VNU. JOURNAL OF SCIENCE, Mathematics - Physics, T.xx. N03AP, 2004
P R O P E R T IE S O F L a -D E F IC IE N T L a0S0C a030M n O 3.s C O M P O U N D
N g u y e n H u y S in h , V u T h a n h M ai, N g u y e n A n h T u a n D epartm ent o f Physics, College o f Science, V N U
H o a n g D ue Q u a n g
D epartm ent o f Physics, C hungbuk N a tio n a l U niversity, K orean
A b s tr a c t: A L a -d e fic ie n t sa m p le o f La050C a 0 30M n O 3.6 w a s p re p a re d b y th e so lid - s ta te re a ctio n m eth od . T h e c rysta l structure o f th e s a m p le is s in g le p h a s e an d o rth o rh o m b ic . T o g e th e r w ith M (T ) curve s, fro m th e Xac'' c u rv e th e C u rie te m p e ra tu re o f th is c o m p o u n d n e a rly 3 0 0 K h a s b e e n d e te rm in e d w h ic h is s ig n ific a n tly h ig h e r th a n th o s e o f th e L a ,.,C a ,M n 0 3^ s y s te m . T h e m a g n e tic e n tro p y c h a n g e re a ch e s a m a xim u m v a lu e o f - A S m = 5 .3 J /k g .K a t th e C u rie te m p e ra tu re u p o n a 5 T m a g n e tic fie ld v a ria tio n . V a lu e s o f 0 .0 1 6 3 a n d 0 .1 5 3 fo r th e o xy g e n d e fic ie n c y Ô a n d th e ra tio o f M n 3*:M n4', re s p e c tiv e ly , h a v e b e en d e te rm in e d . It is s u g g e ste d th a t th is co m p o u n d is a s u ita b le c a n d id a te fo r a p p lic a tio n a s a w o rk in g s u b sta n ce in m a g n e tic re frig e ra tion .
1. I n t r o d u c t io n
M agnetic refrigeration h a s a ttra c te d considerable in te re s t d u e to its ad v a n tag es over gas refrigeration [1]. R ecently, atten tio n s are focused on com pounds w ith a large m agnetic entropy change a t a high tem p eratu re a s well a s L a 1.xC a,M nO s.i com pounds [2]. O n th e o th e r hand, one o f th e in tere stin g problem s of m anga nese perovskites is th e non - stoichiom etry of oxygen - the La-deficient com pounds. In p a rtic u la r, a g ia n t m agnetocaloric effect, an d m agnetic properties ta k e place in La-deficient com pounds. E xperim entally, the charge-ordered s ta te (CO) th e real-space ordering of c h a rg e c a rrie rs M n3*, M n4‘ w as observed in m any of th e hole-doped m anganese oxides. In th is p ap e r, electronic and m agnetic properties o f La deficient La05Ca„ 3M n03.i com pound h av e been presented.
Especially th e charge order s ta te w as observed a t n e a r an tife rro m ag n e tic to param ag n e tic tran sitio n tem perature.
2. E x p e r im e n ta l
T he sam ple of non-nom inal- stoichiom etry com positions of LaO5Cao.3M n03 w as p repared by th e solid state-reaction m ethod, which h a d re p o rted som ew here [3]. The m easurem ents have been carried o u t by X - ra y pow der diffraction (XPD), m agnetization M(T), m agnetocaloric effect, susceptibility (x,c) resitiv ity R(T), oxygen deficiency (Ỗ) an d ratio of M n3*:Mn4*.
3. R e s u lts a n d d i s c u s s io n
The XPD p a tte n reveals th a t th e sam ple is a single p h ase orthorhom bic * perovskite stru ctu re . L attice p a ram eters of the sam ple hav e been d ete rm in e d , w hich a r e a = 5.447,
109
110 N g u y e n H u y S in h , V u T h a n h M ai,..
b = 7.713. an d c = 5.445 which is identified with the crystal s tru c tu re of the p aren t compound L aM n 0 3 [4]. So it is found th a t th e crystal stru ctu re of sam ple h a s been distorted by th e La deficiency.
By th e tritatio n m ethod, th e oxygen concentration in LaosCaoaMnOjs h a s been determ ined. W ith 6 = 0.0163. From the oxygen deficiency 5, th e ratio of Miru : M n4’ was estim ated to be 0.153. It can be seem t h a t the large am ount of M n3* becam e M n4< in com parison w ith th a t of L aM n 0 3, which contained ab out 5% Mn'1* only [4]. This is the m ain reason th a t causes the changes in properties of th e sam ple.
