Solid State Communications 144 (2007) 138–143www.elsevier.com/locate/ssc

Improved magnetotransport in LCMO-Polymer (PPS) composite

Anurag Gaur, G.D. Varma∗

Department of Physics, Indian Institute of Technology Roorkee, Roorkee-247667, India

Received 7 February 2007; accepted 2 August 2007 by G. LukeAvailable online 10 August 2007

Abstract

Polymer embedded, La0.7Ca0.3MnO3/polyphenylene sulfide (LCMO)1−x/(PPS)x (with x ∼ 0, 0.10, 0.20 and 0.30, x is the weight fractionof PPS), composites were prepared and their magnetotransport properties were investigated. X-ray diffraction and scanning electron microscopyobservations indicate that there is no reaction between the LCMO and PPS. It has been observed that the incorporation of PPS phase into theLCMO matrix sharply increases the resistivity and lowers the metal–insulator transition temperature (TIM). Magnetic measurement reveals thatthe ferromagnetic order of LCMO is suppressed by the addition of nonmagnetic PPS. The significant enhancement in magnetoresistance (MR) isobserved at low temperature below 175 K for the composites with x = 0.10 and 0.20 with respect to pure LCMO at magnetic field H ∼ 3 kOe.We suggest that such enhancement in MR is because of spin disorder caused through enhanced spin-polarized tunneling at the grain boundariesin the composite samples.c© 2007 Elsevier Ltd. All rights reserved.

PACS: 71.30.+h; 75.30.Kz; 75.30.Vn

Keywords: A. Manganites; C. Grain boundary; D. Electrical transport

1. Introduction

Magnetoresistive properties of Perovskite manganites ofthe type R1−xAxMnO3 (R is a rare earth such as La, Nd,Y etc.; A is alkaline earth such as Ca, Ba, Sr etc.) havebeen widely investigated in the past few years owing totheir importance in fundamental research and potential use inmagnetic devices [1]. It has been recognized that the largemagnetoresistance (MR), known as CMR, typically exhibitsin the vicinity of insulator to metal transition temperature TIMaccompanied by a simultaneous paramagnetic to ferromagnetic(PM–FM) transition at the Curie temperature (Tc) [2]. Thisis the so-called intrinsic CMR [3]. The intrinsic CMR effect,caused by the double exchange (DE) mechanism proposed byZener in 1951 [4], is observed within a narrow temperaturerange around Tc at high magnetic fields of several teslas, whichrestrains its use for practical applications. Recently growingattention is being paid to polycrystalline manganites in whichthe grain boundary effects dramatically modify their physical

∗ Corresponding author. Tel.: +91 1332 285353; fax: +91 1332 273560.E-mail address: gdvarfph@iitr.ernet.in (G.D. Varma).

0038-1098/$ - see front matter c© 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2007.08.004

properties [3]. An attractive feature of the polycrystallinemanganites is a large MR at very low magnetic field overa wide temperature range below TIM. This grain boundaryMR or extrinsic CMR, which is absent in the single crystal,is related to the natural and artificial grain boundaries [5,6].Spin-polarized tunneling [3] or spin-dependent scattering [5]among neighbouring grains seems to be responsible for thiskind of CMR effects. This extrinsic effect may enhance low-field magnetoresistance (LFMR) in a wide temperature rangeand can be more useful for practical applications to magneticswitching of recording devices.

Various attempts have been made to enhance the low-fieldMR through controlling the grain boundary effects by formingcomposites of the CMR oxides with secondary phases suchas insulating oxides, magnetic materials or with other CMRoxides [7–13]. In addition to these, some studies have alsobeen carried out on magnetic-polymer composites [14–16].Huang et al. [14] observed that in LSMO-PPP composite, thepolymer (PPP) does not modify the conduction mechanismbut certainly modify the grain boundary effect. Amongstthe polymer most commonly used are polyparaphenylene(PPP) [14,15] and polymethyl methaacrylate (PMMA) [16].

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However, no study seems to be available in the literature inmaking the composite with polyphenylene sulfide (PPS). PPSis a semicrystalline polymer with high-temperature resistance,dimensional stability and good electrical characteristics. PPSalso has excellent resistance to a broad variety of chemicalseven at high temperature [17]. Therefore, there is a lesspossibility of PPS to chemically react with manganites and itwill serve only as the transport barrier in the manganites matrixto adjust the tunnel barriers and hence the magnetoresistance.

