Indian Journal of Engineering & Materials Sciences Vol. 10. August 2003, pp. 324-328

Effect of bandwidth and size disorder on the electrical and magnetotransport properties of doped LaMn03 perovskite

eM Thaker', 0 S Rana", Krushna Mavani', S I Patilb, M Sahasrabudheb & 0 G Kuberkar"

"Department of Physics, Saurashtra University, Rajkot 360005, India

bDepartment of Physics, University of Pune, Pune 411 007 , India

Received 4 February 2003; accepted 19 May 2003

Structural , electrical, magnetic and magnetotransport measurements have been carried out on La ' .2x PrxCax MnO,: x = 0.20 and 0.30 manganite compounds. These samples form in a distorted orthorhombic structure and behave as nearly low bandwidth systems. Both these samples exhibit insulator-metal transition be low 120 K with a large co lossal magnetoresistance (CMR) effect at relatively weaker fields (MR=75% at 1 Tesla for x = 0.30 sample). We have also studied the structural, transport and mag netotransport properties of a larger size cation, Sr2

+, substituted LUo.SPrO.2CUo 3.xSr, MnO-, (0.O:O;x:O;O.20) compounds. It is observed that increasing Sr2+ substitution results in an enhancement in transiti on temperature with a subsequent fall in MR%. The insulator-metal transitio n temperature for these samples increases fro m 138 K for x=O.O to 252 K for x=0.20. The CMR effect fall from 46% for x=O.O to less than 10% for x=0.20 at a field of I Tes la. Al so. for samples with higher Sr conte nt the MR % at low temperature is comparable to the MR % in the vicinity of peak resistance. The resistance data in the semiconducting region of all the samples obeys the Mott's Variable Range Hopping type (InR oc T o.2,) of conduction mechani sm.

The divalent cation doped LaMn03 perovskites have sparked a renewed interest in material research due to the widely known colossal magnetoresistance (CMR) effect under the applied magnetic field and various other inter-related phenomena like concomitant insul ator-metal transition (I-M) and paramagnetic­ferromagnetic transitIOn (PM-FM), Zener-Double exchange, charge ordering, orbital ordering, Jahn­Teller effects and electron lattice coupling effects ' ·3

exhibited by them. The I-M and PM-FM transition temperat ures, which are due to spin-polarized conduction of carriers through Zener-Double exchange depends upon the eg electron bandwidth and carrier density . Lao7Cao.3Mn03 (LCMO) and Pro.7C3(u Mn03 (PCMO) are intermediate and low bandwidth systems respectively2. The dependence of transition temperature is ascribed to the amount of divalent cation doping and average A-site cation radius which is quantified by a tolerance factor4

, t = <A-O>ICvl2<Mn-O» . Rodriguez-Martinez et ai. 5

defined a quantitative factor called size variance,

.. . (1)

where X i is the fractional occupancy of the cation with ionic radius rj and <rA> is the average A-site cation radius: characteristic of the mismatch of size of the different cations occupying A-site and studied its

effect on the structural and transport behaviour. It is an established fact that, the optimal divalent cati on doping of 33% in the absence of any cation size mismatch induces the largest possible concomitant electrical and magnetic transition temperatures. The La' .2xPrxCaxMn03 (LPCMO); x 0.20. 0.]0 manganite system is aimed to study the magneto transport and magnetic properties in the region lying between LCMO and PCMO with different carrier concentrations. The effect of substitution of larger cation Sr2+ is interesting to observe the bandwidth modification and cation size mismatch effects. We have studied the Lao.5 PrO.2Cao.3.xSrxMn03 (LPCSMO); x = 0 .0-0.20 (0.05) system with an aim to investigate the modifications in the transport and MR behaviour with increasing bandwidth and size disorde r.

