The resistivity is strongly dropped from th is point tow ard to h igher tem peratures. Raychaudhuri, World Scien[r]
Trang 1P 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 -
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
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,
Trang 2b = 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
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
Trang 3tem 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