T his value is not sm aller th a n those of un-doped sam ples, however,... CMR response is g re atly broadened tow ard low tem p eratu re.[r]
Trang 1IN F L U E N C E O F T H E S U B S T IT U T IO N O F M n -S IT E ON T H E
P R O P E R T IE S O F T H E L a0.fl7C a0.3SM n0JA ,.iO 3 (A = N i, A l, C u)
CO M PO U N D S
N g u y e n A n h T u a n , N g u y e n H uy S in h , T r a n D in h T h o , D o V ie t T h a n g
D epartm ent o f Physics, College o f Science, V N U
A b s tr a c t: T h e in flu e n c e o f th e M n-site su b stitu tion (10 % ) b y Ni, A l an d C u on
in v e stig a te d F e rro m a g n e tic-p a ra m a g n e tic an d m e ta l-in s u la to r tra n sitio n s w ere
w e re fo u n d in th e ir c rysta l structures, T h e C urie te m p e ra tu re T c is lo w e rm o st fo r
N i-d op ed s a m p le H ow e ver, m ag n e tic-fie ld -in d u ce d re s is tiv ity o f th e N i-d op ed
m a g n e to re s is ta n c e ra tio (C M R = (R (0 )-R (H ))/R (0)) re a ch e d 17% in th e N i-d op ed
d o u b le e x c h a n g e (D E ) an d s u p e r e xch a n g e (S E ) inte ra ctio n s p la y v e ry im p o rta n t role in C M R fo r th e se c om p ou nd s
1 I n t r o d u c t io n
In recently years, a gre at deal of work h as been devoted to th e mixed-valence
m anganites R|.,.AvM nO :, (R = R are Earth elem ents, A = Ca, Sr, Ba, Pb) exhibiting colossal
m agnetoresistance (CMR) due to its significance both for fundam ental research and for practical application in m agnetoelectronics [1-2] I t w as show n t h a t two factors govern essentially th e colossal m agnetoresistance (CMR) properties of th ese com pounds, the size of
th e interpolated cation an d th e hole c a rrie r density characterized by th e mixed valence
M n(III) : Mn(IV) M any issue of th is system have focused to revolving of th e principal factors to d ete rm in e th e C urie tem p eratu re an d th e m agnetoresistance Some of them have shown th a t Tc a n d m agnetoresistance are optimized w hen about 30% M n3+ ion is converted
to M n4+ by su b stitu tio n o f stra n g e elem ents [3] For th e case o f doping hole, th ere are large variations in th e observed Tc an d in th e m agnitude of th e m agnetoresistance in this system T hese discrepancies h av e been ascribed to
chem ical disorder, oxyen deficienties, grain boundary
effects, lattice co n stan t effects, etc
It is m ore i n te r e s t to s tu d y th e effects of Mn-
site s u b s titu tio n , w hich is d ire c t su b titu tio n o f a
stra n g e e le m e n t for M n in a n octa h ed ra l s tru c tu re
fram e T h is m av provide clues for both exploring
novel CMR m a te ria ls a n d concerning th e
m echanism o f CMR F or t h is re aso n , we stu d y th e
Lao^C aoiiM no^uO -j (w ith A = Ni, Al a n d Cu)
compounds
* LaMjCa^tjMnafAn/O,
J L
i 1
20 (degree)
F ig l XRD patterns of samples
Trang 22 E x p e r i m e n t a l
S am ples w ith th e nom inal com positions of
La067Ca033M n09A010 3 (A = Ni, Al a n d Cu) w ere prepared
by using th e so lid -sta te re actio n m ethod T he crystal
stru c tu re s of sam p les w ere checked by powder X-ray
diffraction T h e m agnetic a n d electronic properties of
th e sa rrp le s h a v e bee n in v eatigated by the
m agnetization a n d re sistiv ity m easurem ents
3 R e s u lts a n d d is c u s s i o n
Fig 1, XPD p a tte r n prove th a t all sam ples were
single p h ase w ith orthorhom bic perovskite stru ctu re
ISO 200 250 300 T(K) Fig.2 M(T) curves of samples Laa (i7Ca0 3aMn09A0 j0 3 (A = Ni, Al, Cu)
SO 100 150 200 250 300
TOO
50 100 ISO 200 250 300 TOO
50 100 150 200 250 300 T(K) Fig.3 R(T) curves of samples a t H = 0 and 0.4 T (a) A = Ni, (b) A = A] and (c) A = Cu Fig 2 show s th e te m p e ra tu re dependence o f m agnetization of all th e sam ples These curves M(T) show t h a t th e re ex ist a m agnetic ordering tran sitio n from a p aram agnetism to
ferrom agnetism a s T decreases T he C urie te m p e ra tu re Tc 18 160, 245 an d 200 K for A = Ni,
