t i n g o f t h e B r a g g r e f l e c t i o n s , i n d i c a t i n g t h e p h a s e s e p a r a t i o n ( s e e i n s e t o f F i g . 2 ( b ) ) . W e have analyzed the powder patterns below 200 K using a two-phase model with two distorted-perovskites ( Pbnm ; Z = 4). The R i e t v e l d r e f i n e m e n t s a r e s a t i s f a c t o r y , i n which R wp and R I (reliable factor based on the integrated intensities) are fairly typical o f p u b l i s h e d s t r u c t u r e s . T h e s e t w o p e r o v s k i t e p h a s e s c a n b e c h a r a c t e r i z e d by a lattice constant c . Hereafter, we will refer to the respective phases as ‘short- c ’ (7.54 - 7.58 Å) and ‘long- c ’ (7.60 - 7.62 Å) phases. Figure 3 shows temperature variation o f ( a ) r e s i s t i v i t y ρ , ( b ) l a t t i c e c o n s t a n t s a n d ( c ) i n t e n s i t y o f t h e m a g n e t i c B r a g g reflections of Nd 0.55 ( Sr 0.17 Ca 0.83 ) 0.45 MnO 3 . The most important point here is that the lattice constants indicate a discontinuous c h a n g e a t T C O . I n o t h e r w o r d s , t h e s y s t e m i s t r a n s f o r m e d i n t o a t w o - p h a s e state, both of which differ from the room t e m p e r a t u r e p h a s e . S u c h a s t a t e i s p e r h a p s a s c r i b e d t o t h e r a n d o m n u c l e a t i o n o f a l o w - t e m p e r a t u r e p h a s e and subsequent stress-induced growth of t h e s e c o n d a r y p h a s e ( s t r e s s - i n d u c e d p h a s e s e p a r a t i o n ) . W i t h f u r t h e r temperature decrease, an insulator-metal transition takes place at 157 K (= T IM ). 14.5 15.0 15.5 16.0 0 2 2 θ θ (degree) 10 15 20 25 30 35 40 0 2 4 6 8 10 0 2 4 6 8 Pbnm R I = 4.88% R wp = 2.53% Intensity (10 4 counts) R wp = 2.29% R I = 4.79% (short-c) R I = 3.54% (long-c) Intensity (10 4 counts) 2 θ (degree) (b) at 110 K λ = 0.5051 Å (a) at 265 K Nd 0.55 ( Sr 0.17 Ca 0.83 ) 0.45 MnO 3 Fig. 3. Temperature dependence of (a ) resistivity ρ , (b ) lattice constant a n d ( c ) i n t e n s i t y o f t h e m a g n e t i c B r a g g r e f l e c t i o n s o f N d 0 . 5 5 ( S r 0 . 1 7 C a 0 . 8 3 ) 0 . 4 5 M n O 3 . O p e n a n d c l o s e d s y m b o l s r e p r e s e n t t h e short-c and long-c phases, respectively. T h e b o t t o m p a n e l o f F i g . 3 s h o w s i n t e g r a t e d intensities of the magnetic Bragg reflections. The th re e ma gn et ic re fl ec ti on s, th at is , F- , A- an d CE - t y p e s , s e e m t o a p p e a r a t t h e s a m e t e m p e r a t u r e near T IM . The F- and A-type ( CE-type ) components can be ascribed to the long- c (short- c ) phase based on the lattice constants. The magnetic ordering at l o w e r t e m p e r a t u r e s b e l o w T C O c o n t r a d i c t s t o t h e percolation model. In addition, the volume ratio of the insulating short- c component rather decreases with cooling. A new scenario for the insulator-metal behavior 100 200 300 0 100 200 0 200 400 600 800 F-type + nuclear A-type (0,0,1) CE-type (1/2,1/2,1) Temperature (K) (0,2,0) + (2,0,0) + (1,1,2) Intensity ( arb. units) Intensity ( arb. units) 5.35 5.40 5.45 a b c / ÷ 2 two-phase single phase Lattice Constant (Å) 0 5 10 Nd 0.55 ( Sr 0.17 Ca 0.83 ) 0.45 MnO 3 T CO T IM ρ ( Ω cm ) (c) (b) (a) (a) COI COI PI FM (b) (c) T T CO T C ~ T IM Fi g. 4. Sc he ma ti c il lu st ra ti on s of th e me ch an is m fo r th e in su la to r- me ta l behavior: (a ) T > T CO , (b ) T IM < T < T CO and (c) T < T IM . COI, PI and FM stand for the charge- ordered insulat ing, paramag netic insulat ing and ferromagnetic metallic phases, respectively. i s a s f o l l o w s . W i t h a d e c r e a s e i n t e m p e r a t u r e below T CO , the system is transformed from a single phase ( Fig. 4(a) ) to the two-phase state ( Fig. 4(b) ), p o s s i b l y d u e t o t h e s t r e s s - i n d u c e d p h a s e separation. These phases, that is, the short- c and l o n g - c p h a s e s , c a n b e a s c r i b e d t o t h e C O I a n d pa ra ma gn et ic in su la ti ng (P I) ph as es , re sp ec ti ve ly . With further decrease of temperature below T IM , the long- c phase indicates a PI to FM phase transition ( Fi g. 4( c) ). If th e me ta ll ic re gi on we re co nn ec te d, t h e a p p a r e n t i n s u l a t o r - m e t a l t r a n s i t i o n w o u l d b e observed. Yutaka Moritomo and Akihiko Machida Nagoya University E-mail: moritomo @ cirse.nagoya-u.ac.jp References [1] M. Uehara, et al ., Nature 399 (1999) 560. [2] A. Mac hid a, et al ., Phy s. Re v. B 62 (2 000 ) 3 883 . [ 3 ] Y . M o r i t o m o e t a l . , P h y s . R e v . B 6 0 ( 1 9 9 9 ) 10374.