In this paper, we attempt to investigate the magneto-transport properties of La0.7Ca0.3MnO3/polyphenylene sulfide(LCMO)1−x/(PPS)x composite. Incorporation of nonmagneticPPS phase increases the resistivity of (LCMO)1−x/(PPS)xcomposite samples and suppresses the metal–insulator transi-tion temperature. However, an enhanced low-field magnetore-sistance has been observed in the composite samples for lowerPPS content.

2. Experimental procedure

The La0.7Ca0.3MnO3 (LCMO) samples were prepared viasol-gel method. The aqueous solution of high purity nitrates ofLa, Ca and Mn has been taken in the desired stoichiometricproportions. An equal amount of ethylene glycol has beenadded to this solution with continuous stirring. This solutionis then heated on a hot plate at temperature of ∼80–100 ◦Ctill a dry thick brown sol is formed. This has been furtherdecomposed in an oven at a temperature of 250 ◦C to get the dryfluffy material. The obtained polymeric precursor is calcinedat 600 ◦C for 12 h. The calcined mixture was reground andsintered at 1000 ◦C for 12 h. The obtained LCMO powders withsingle phase perovskite structure were completely mixed withPPS in an appropriate weight ratio and subsequently pressedinto pellets at a pressure of 5 MPa cm−2. Then the pellets weresintered at 400 ◦C for 2 h in air in order to make the well-connection between adjacent LCMO particles and polymer.The low sintering temperature for small duration was chosento avoid inter-diffusion of LCMO and PPS. The structuralcharacterization was examined by employing X-ray diffraction(Bruker AXS D-8 advance, Cu Kα radiation) technique atroom temperature and surface morphology was investigated byfield emission scanning electron microscope (FESEM, ModelFEI Quanta 200F). Resistivity as a function of temperaturewas measured by a standard four-probe method using Keithleyinstruments without or with magnetic fields (0–3 kOe). TheDC magnetization measurements were done by using vibratingsample magnetometer (VSM Model 155, Princeton AppliedResearch).

3. Results and discussion

Fig. 1 shows the thermal gravimetric analysis (TGA) curvesof (LCMO)1−x(PPS)x composite samples with (a) x = 1.0 and(b) 0.30. It is found that the decomposition of pure PPS andcomposite with x = 0.30 starts at about 465 ◦C and 450 ◦C,respectively. In both cases, the dissociation temperatures areabove the sintering temperature (400 ◦C) of the composite

Fig. 1. Thermal gravimetric analysis (TGA) curves of (LCMO)1−x(PPS)xcomposite samples: (a) x = 1.0 and (b) x = 0.30.

Fig. 2. X-ray diffraction patterns of (LCMO)1−x(PPS)x composite sampleswith x = 1.0, 0.0, 0.10, 0.20 and 0.30. The asterisks represent the diffractionpeaks arising from PPS.

samples. This indicates that the final sintering of the compositepellets at ∼400 ◦C does not decompose the polymer.

The phases of samples were characterized by X-raydiffraction (XRD) with Cu Kα radiation. Fig. 2 shows theXRD patterns of (LCMO)1−x(PPS)x composite samples withx = 1.0, 0.0, 0.10, 0.20 and 0.30. The XRD pattern ofthe composites with x = 0.20 and 0.30 show two differentsets of diffraction peaks, corresponding to orthorhombicLCMO perovskite and PPS phases, respectively, which clearlyindicates the coexistence of LCMO and PPS phases. Thepure LCMO sample has an orthorhombic unit cell with latticeparameters: a = 5.487 A, b = 5.436 A, c = 7.764 A. Thelattice parameters of LCMO in the LCMO-PPS composite donot change within the accuracy of diffractometer. This showsthat LCMO maintains its identity and there is no reactionbetween LCMO and PPS and PPS is introduced as second phaseof the composite mainly segregates at the grain boundariesand on the surfaces of the LCMO grains. The direct evidence

140 A. Gaur, G.D. Varma / Solid State Communications 144 (2007) 138–143

Fig. 3. Scanning electron micrographs of (LCMO)1−x(PPS)x compositesamples: (a) x = 0 and (b) x = 0.20. Inset shows the EDAX spectra of samplex = 0.20.

of the coexistence of two phases also comes from SEMmicrographs. The representative SEM micrographs of LCMO-PPS composites with x = 0 and 0.20 are shown in Fig. 3(a)and (b), respectively. The interfaces between PPS and LCMOcan be distinguished clearly. The bright regions in Fig. 3(b)are assigned to PPS and grey regions are the LCMO matrix.Moreover, the PPS is easily observable in the compositesalthough its distribution within the LCMO matrix is not veryuniform as shown in Fig. 3(b). Moreover, energy dispersive X-ray (EDAX) spectra of the doped composite for x = 0.20 (asshown in the inset of Fig. 3(b)) shows the carbon peak alongwith La, Ca, Mn and O peaks, which also supports the presenceof polymer in the doped composites. The Au peak in the EDAXspectra is due to coating of gold over surface of the sample toavoid charging.