Experimental Procedure Polycrystalline samples of La ' .2x PrxCa.,Mn0.1 :

x=0.2 and 0.3 and Lao.5Pro.2Cao.3.xSrxMnO:l; x=O.O. 0.05, 0. 10, 0. 15 and 0.20 were synthesized by conventional solid-state reaction route . Stoichiometric quantities of high purity (>99.9%) La20 ), Pr60 ". CaC03, SrC03 and Mn02 powders (Aldrich , USA make) were thoroughly mixed and calcined at 950°C for 24 h. The samples were ground and pelleti zed in disc shape with 10 mm diameter and approx im ately

TAHKER et al .: ELECTRICAL AND MAGNETOTRANSPORT PROPERTIES OF DOPED LaMn03 PEROVSKITE 325

I mm thickness by applying a pressure of 4 Ton. The pellets were sintered at 1 100°C for 24 h and at l200°C for 100 h with an intermediate grinding. The samples were reground and pelletized using the same procedure and final sintering was carried out at 1375°C for 24 h. XRD measurements using Cu-Ka radiation were performed for the structural determination and phase purity. Electrical transport and magnetotransport properties were studied by standard four-probe method up to a field of 5 T. a.c and d.c. magnetic susceptibility measurements were carried out for the determination of the Curie temperature (Tc).

Results and Discussion Rietveld refinement using FULPROOF program for

phase purity and structural investigations of all the samples reveals a good phase formation with an impurity -1-2% of constituent oxides in some of the samples. Both the stoichiometric compositions of LPCMO fit to a distorted orthorhombic structure whereas in LPCSMO, the structural transition from orthorhombic to monoclinic structure is noticed at x = 0.15 . The cell volume increases with increasing Sr concentration (Table 1), which can be attributed to larger size cation substitution effect.

It is clear from the R-T and Magneto R-T plots in Fig. 1 that, the LPCMO samples show transition around 60 K and 120 K for x = 0.2 and x = 0.3 respectively. The peak resistance increases with increasing doping concentration. The low Tc behaviour can be attributed to the effect of average A­site cation radius on the buckling of Mn-O-Mn bond angles6

. Fig. 2 depicts the R-T and magneto R-T behaviour of LPCSMO samples up to a field of 5 Tesla. The electrical transport behaviour of the LPCSMO system reveals that as Sr2+ concentration increases the transition temperature increases with a sharp fall in peak resistance for x = 0.05 sample and remaining nearly constant" value for x = 0.10, 0.15 and 0.20 samples. The sharp fall in peak resistance due to the substitution of Sr2+ has its origin in the modification of eg electron bandwidth as the average

~ 3 o

La'.2,Pr,Ca,MnO , (la)

x=0.2

_OT _IT -o- 2T -o-5T

O ~~~~ ~--.-~~~

~40 E

(1 b)

..c o 30 ~ ., ~ 20

~ .~ 10 a:

x=0 .3

_OT _IT -0-2 T -o-5T

Fig. 1 - R-Tand magneto R-Tplots for Lal .2,Pr,Ca, MnOl for x = 0.20 (1 a) and x = 0.30 (1 b) samples.

7500 ____ OT

5000 ____ 1 T

-o-2T x =0.0 -o-5T

2500

0 750 ........ OT

...... , T x = 0.05

500 -o-2T -o-5T

I 250 ,£. 8 c: 19

. ~ IX

0

75

50

25

0 75

50

25

____ 0 T x = 0.2

120 ____ ~T 80 -o-2T

-o-5T

40

00 50 100 150 200 250 300 T(K)

Fig. 2 - R-Tand magneto R-Tplots for Lao."PrO.2Cao.3., Sr, Mn0 1: x = 0.0 - 0.20 (0.05) samples.

Table 1- Structure. cell parameters and cell volume of Lao.SPrO.2Cao.3.,SrxMn03 (0<.J$0.20) sample.

x Structure a (A) b (A) c(A) Voll)me(A3)

0 Orthorhombic (Pnnw) 5.458(4) 7.722(4) 5.444(3) 229.426

0.05 Orthorhombic (Pnma) 5.448(2) 7.729(4) 5.463(3) 230.078

0.10 Orthorhombic (Pnnw) 5.456(1 ) 7.715(3) 5.477(4) 230.532

0.15 Monoclinic (l2IA) 7.715(3) 5.486(1 ) 5.459(2) 231.045

0.20 Monoclinic (I2IA) 7.717(4) 5.499(2) 5.46f(3) 231.764

326

T " 0.0030 0.0045 0.0060

8 x = 0 .20

6

a: E 4

T -t

INDIAN J. ENG. MATER. SCI. , AUGUST 2003

-4

-6

~0~.OTO~4 __ ~0~.Or06~ __ 0~.~00~8~ 8

A-site cation radius increases by Sr2+ substi tution. In order to understand the conduction mechanis m of polarons created as a result of electron-phonon coupling in the semiconducting region, we fitted the R-T data of LPCMO samples in Fig. 3 and LPCSMO samples in Fig. 4, to nearest neighbour small polaron hopping model7

.8 corresponding to P = Po exp(E) k7)

and the Mott's type Variable Range Hopping (V RH ) model9.

