VNU. JOURNAL O F SCIENCE. Mathem atics - Physics, T.xx, N03AP, 2004
E X T R A O R D I N A R Y C O L O S S A L M A G N E T O R E S I S T A N C E I N L a 0 67C a033M n09C r0 ,(>3 C O M PO U N D
N g u y e n A n h T u a n , N g u y e n H u y S in h , Do H o n g M in h , V u T h a n h M ai D e partm ent o f Physics, College o f Science, V N U
Abstract: The magnetic and magnetotransport properties of La06,Ca033Mn09Cra,O3 c o m p o u n d h a v e b e e n in v e s tig a te d . T h e resista n ce o f th e sa m p le s tro n g ly c h a n g e s b y m a g n e tic fie ld v a ria tio n in lo w e r te m p e ra tu re re g io n o f th e m etal- s e m ic o n d u c to r tra n s itio n te m p e ra tu re . The m a xim u m m a g n e to re s is ta n c e ratio (M R = (R {0 )-R (H ))/R (0 )) o f 1 2 .5 % in th e sa m p le is ob ta in e d in a m a g n e tic fie ld of 4 kO e . It is fo u n d th a t th e e x tra o rd in a ry c o lo ssa l m a g n e to re s is ta n c e be ha vior, c h a ra c te riz e d b y d o u b le p e a k s o n th e curve o f th e m a g n e to re s is ta n c e versu s te m p e ra tu re , c o n c e rn in g b ro a d e n e d te m p e ra tu re ra n g e o f C M R . T h e e x p la n a tio n o f th e s e re s u lts b a s e d o n s u p p o s e o f th e e x iste n ce o f th e fe rro m a g n e tic cou p lin g b e tw e e n C r a n d M n ion s.
1. I n t r o d u c t i o n
A fter th e discovery o f m agnetoresistance effect in th e neodym ium m anganite N dODPb(|6M n 0 3 [1], a sy stem atic exploration of the lan th an u m perovskites, La, sBsMnOa w ith B = Ca, S r, Ba, Pb w a s perform ed, dealing eith er w ith th in film s [2] or ceram ics [3]. It w as show n th a t two factors govern essentially th e colossal m agnetoresistance (CMR) properties o f th e s e com pounds, th e size of the interpolated cation an d th e hole carrier d ensity charac terize d by th e m ixed valence M n(III) : Mn(IV). Besides, it is now clear th at lattice stra in an d deform ations, w hich affect the M n "-0 -M n 14 bond angle an d length, have d ra m a tic consequences for th e pro p e rties of th ese system s Ị4]. In term s of th e crucial role of M n site, it would be in te re s tin g an d w orthw hile to study th e effects of doping the M n-sites with a foreign cation, w hich m ay provide clues foi- both exploring novel CMR m aterials and concerning w ith m echanism of CMR. In th e last years, th ere hav e been considerable reports on th e effects of M n-site su b s titu tio n by foreign elem ents such a s Fe, Ni, Co [5], A] [6], Cr [7], etc. Am ong th e s e doping elem ents, Cr has a spectacular effect and a ttra c ts more a tten tio n . For instance, th e doping on M n site with 5% of C r in th e charge ordered (CO) in su la to r P r0 5Ca0 6M n 0 3 can destroy th e CO antiferrom agnetic s ta te and induces an in sula tor-m etal tran s itio n [7]. F or th is reason, we study th e m agnetic and m a g n e to tra n sp o rt pro p e rties o f th e LaOU7Cao33Mn0 flCro ,0 3 compound.
2. E x p e r i m e n t a l
Sam ple w ith th e nom inal com positions of La0C,Ca033M n09Cr0 ,O3 w ere prepared by using the so lid -sta te re actio n m ethod. The stru ctu re and p hase p u rity of as-prepared sam ple w as checked by pow der X-ray diffraction (XRD) usin g Cu Ku ra d iation a t room tem p eratu re. T he m ag n e tiz atio n m ea su rem en ts w ere carried o u t using an extracted sample m agnetom eter (ESM). R esista n ce w as m easured by sta n d a rd four-probe method.
140
E x tr a o r d in a r y c o lo s s a l m a g n e to r e s is ta n c e in... 141
3. R e s u lts a n d d is c u s s io n
Fig. 1, XPD p a tte rn prove th a t the sam ple w as a single phase with orthorhom bic perovskite structure.
The te m p e ra tu re dependence of m agnetization of th e sam ple registered in a m agnetic field of 80 Oe in fig. 2 shows t h a t th e re exists a m agnetic ordering transition from param ag n e tism to ferrom agnetism a t about 215 K a s T decreases. Interestingly, when T<TC, the m agnetization decreases w ith cooling. This im plies th a t the an tiferrom agnetic (AFM) com ponent m ay develop a t low tem p eratu re. It is also clear th a t the zero-field cooled (ZFC) m agnetization d ata do not coincide with FC d a ta below T c, which is a characteristic of clu ster glass [8].