The temperature dependence of magnetization at 5 kOefor (LCMO)1−x(PPS)x with x = 0, 0.10, 0.20 and 0.30 isshown in Fig. 4. The value of magnetization (M) at 80 Kare 79.82, 68.75, 60.00, and 51.67 emu/gm for x = 0.0,0.10, 0.20 and 0.30, respectively. This successive decrease inM with increasing PPS concentration is due to decrease inthe volume fraction of ferromagnetic LCMO phase and extramagnetic disorder caused by PPS in these composite samples.Magnetic measurement shows that PPS is nonmagnetic inthe whole measured temperature range. This suggests thatthe magnetization of the composites comes up only fromLCMO. According to the magnetization of parent LCMO

Fig. 4. Temperature dependence of magnetization at 5 kOe for (LCMO)1−x(PPS)x composite samples. The solid lines represent the estimated M accordingto the M and weight fraction of LCMO for x = 0.10, 0.20 and 0.30.

and LCMO weight fraction in the composites, the magnitudeof magnetization (M) of composites with x = 0.10, 0.20and 0.30 are estimated and their variations with temperatureare shown by the solid lines in Fig. 4. The experimentalcurves are obviously lower than the as-estimated ones. Thisdiscrepancy in M leads us to take into account the extramagnetic disorder caused by PPS in the composites. This extramagnetic spin disorder is induced by grain boundaries in thecomposites and suggests the suppression of DE mechanismresulting suppression of the ferromagnetic alignment of Mnions. Since the PPS is not incorporated into the LCMO latticeand it segregates into the grain boundaries or interfacial regions,which blocks the Mn spins at grain boundaries and increasesthe anisotropy in the interfacial regions and misalignment ofthe magnetic moments of the neighbouring FM domains [18].Therefore, despite the nonmagnetic character of PPS, it isexpected to increase the magnetic disorder by disruptingthe Mn–O–Mn bonds in the interfacial regions and hencesuppression of the long range FM order. All the composites,studied in the present work, have almost similar behaviourof magnetization as a function of temperature. Moreover, theparamagnetic (PM) to ferromagnetic (FM) phase transitiontemperature (Tc) determined from the peak in dM/dT –Tcurves is almost independent of PPS content (x) and is ∼279 Kfor all the samples. This is due to the fact that the PM–FM phasetransition is an intrinsic and intragrain property. The observedconstancy of Tc also indicates that stoichiometry of LCMOphase within the grains remains essentially unchanged as PPSis not accommodated within the perovskite structure and it sitsonly at the grain boundaries and on the surfaces of LCMOgrains. The magnetic hysteresis loops for the LCMO/PPSsamples with x = 0.0, 0.10, 0.20 and 0.30 are displayed inFig. 5. M–H curves show that the magnetization of the samplesincreases rapidly at low fields and then tends to saturate athigher field. The value of magnetization of composites againdecreases with x because of reducing the volume fraction ofLCMO phase and extra magnetic disorder due to PPS content.

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Fig. 5. Field dependence magnetization (M–H ) curves at 80 K for(LCMO)1−x(PPS)x composite samples.

This demonstrates that ferromagnetic order is weakened andmagnetic disorder increases with PPS content.

The temperature dependence of resistivity without and withan applied field H = 3 kOe, for the (LCMO)1−x(PPS)xwith x = 0, 0.10, 0.20 and 0.30 is shown in Fig. 6. Themetal like conductivity is found in pure LCMO while withincreasing PPS concentration x , the zero field resistivity of