Al an d Cu, respectively T h ese v alues a r e lower th a n th a t of un-doped sam ple (about 260 K) and Tc is low erm ost for N i-doped sam ple T his is due to su b stitu tin g Ni, Al an d Cu for Mn
d ilutes M n sub-lattice cau ses decreasing in ten sity of double-exchange (DE) interaction betw een M n3' a n d M n4\ H ow ever, su b stitu tin g a n ion w ith the highest m agnetic mom ent (Ni) produces th e low est T c T his strongly im plies th a t a su p er exchange-like interaction could ta k e place th ro u g h M n3+/M n l+-0-N i2* [4]
T he te m p e ra tu re dependence of re sistan c e curves u n d e r zero an d 0.4 T m agnetic field
a r e d em o n stra te d in Fig 3 From these curves, we have determ ined th e tra n s itio n from a ferrom agnetic m etallic (FMM) s ta te to a p aram agnetic sem iconducting (PM S) s ta te as T
increases T his tran s itio n occurs a t te m p e ra tu re ab out Tp = 125, 242 a n d 153 K for dopants
of Ni, Al an d C u, respectively U nder m agnetic field of 0.4 T, Tp displaces tow ard higher
tem p eratu re, from 125, 242 a n d 153 K to 131, 245 and 157 K for Ni, Al a n d Cu-doping sam ples, respectively D isplacing to h ig h e r tem p eratu re the m etal-sem iconductor (M-SC) tran sitio n cau ses th e high m ag netoresistance ra tio in th e vicinity of th is M-SC tran sitio n as show n in fig 4 In cases of A = A1 o r Cu, m axim a of CMR in o u r sam ples a r e a b o u t 10% a t
Trang 3it is significantly low er th an those of A = Ni (CMR = 17% a t 100 K), alth o u g h , Tc in case of
A = A1 an d Cu a r c higher th a n those of Ni sam ple T hese re s u lts prove m ore th a t
m agnitude of CMR closely concerns to SE interactions
O ne an o th e r hand, in case of A = Ni CMR response is g re atly broadened tow ard low tem p eratu re T his may be re s u lt from the clu ster glass n a tu re of L a„0-C aOMM n09Ni0 ,Oj Ni substitution induces AFM interaction betw een Ni2+-0-M n37M n4< an d prom otes the proportion of M n^-O -M n44 AFM interaction T he random d istrib u tio n of FM an d AFM exchange in tera ction would favor the form ation of c lu s te r glass, a s evidenced in the tem p eratu re dependence of m agnetization of the sam ple a t low m agnetic field [5] D ue to
th e form ation of FM clusters an d th e ir random ly frozen m om ent as well a s th e large spin fluctuation, th e re should be severe sp atial m agnetic d isorder th a t m ay play a key role in electron localization an d lead to high resistivity s ta te a t low tem p eratu re W hen applied
m agnetic field, th e whole m om ent of th e frozen FM clu sters expand an d th e ir orientation are forced to align uniform ly so th a t th e sp atial m agnetic d isorder is reduced, w hich favors the electron delocalization an d consequently re s u lts in a significant drop of th e low tem p eratu re resistivity This m ay be the reason th a t a low te m p e ra tu re CMR effect is usually observed in a clu ster glass state
In conclusion, th e suppression of T,- an d
a large v aria tio n o f m agnetoresistance have
related to th e effects of Ni, Al and Cu-
substitution (10% a t) for Mn in L a ^ C a ^ M n O j
compound T he responsible for th is case may
be th ere is n o t only by th e decline of DE
interaction re s u lts from diluted M n-sublattice
bu t also com petition between double exchange
and superexchange interactions, especially, the
Ni-O-Mn bond an d influence of m agnetic
m om ent of Ni ions is tak e n account Therefore,
the m agnetic n a tu re of su b stitu tin g elem ents
cannot be neglected in u n d ersta n d in g the
properties of LaCaM nAO system s
Acknow ledgm ents T he N ational F u ndam ental R esearch P roject 421.104/2004-2005 and
TN.04.07 su p p o rt th is work
R e fe re n c e s
1 R M ahesh, R M ahendiran a t al J Solid State Chem., 114(1995) 297.
2 H Song, w , Kim S.-J Kwon J Kang Jour Appl Phys., 89(2001) 3398.
3 P Schiffer, A p Raminez, w Bao, a t al„ Phys Rev Lett 75(1995) 3336
4 H Song, w Kim, S.-J Kwon, J Kang, Jour Appl Phys., 89(2001) 3398.
T (K ) Fig.4 The CMR(T) curves of samples LaowCaojjMnU9Ao |0 3 (A = Ni.Al, Cu)