The tem p eratu re dependence of th e resistance u nder zero and 4 kOe m agnetic field is dem onstrated in fig. 3. From th ese curves, we have determ ined th e transition from a ferrom agnetic m etallic (FMM) s ta te to a param agnetic sem iconducting (PMS) s ta te a t about Tp = 145 K a s T increases.
The corresponding te m p e ra tu re dependence of CMR is shown in fig. 4. T he MR ratio is defined a s MR - {R(0)-R(4 kOe)}/R(0), w here R(0) is th e zero-field resistance and R(4 kOe) is th e re sistan c e under 4 kOe m agnetic field. From fig. 4, one can see th a t Cr doping leads to ex traordinary m agnetoresistance behavior, characterized by double peaks on the MR(T) curve. I t is contrast to th e case in La0 67Ca„ 33Mn03 in which CMR occurs merely n e a r Tf, th e te m p e ra tu re range of CMR response is greatly broadened by C r doping. This extraordinary CMR effect is beneficial to practical application in term s of th e very broad tem p eratu re range of CMR response. T herefore, it suggested t h a t Cr doping could be a p o ten t way in tu n in g CMR. The explanation of these re s u lts based on suppose of the existence of th e ferrom agnetic coupling between C r and Mn ions. One possible m echanism m ay be th a t a double exchange-like interaction could ta k e place through Mn3*-0-Cr3* du e to th e identical electronic configuration betw een C r3t an d M n4*. I t is well known th a t double exchange co rrelates electrical tran sp o rt to m agnetic configuration an d consequently plays a key
20 (degree) F ig .l. XPD pattern of the
Lao S7Cao ^Mno 9Cr0 j0 3
50 100 150 200 250 300
Fig. 2. M(T) curves of the La067Ca0 33Mn0 9Cr0 ,0 3
s o 100 ISO 200 250 300 T(K ) Fig.3. R(T) curves of the
Lao GvCao 3.iMn0 9Cr0|O,
50 100 150 200 250 300 T(K) Fig.4. CMR(T) curves of
sample at H = 0.4 T
14 2 N g u y e n A n h T u a n , N g u y e n H u y S in h ,..
role in CMR. The ap p earance of an additional CMR peak in La0fl7Ca033M n09C r0 |0 j im plies th a t the interaction through M n3*-0-Cr3* could be double exchange-like ra th e r th a n su p er exchange because s u p e r exchange generally does not give rise to a CMR. Due to the different coupling co n stan t betw een M n3*-O M n4* and M ns*-0-Cr3\ th e delocalization of carriers by applied m agnetic field m ay happen a t sep a rate d te m p e ra tu re ranges, consequently re su ltin g in two CMR peaks.
One an o th e r hand, th e clu ster glass n a tu re of Lau67CaO33M n0i9C r0 , 0 3 also plays a key factor in th e origin of low tem p eratu re m agnetoresistance. C r su b stitu tio n induces AFM interaction betw een C r3*-0-Cr34 an d prom otes th e proportion of M n‘*-0-Mn'* APM interaction. T he random d istribution of FM and AFM exchange interaction would favor the formation of clu ster glass, a s evidenced in the tem p eratu re dependence of m agnetization of the sam ple. Due to th e form ation of FM clusters an d th e ir random ly frozen m om ent a s well as the 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 and lead to high resistivity s ta te a t low tem p eratu re.
U nder an applied m agnetic field, th e w hole mom ent of th e frozen FM c lu s te rs expand and th eir orientation a re forced to align uniform ly so th a t the spatial m agnetic disorder is reduced, which favors the electron delocalization and consequently re su lts in a significant drop of the low tem p eratu re resistivity. This m ay be the reason t h a t a low tem p eratu re CMR effect is usually observed in a cluster glass state.
In conclusion, o u r obtained re su lts of La067Ca0_33M n09Cr0 |0;j sam ple show ed th a t the extraordinary CMR effect a s well a s clu ster glass behaviors have been observed in the sam ple. CMR occurs n e a r T f an d the tem p eratu re range of CMR response is greatly broadened by C r-doping content. The double exchange interaction through M n^-O -Cr3* m ay be th e m ajor reason which is concerning with iso-electronic configuration betw een Cr3* and Mn“ , The form ation of FM clu ster an d spatial m agnetic disorder in th e sam ple plays role to change the resistivity s ta te a t low tem perature.
Acknow ledgments. The N ational F undam ental R esearch Project 421.104/2004-2005 and TN.04.07/2004 support th is work.
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