ARTICLE IN PRESS

Physica B 404 (2009) 3602–3607

Contents lists available at ScienceDirect

Physica B

0921-45

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/physb

Study of structural, transport and magneto-resistive propertiesof La0.7Ca0.3�xCexMnO3 (0rxr0.2)

Amit Khare a,�, Ashish Bodhaye a, Deepshikha Bhargava a, R.J. Choudhary b, Sankar P. Sanyal a

a Department of Physics, Barkatullah University, Bhopal 462 026, Indiab UGC-DAE Consortium for Scientific Research, Indore 452 017, India

a r t i c l e i n f o

Article history:

Received 31 March 2009

Received in revised form

5 June 2009

Accepted 5 June 2009

PACS:

75.47.GK

75.47.Lx

71.30.th

74.25.Fy

Keywords:

CMR

Manganites

Metal–insulator transition

Ferromagnetic–paramagnetic transition

26/$ - see front matter & 2009 Elsevier B.V. A

016/j.physb.2009.06.008

esponding author. Tel.: +919827824520.

ail address: khareamit21@gmail.com (A. Khar

a b s t r a c t

We have synthesized a series of La0.7(Ca0.3�xCex)MnO3 (0rxr0.2) by standard solid-state reaction

method. X-ray diffraction (XRD) measurement was carried out for structural studies and Rietveld

refinement was done for structural analysis. The transport properties were studied using four probe

technique. The temperature dependence of the resistivity was measured in the temperature range of

20 K to room temperature. It is found that all samples show a systematic variation in metal to insulator

transition at transition temperature (TP) and resistivity (r) with the relative concentration of hole and

electron doping in the system. The samples showed varying amounts of colossal magnetoresistance

depending upon temperature and applied magnetic field. The magnetoresistance values as high as 72%

were observed in x ¼ 0 sample.

& 2009 Elsevier B.V. All rights reserved.

1. Introduction

Perovskite oxides have drawn considerable attention owing totheir interesting properties such as colossal magnetoresistance(CMR) [1–3], charge ordering [4,5], metal–insulator transition andindication towards technological applications. Doped perovskite-type manganites RE1�xAxMnO3, with RE ¼ La, Nd or Pr and A ¼ Ba,Sr, Ca and Pb [6–9], are promising magnetoresistive materials inwhich the change in resistivity by applying magnetic field is so largethat this effect is described as ‘colossal’. In optimum doping range,these materials exhibit ferromagnetic to paramagnetic (FM–PM) aswell as metal to insulator (M–I) transition as the temperature isincreased. The parent compound LaMnO3 is an antiferromagneticinsulator (energy gap 1.1 eV and Neel temperature 150 K) withdistorted perovskite structure [10]. The divalent ion substitution atLa site changes some of the Mn+3 ions into non-Jahn–Teller ionMn+4 ions and creates holes in eg band, which induces doubleexchange interactions [11], wherein an electron from eg level ofMn+3 ion hops to ferromagnetically alligned Mn+4 ion giving rise tometallicity. Thus, the existence of metallicity and ferromagnetismcan be attributed to mixed valency of manganese ions. Moreover, inthis regime, the spin-dependant scattering and electron–phonon

ll rights reserved.

e).

interactions arising from Jahn–Teller splitting of Mn 3d levels [12]play a crucial role in charge transport and the material exhibits thenovel phenomenon of colossal magnetoresistance. It has also beenfound that the bond angle and bond length of Mn3+–O–Mn4+ alsoplay crucial role in controlling the CMR properties of thesemanganites as the geometric quantity and the tolerance factor aremodified when suitable ions are substituted for La to fill the 3Dnetwork of MnO6 octahedra [13]. These compounds have a half-metallic ferromagnetic ground state with a large magnetoresistance(MR) associated with the ferromagnetic transition temperature.

There are also reports on electron doping by substitution oftetravalent ions like Ce [14–20] in parent LaMnO3 system. In sucha system the network of Mn3+–O–Mn2+ is proposed to explain theelectrical and magnetic behavior. In the light of all these findings,we undertook a detailed study of the Ca and Ce doped LaMnO3

and synthesized the series of La0.7Ca0.3�xCexMnO3 (0rxr0.2) toanalyze the structural, transport and magnetoresistive properties.This will provide us a system to understand the effect of co-doping of electrons and holes on the system with a network ofMn3+–O–Mn4+ and Mn3+–O–Mn2+.

