T his observation is special case because according to the phase diagram of La^Ca^M nO^s have been reported by Shiffer et. Shraim an, Phys.[r]
Trang 1M A G N E T IC P R O P E R T I E S AND E X IS T E N C E O F C H A R G E-
O R D E R IN G S T A T E IN L a,.xCaxM n 0 3 SY STE M
V u T h a n h M ai, N g u y e n H u y S in h , N g u y e n A nh T u a n , D o H o n g M in h
D epartm ent o f Physics, College o f Science, V N U
A b s tr a c t: M a g n e tic p ro p e rtie s o f L a ,.„C a ,M n 0 3 (x = 0 0 0 -r 0 7 0 ) s y s te m ha ve
p a ra m e te rs d e c re a s e d b y C a -d o p in g con cen tratio ns T h e C u rie te m p e ratu re s,
de te rm in e d T h e re a l c o m p o s itio n s o f La , C a, M n w e re e s tim a te d b y E D S -
fo u n d in lo w -d o p in g s a m p le s o f X = 0 1 0 and 0.30 a t T co » 2 0 0 K a n d 110 K,
re s p e c tiv e ly
1 I n t r o d u c t i o n
T he discovery of th e m agnetoresistive behaviour of the mixed valence m anganites A,.xB ,M n 0 3 (w ith A = La, B = C a, Sr, B a ) in 1993 [1], opened a very in tere stin g field of research from th e fu n d a m e n tal a s well a s from th e practical p o int of view T he hole-doped
m ixed-valence m an g a n ite perovskites can exhibit a m agnetic tran sitio n a t a Curie
te m p e ra tu re (Tc), from a param ag n e tic s ta te (PM) a t high te m p e ra tu re to a ferrom agnetic
s ta te (FM) a t low te m p e ra tu re s Also, they pre sen t a transition from a n in su la to r state to a
m etallic conducting s ta te , w hich is com panied by a peak in th e re sistiv ity a t T,, [2] In addition, th e colossal m ag n etoresistance th e phase separation and th e ch a rg e ordering effects [3] in d ifferent regions of th e hole concentration (x) hav e been found These
in te re s tin g pro p e rties o f th e La|.xCaxM n 0 3 system are governed by com peting M n3*-Mn3* superexchange (SE) an d M ns*-Mn'" double exchange (DE) in tera ctio n s accom panied with local J a h n -T e lle r disto rtio n s a n d form ation of a mobile sm all-polaron s ta te [4]
T his p a p e r in v estig ated th e m agnetic properties of th e L a1„C asM n 0 3 system, especially, th e existence of th e charge-ordering s ta te in low doping region (x = 0.10 and 0.30) w as shown
2 E x p e r i m e n t a l
S am ple of La,.xCaltM n 0 3 w ere pre p are d with high-purity oxides of C aC 0 3, MnO^, La.,0;, by solid s ta te re actio n m ethod T h e m ixtures were p re sinte red a t 950"C for 10 hours and th e n t h a t m ateria ls w ere grounded an d pressed into th e pillets A fter th at, the
s in te rin g process w as c a rrie d o u t a t 1340"C for 30 hours and th en an n e aled a t 650°c for 48 hours T he sam p les w ere cooled in a ir by th e furnace turned off
Trang 23 R e s u lts a n d d is c u s s io n
T he XPD p a tte rn s revealed th a t th e
sam ples w ere a single p hase with orthorhom bic
perovskite stru ctu re From XPD peaks, the
lattice p a ram eters (a, b, c) of th e sam ples have
been calculated From obtained re sults, it can
be seen th a t w ith increasing of the doping
concentration Ca2*, the a an d b lattice
p aram eters w ere reduced One of the m ajor
re asons for this reduce m ay be ra d iu s of ion
Ca2* (0.99Ả) sm aller th a n th a t of La3* (1.016 Ả)
Also because of th is reason th e volum e of cell
u n its decreases by increasing Ca-doping
concentrations T hus, th e increase in doping Ca
concentration causes the lattice distortion and
th u s is Jah n -T eller distortion typical W hen
Las‘ is su b stitu ted by Ca2*, th e proportion of
M n4< increases and th e orthorhom bic distortion
decreases T he ra tio of Mn3‘:Mn44 ions
dependences on th e doping concentration (x) in