10 corresponding P = Po exp(T ) 7) 1/4. As shown in Figs 3 and 4 , the data fits linearly in In p versus T 0.25 plot, suggesting that the conduction of the carri ers in the semiconducting region obeys the Mott ' s Variable Range Hopping model. To in thi s equation is related to localization length by a relation,

o

T -0.25

. . . (2)

where L is the localization length of the carri ers and N(E) is the density of states.

Fig. 3 - Mott ' s VRH model (In R versus T -j).25) and small polaron model [In (RIT) versus T - I] fits for La l.2xPrxCaxMn03 samples.

The magnetotransport measurements have been carried out under 1 T, 2 T and 5 T fields for all the compositions. It is observed that, large C MR effect at relatively weaker fields, 75% at 1 T for LPCMO (x = 0.30), occurs for low bandwidth system. The origin o r this large CMR effect at relatively weaker fields li es in the magnetic disorder at Mn-O couplings and grain

Tl Tl OOB Q!m Qtm Qtm OOB Q!m Qtm D

x=uo 7 x=QQ;

8 2 6 0

~ o~~ -1~ 6

-2 4 -2 4

Q2i yillI <LB (3) 3 ~

5.0 00B'5 Q(D() Tl Q<Dt5 QOliI OOID -10 :19

x=U15 45 x=UlO

:16 -15 40

~~ ~ ~15 -2 :l3 -20

:1

-3 -25 25

~ yillIQ25 Q2i QJV Qlt5 yillI Q2'I) Q2'D

Fig. 4 - Mott 's VRH model (In R versus T -j)·25) and small polaron model [In (RIT) versus T - 1] fit s for

L30s Pro2C30.3_xSrxMn03; x = 0.0 - 0.15 (0.05) samples.

TAHKER e( at. : ELECTRICAL AND MAGNETOTRANSPORT PROPERTIES OF DOPED LaM nO) PEROVSKITE 327

boundary effects II. The Tc depends upon the transfer integral of eg electron, which further depends upon the spin arrangements on the adjacent Mn ions. If the Mn­O-Mn bond angle is towards 1800 then the spin arrangement on Mn ions is ferromagnetic and favours the transfer of eg electron easily but deviation from thi s angle obstructs the conduction of eg electron and hence Tc occurs at low temperatures. The application of magnetic field forces parallel alignment of the spins and eg electron transfer increases to show a drop in resistivity and hence CMR effect occurs when spins order ferromagnetically on Mn-O couplings. The grain boundaries also contribute to resistivity by the spin dependent scattering of carriers . The application of magnetic field reduces the spin dependent scattering of carriers and contributes to CMR effect. The large sensitivity of the LPCMO system at weaker fields prompted us to modify the system to a larger bandwidth by the substitution of larger size cation Sr. It is observed from the magneto R-T measurements on Sr substituted LaO.5PrO.2Cao.3_xSrxMn03 system (Fig. 2) that, insulator-metal transition temperature (Tp) increases with increasing Sr content and a subsequent fall in M R %. The transition temperature is revived from 138 K to 252 K and CMR effect falls from 46% for x = 0.0 to 7% for x = 0.20 at a field of 1 Tesla. Sr2+ (1.31 A) has larger size than Ca2+ (1.18 A). Hence, substitution of Sr2+ for Ca2+ increases the Mn-O-Mn bond angle towards 1800 and the magnetic disorder at the Mn-O couplings falls. This accounts for the decreasing CMR effect with increasing Sr2+ content throughout the series. Interestingly the CMR effect increases consistently at low temperature below transition temperature with increasing Sr content. This

low temperature large CMR effect is due to the intergrain tunneling magnetoresistance 12