2. Experimental details

A series of polycrystalline samples of La0.7Ca0.3�xCexMnO3

(0rxr0.2) were synthesized by the standard solid-state reaction

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A. Khare et al. / Physica B 404 (2009) 3602–3607 3603

method. The stoichiometric amount of high purity chemicalsLa2O3, CaCO3, CeO2 and MnO2 with purity 499.99% were wellmixed and ground for several hours. The mix was then calcinedin an open atmosphere at 1100 1C for 24 h with intermediategrinding. The reacted powders were reground and calcinatedat 1150 1C for another 24 h. The well calcined powders werereground again and pressed into circular pellets by applyinghydraulic pressure. The pellets were sintered at 1250 1C for 24 h.X-ray diffraction (XRD) measurement of all the samples werecarried out for structural studies using CuKa radiation at roomtemperature. The resistance as a function of temperature wasmeasured by four probe method in the temperature range of20–300 K. The magnetoresistance, defined as MR% ¼ ½ðR0 � RHÞ=

R0� � 100, where R0 and RH are resistance values without and withmagnetic field, respectively, of the samples, was measured as afunction of temperature by standard four-probe technique in asuperconducting magnet with the maximum applied field of 8 T inthe temperature range 20–300 K.

Table 1Lattice parameter and unit cell volume for different compositions of La0.7-

Ca0.3�xCexMnO3 (0rxr0.2).

Composition Crystal

parameters

a (A) b (A) c (A) Unit cell

volume V (A3)

La0.7Ca0.3MnO3 5.46723 7.70471 5.45287 229.69354

3. Results and discussion

The Rietveld fitted room temperature powder X-ray diffractionpatterns for all the samples studied, are shown in Fig. 1. Thedetailed structural analysis of the XRD data, carried out usingRietveld refinement by employing FULLPROF code [21], clearlyshows that all the samples posses orthorhombic structure havingPnma space group and the presence of phase segregated CeO2 inthe samples with the nominal compositions of Ce 40.05. It can beseen that with the increase in Ce-concentration, the intensity ofthe XRD peaks related to CeO2 increases, which indicates that thesystem does not remain in single phase with higher concentrationof Ce, similar behavior has already been reported earlier [22–27].Further, we have calculated the lattice parameters and tabulatedthem in Table 1. The lattice parameters of La0.7Ca0.3�xCexMnO3

(0rxr0.2) change considerably with Ce doping. The unit cellvolume increases with increase in the Ce doping in theLa0.7Ca0.3�xCexMnO3.

Fig. 1. Rietveld fitted X-ray diffraction patterns for La0.7Ca0.3�xCexMnO3

(0rxr0.2). The open circles are the observed data and the continuous line is

the Rietveld refined pattern. The difference pattern between the observed and

calculated patterns is shown below of each pattern. The vertical bars are the

positions of the allowed Bragg peaks.

The plot of resistivity of all the samples measured as a functionof temperature is shown in Fig. 2. All samples show a metalto insulating transition, the transition temperature TP beingdependent on the Ce concentration. The x ¼ 0 sample showscharacteristic metal–insulator transition at 265 K. The value ofresistivity increases with increasing concentration of Ce and TP

decreases with increasing concentration of Ce upto 0.15; however,for x ¼ 0.2, TP increases. At this juncture it is important to discussthe possible valence states of Ce and Mn ions, since TP of thesematerials are very sensitive to the relative concentration of Mn+2,Mn+3 and Mn+4 as well as oxygen vacancies. It is well known thatin doping of Ca in LaMnO3 drives Mn from +3 to +4 valence state.The experimental procedure for Ce doping in La0.7Ca0.3MnO3 isclose to that reported in Ref. [28]. It is observed by the X-rayabsorption near edge spectra of Ce doped LaMnO3 samples that Ceis in +4 valence state and it drives Mn in +2 valence state. Theyalso suggested that Ce doping induces holes in O 2p derivedstates. Considering these facts, it is expected that in hole dopedsystem of La0.7Ca0.3MnO3, Ce doping is equivalent to electrondoping. Therefore, when Ce is doped in La0.7Ca0.3MnO3 samples,the relative concentration of holes decreases and we find theenhancement in resistivity and decrease in TP. However, for x40.10,we have a system where relative concentration of electron doping ishigher as compared to hole doping and hence the system behaves aselectron doped system resulting in increase in TP value. Thoughentire Ce is not substituting in the lattice in samples beyondx ¼ 0.05, but through Rietveld we have evaluated that all Ce is not