system showed in Fig 1 T his curve looked like
a fitting hyperbola T he real com positions of La,
Ca an d Mn w e re estim ated in Ca/La u n its by
EDS m ea surem ent m ore sm aller th a n nom inal
com positions in all sam ples (these values are
not shown here) The tem p eratu re dependence
of th e m agnetization in all sam ples have been m ea su red in field cooled (FC) an d zero-field cooled (ZFC) u n d e r a n external field of 1.1 kOe From M(T) curv es we ex tra c t th e Tc values
of th e sam ples a s th e tem p eratu re w here th e cu rve h a s its inflexion point T he C urie tem p eratu re show ed in table 2
The m ea su rem en t of th e m agnetization dependence on th e v ario u s field have been perform ed u p to 5 T, in various tem p eratu re ra n g in g from 200 K to 300 K From th ese curves, the o b tained m agnetic entropy change A S ^ a s a function of te m p e ra tu re an d th e ir values for each sam ple was showed in Fig 2 I t is clear t h a t th e larg e m agnetic entropy changes in th is system originate from th e considerable ch ange of m ag netization n e a r T c The tem p e ra tu re dependence on the susceptibility of th e s am p les h a s been m easured (which are not show n here) From these curves, th e C u rie-tran sitio n te m p e ra tu re h a s been obtained a t peaks of susceptibility curves T he v alues of th e tran s itio n te m p e ra tu re s are corresponding to th e sam e obtained those from m agnetization curves, w hich showed tran sitio n from ferrom agnetism to param agnetism
The te m p e ra tu re dependence of th e re sistiv ity curves in th e sam p les h av e indicated
F ig l Ratio’of Mn3*:Mn° dependences on doping concentrations
T(K ) Fig.2 The tem perature dependence of
of La,.,Ca,Mn03
Trang 3m axim um on resistivity curves T hese tran sitio n
tem p eratu res (Tp) have been determ ined 251 K,
265 K and 257 K, respectively, w hich are nearly
the Tf tem p eratu res The resistivity curves 5
showed the charge-ordering s ta te in o th er X
sam ples A part from th e charge-ordering in
sam ple of X = 0.50, it is found th a t the existence
of charge-ordering s ta te in sam ples of x=0.10 and
0.30 Fig 3a an d 3b d em onstrated th e existence
of charge-ordering in suddenes of drop on
resistivity curves of the sam ples with X =0.10 and
0.30 T his observation is special case because
according to the phase diagram of La^Ca^M nO^s
have been reported by Shiffer et al [3], th e CO-
appeared a t X = 0.51 W hile K um ar et Al [8] had SI
found th e C O -state in region of 0.63 < X < 0.67 x
Hyroyuki et Al [9] observed CO a t X < 0.20 This
o ur re su lt contributed to clear th a t th e charge-
ordering not only occurred in high-doping
concentration w ith X > 0.50, b u t th e re is a very
fair possibility of C O -existent in th e lower doping
concentration of X < 0.50 in La^CajM nOa.*
system , also
4 C o n c lu s io n s
Ca2' su b stitu tio n for La3* causes the increasing of M n“ an d decreasing of orthorhom bic distortion in crystal s tru c tu re of sam ples The values of m agnetic entropy change indicated in tab le 2 a re re lative large in th is system Especially, th e existence of
C O -state in low-doping concentration of C a with X = 0 1 0 an d 0.30 h ad invented in this system
Acknow ledgm ents T his work is supported by the National F u n dam ental R esearch Project
421.104/2004-2005
R e fe re n c e s
1 R Von Helmolt, J Wecker e t al, Phys Rev Lett 71 (1993) 2331.
2 A P Ramirez, J Phys: Condense M atter 9(1997) 8171.
3 Y Moritomo, Phys Rev B 60(1999) 10374.
4 A J Mills, P B Littlewood, B I Shraim an, Phys Rev Lett 74(1995) 5144.
5 N Kumar e t al Jour Solid State Chem., 129(1997) 363.