. The drop in the resistance in the higher magnetic field also Increases with doping concentrations. The magnetoresistance also occurs as a result of the reduction of the scattering of the carriers at the interfaces due to application of magnetic field. The scattering of the carriers at the interfaces can be understood in the following way' 3. An electron moving from initial grain to the next magnetic grain crosses two interfaces in its journey, each of which may scatter the electron. This acts as an insulating barrier to the conduction of the carriers through the grains. Since A-site is occupied by different size cations, so unlike cations may occupy the positions at the faces of the adjacent grains. Hence, due to

increasing size variance, the Mn environment vanes at the interfaces of the connecting grains , which results in equal probability of weak and strong scattering of electrons at the interfaces . The strong scattering can be suppressed by the application of larger magnetic fields , to account for the larger drop in resistance as size disorder increases with Sr content. Another notable factor is that, the trend in the increase in Tc is not linear with Sr concentrati on, being larger for lower compositions and smaller for higher Sr content. This leads to deviation in the ideal relationship of Tc and tolerance factor4

• It can be explained by the mismatch of the cations at A-si te. which increases with Sr content. The large size variance, as calculated from Eq. (1), induces the strain in the lattice by random displacement of oxygen ions and hence obstructs the Zener-Double exchange. The magnetic susceptibility measurements have been carried for both Lal _2xPrxCaxMnO); x = 0.2 and 0.3 and Lao.SPrO.2CaO.3-xSrxMnOJ (0.0 :S x :S 0 .15) se ri es. Fig. 5 shows d.c. magnetic susceptibility (at · H = 50 Oe) and a.c. magnetic susceptibility (at H = 6 Oe) measurements carried out on x = 0.20 and x = 0.30 samples respectively for LPCMO series. The large difference in the magnetization values fo r these samples is due to the difference in applied magnetic field in both the measurements. The plot of a. c. susceptibility measurements (at H = 6 Oe) for LPCSMO (x = 0.0, 0 .05 and 0 .10) samples are shown in Fig. 6. It is clear from the figure that Curi e temperature shows an increasing trend with increasing Sr content. The field-cooled (FC) and zero-field­cooled (ZFC) magnetization plots (at H = 50 Oe) fo r .r = 0.0 and 0.15 shown in inset. Fig. 6 reveal a larger magnetic irreversibility for x = 0.0 sample as compared to x = 0.15 sample. This is indicative of dependence of the magnetization on the applied magnetic field . It is inferred that the magnetic

9

-3' E ~" 6 b

4 x=0.30 (H=6 Oe) s::

3~

100 ISO 200 2500

T(K)

c,

Fig. 5 - d.c. magnetization (M) versus temperature en plot for x=0.2 and a.c. magnetization (M) versus temperature (n pl ots fo r for x=0.3 samples of La l.2,Pr, Ca, Mn03 series.

328 INDIAN 1. ENG. MATER. SCI., AUGUST 2003

'""' bO '3 4 E ~

'" 3 '=' 100 200 300 ...... '-'

2 T(K)

::E -o-x=O.O -+- x=O.05 -D- x=O.lO

0 300

Fig.6 - a.c . IJagneti zation (M) versus temperature (7) plots for Lao , Pro."Ca . ,SrxMnO) (x=O.O, 0.05 and 0.10) samples. Inset fig ure , lh>\vS the ZFC and FC magnetization (M) versus temperature (7) plots for x = 0.0 and 0.15 samples.

irreversibility decreases with increasing transition temperature.

Conclusions The Pr-Ca substitution in LaMn03 results in low

bandwidth system with large CMR effect at relatively weaker applied magnetic fields at low transition temperature. The bandwidth of the system was modified by substitution of Sr. The fitting of the electrical transport data in the semiconducting region reveals that conduction in this region occurs through the Mott 's Variable Range hopping of polarons. CMR effect is found to have an inverse relationship with the transition temperature. The A-site disorder increases with Sr concentration and leads to microscopic in homogenieties in the samples through the random

displacement of oxygen anion and shows its intluence on the electrical, magnetic and magnetotransport properties.

Acknowledgement This work is financially supported by UGC, New

Delhi , in the form of major research project. One of the authors (DSR) is thankful to UGC for providing research fellowship. Authors also acknowledge IUC, Indore, for providing experimental facility for a.c . susceptibility and magnetoresistance measurements.

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