La0.7Ca0.25Ce0.05MnO3 5.46748 7.72948 5.46151 230.80773

La0.7Ca0.2Ce0.1MnO3 5.47853 7.73021 5.48040 232.09597

La0.7Ca0.15Ce0.15MnO3 5.48753 7.75043 5.48413 233.24398

La0.7Ca0.1Ce0.2MnO3 5.50137 7.77014 5.48751 234.57766

Fig. 2. Temperature dependence of resistivity of La0.7Ca0.3�xCexMnO3 (0rxr0.2)

in zero field.

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contributing to formation of CeO2. We note that x ¼ 0.10, 0.15 and0.20 is essentially equal to 0.07, 0.10 and 0.12, respectively, asobserved from Rietveld data. In addition, it can be emphasized that

Fig. 3. Temperature dependence of resistivity of La0.7Ca1�xC

the transport behavior of Ce-doped LCMO is not affected much dueto the cerium oxide remaining in phase segregated manner inhigher concentration of Ce doped samples (Ce40.05).

exMnO3 polycrystalline samples under magnetic field.

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The insulator–metal transition in La0.7Ca0.3�xCexMnO3 is verysensitive to the applied magnetic field. From Fig. 3, one can seethat the resistivity peak is significantly depressed by appliedmagnetic field, which leads to an enormous enhancement of theCMR effect. The variation of MR value with temperature measuredat 5 and 8 T magnetic field is shown in Fig. 4. All the samples showquite similar behavior except the change in the magnitude. It isclear from Fig. 4 that the MR value increases gradually with

Fig. 4. The variation of magnetoresistance as a function of temperature at 5 T (�3�) an

x ¼ 0.05, (c) x ¼ 0.10, (d) x ¼ 0.15 and (e) x ¼ 0.20.

temperature and a higher value of MR is observed at transitiontemperature (TP) for both 5 and 8 T magnetic fields. The samplex ¼ 0 shows highest MR value of �72% for 5 T and �80% for 8 T attransition temperature Tp.

The samples show a significant change in resistance with theapplied magnetic field. In other words, they bear varying amountof MR. We show in Fig. 5, the variation of MR value with appliedmagnetic field at different temperatures. It is clear from Fig. 5 the

d 8 T (�d�) magnetic field for the La0.7Ca0.3�xCexMnO3 samples with (a) x ¼ 0, (b)

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MR value increases gradually with magnetic field for all samples.The trend of MR for x ¼ 0.05 sample is consistent with itsmagnetic phase. In paramagnetic phase the variation of MR isconcave type while in ferromagnetic phase the variation of MR isconvex in nature. Similar trend is observed for all the samples.

Fig. 5. The variation of magnetoresistance with applied magnetic field at temperatures 3

x ¼ 0.10, (d) x ¼ 0.15 and (e) x ¼ 0.20.

4. Conclusions

The structural, transport and magneto-transport properties ofthe series of La0.7Ca0.3�xCexMnO3 (0rxr0.2) are reported. Wehave successfully synthesized a series of polycrystalline samples

00, 200 and 5 K for the La0.7Ca0.3�xCexMnO3 samples with (a) x ¼ 0, (b) x ¼ 0.05, (c)

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with nominal composition La0.7Ca0.3�xCexMnO3 (0rxr0.2). It isobserved that, in the samples with Ce40.05, there is presence ofCeO2-phase remaining in phase segregated manner in the lattice,having orthorhombic structure crystallize in Pnma space group.All samples show a systematic variation in metal to insulatortransition temperature and resistivity with the relative concen-tration of hole and electron doping in the system. The substitutionof Ce on Ca site results in the decrease in Tp and increase inresistivity depending upon the relative concentration of chargecarriers. A varying amount of MR values were observed in thesesamples depending upon the applied magnetic field and tem-perature.

Acknowledgments

Authors are grateful to Prof. D.G. Kuberkar for extendingexperimental facilities and fruitful discussion. Authors are alsothankful to Dr. R. Rawat for providing magneto-resistancemeasurements and to UGC-DAE-CSR, Indore, for providingexperimental facilities and partial financial support. The authors(A.K. and S.P.S.) are also thankful to CSIR [80(0064)/06/EMR-II]and UGC [F.No.31-4/2005 (SR)] for financial support.

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