From EDS p attern , th e real compositions of the sam ple h a s been estim ated wish La = 66.475%, Ca = 3.70% and M n = 29,83%.
Fig. 1 show s th e tem p eratu re dependence of the m agnetization m easured up field of 1000 Oe under zero - field (ZFC) and field cooled (FC) conditions. It is found th a t th e magnetic mom ent curves of th e sam ple in th e ZFC coinciding with th a t is FC. T his suggests that the spin o rder does not depend on external m agnetic filed until 1000 Oe. The Curie tem perature Tr is determ ined a s 292K on both the tem p eratu re dependence of the m agnetization and of th e susceptibility m easurem ents. From the Xuc'1 curve in fig 2, the param agnetic-ferrom agnetic tran sitio n a t Curie tem p eratu re h a s indicated. The Tc of this sam ple is re lative large th a n th a t of La,.
xCasMnO;j system . The key m echanism is connected to th e charge tra n s fe r induced by eith er stoichiom etric defects, such a s action vacancies or oxygen co ntent o r hole doping by partial substitution on th e Mn - site. That m eans the su b stitu tio n of Ca 2* for La*5 in La - deficient case leads to an increase of th e Mn3*- M n1* localized states, accom panying an enhancem ent of the ferrom agnetic double exchange interaction. Therefore, th e Curie
P nm a s tru c tu re in com parison w ith th e
ZFC FC
ion 150 200 250 300 3!
T(K) F ig.l. The magnetization curves of FC
and ZFC conditions
Fig.2. The X„,:(T) curve of the sample (insert is the xac'(T) curve)
entropy change il oua fields
P r o p e rtie s o f L a -d e fic ie n t L a 0 S0Ca030M nO 3.s c o m p o u n d 111
tem p eratu re has increased. From th e M(H) curves of m agnetization in the dependence on applied field up to 5 T, th e m agnetic - entropy change ASmn(, can be approxim ately calculated an d the obtained value a s Õ.3 J/kg.K (fig. 3).
T his large value originated from the considerable change of th e m agnetization nea r Tc. A nother possible reason is th a t higher m agnetic field, th e m agnetic m om ents are oriented b etter th an a t low er m agnetic field and w hen the ratio of M n;l7M n4* w as reduced, the com petition between DE an d SE interaction increased, th is S E interaction will be dom inated.
The resistivity curve of the sam ple in fig 4 showed the m axim um value a t TM| = 185 K w here the in su la to r - m etal tran sitio n occurs. T he n atu re of th e I-M generally transition can be understood th a t th e J a h n - T e lle r distortion due to th e M n:l* ions plays a key role in m anganese. T he creation of M n1' ions removes the distortion leading to move cubic stru ctu re s. Therefore, across th e I- M tran sitio n appearing a t T|M, th e J -T distortion decreases [5], and th e distortion becomes more prom inent in in su la tin g phase. Especially, on th e rensitivity curve, we found an anom alous point a t ab out 285 K. The resistivity is strongly dropped from th is point tow ard to h igher tem peratures. It is suggested th a t may be th is point th e charge - ordering, because of charge ordering can also possible occurs around C urie -te m p e ra tu re tran sito n (292 K).
4. C o n c lu s io n s
The obtained re su lts revealed in trinsic processes in th is com pound. I t is found th a t the tran sitio n from ferrom agnetism to param agnetic showed a t C urie tem p eratu re Tc * 292K, which is n e a r room tem p eratu re, and in su la to r - m etal tran sitio n occurs in low tem p eratu re region a t 185K and large m agnetic entropy changes aro u n d Te.
Especially, it is observed th e anom alous point at 285K, which we suggested the change ordering around C urie tem perature.
Acknowledgments. T his work w as supported by The N ational F u n dam ental Project 421.104/2004-2005 an d TN .04.07 (2004).
R e fe re n c e s
1. K.A. Gschneider J r and V.A. Pecharky, J. APPL. Phys. Vol 85(1999) 5365.
2. X. Bohigas, J. Tejada, M.L. M asinez-Sarriou, s . Tripp, R. Black, J. Magn. Magn. Mater, 208(2000)85
3. N.H. Sinh, N.p. Thuy, J. Magn. Magn. Mater, 262(2003) 502 4. C.N.R. Rao, A K . Raychaudhuri, World Scien. Pub. Singapore (1998)147.
5. C.N.R. Rao, A-K. Raychaudhuri, World Scien. Pub. Singapore (1998) 3.
T(K) Fig.4. The R(T) curve of