(LCMO)1−x(PPS)x composite samples increases within two orthree orders of magnitude even at room temperature, when xvaries from 0 to 0.30. It can be seen from Fig. 6 that the valueof resistivity at room temperature (300 K) increases from 1.24to 617.4 � cm when x increases from x = 0 to x = 0.30.At the same time, it is also observed that the resistivity has areduction under the applied field especially at lower values oftemperatures. Meanwhile, the virgin LCMO (x = 0) showsthe clear insulator (dρ/dT < 0) to metal (dρ/dT > 0)transition at a temperature 265 K while the value of transitiondecreases from 265 to 105 K when x increases from x = 0to x = 0.30. The value of transition temperature is ∼265,∼134, ∼121 and ∼105 K for the samples with x = 0, 0.10,0.20 and 0.30, respectively. The strong suppression of theTIM is caused by the PPS induced disorders and also by theincrease in the nonmagnetic insulating PPS phase fraction inthe composites. This causes the increase in the carrier scatteringleading to a corresponding enhancement in the resistivity.Moreover, in pure LCMO, the electrical transport is achievedthrough a direct contact between LCMO grains. However, indoped composites, there are two kinds of conduction channelsconnected parallel to each other [18,19]. One is related tosmall LCMO grains, which determines the transport propertiesof the system through direct contact between LCMO grainsand the other is related to embedded PPS. Since the PPS wasmainly distributed at the grain boundaries and on the surfaces of

Fig. 6. Temperature dependence of resistivity at zero and an applied field (H = 3 kOe) for (LCMO)1−x(PPS)x composite samples.

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Fig. 7. Temperature dependence of magnetoresistance (MR) in a field of 3 kOeof (LCMO)1−x(PPS)x composite samples. Inset shows the MR versus x curve.

LCMO grains producing energy barriers to electrical transportprocess, an obvious higher resistivity was observed for thedoped composites.

The temperature dependence of magnetoresistance (MR)in a field of 3 kOe for the (LCMO)1−x(PPS)x compositesamples is shown in Fig. 7. The MR ratio is defined asMR(%) = [ρ(0, T )–ρ(H, T )]/ρ(H, T ) × 100%, whereρ(0, T ) and ρ(H, T ) are the resistivity values for zero andapplied fields, respectively. We find that (LCMO)1−x(PPS)xcomposite samples having low PPS concentration, viz., x ∼

0.10 and 0.20 exhibit an enhanced MR compared with parentLCMO in low temperature region (T < 175 K). We plot thex dependence of MR in the inset of Fig. 7. The maximumMR ratio at 80 K is ∼16.73% and ∼18.45% for the compositesamples with x = 0.10 and 0.20, respectively while ∼15.73%for pure LCMO at 3 kOe. So, the observed enhancement inMR at 80 K with respect to pure LCMO is ∼7% and ∼17%for the composites with x = 0.10 and 0.20, respectively,at 3 kOe. This enhancement in MR at low temperature iscommonly interpreted within the frame work of spin-polarizedtunneling across LCMO grain boundaries and additional grainboundary effects introduced by PPS grains. As proposedby Hwang et al. [3], the spin-polarized tunneling betweenneighbouring grains of manganites is responsible for the low-field magnetoresistance effect in polycrystalline samples. Thistunneling takes place across the grain boundaries or interfaces,which produce the spin disorder. The magnetic measurements(Fig. 4) also indicate the extra magnetic disorder induced byPPS content at the grain boundaries in the composites andthis spin disorder is suppressed by applying the magnetic field,resulting the enhancement in MR. As Fig. 7 indicates, we getthe enhancement in MR for the composites with x = 0.10and 0.20 at temperatures below 175 K which suggests that thisenhanced MR basically comes from the grain boundary effectsin our samples because it is well-known that LFMR is mainlyattributed to a grain boundary effect especially at temperatureT < Tc [3]. However, with further increase in PPS content fromx = 0.20, the MR decreases with respect to pure LCMO even at

Fig. 8. Magnetic field dependence of magnetoresistance (MR) in magnetic field(0–12 kOe) at 80 K for (LCMO)1−x(PPS)x composite samples.

low temperature which may result because the grain boundariesbecome too thick for electron tunneling.

The magnetic field dependence MR for the (LCMO)1−x(PPS)x composite samples measured in magnetic field range of0–12 kOe at 80 K is shown in Fig. 8. Analysis of Fig. 8 showsthat with an increase in the magnetic field from 0 to 12 kOe,the value of MR for low PPS doping (x = 0.10 and 0.20) islarger than pure LCMO (x = 0.0) and smaller for x > 0.20.It suggests that magnetic field sensitive MR can be enhancedwith PPS as a secondary phase impurity at low concentrationonly (up to x = 0.20 in the present case). The percent change inMR for the composites with x = 0.10 and 0.20 with respect topure LCMO is more at low fields (up to 3 kOe). The percentagechange in MR with respect to pure LCMO at 80 K is ∼7% and∼17% at 3 kOe field while the same is ∼3% and ∼6% at 12 kOefield for the composites with x = 0.10 and 0.20, respectively.This indicates that large change in MR is produced at low fieldsand this LFMR is caused through spin disorder by the tunnelingprocess at the grain boundaries and when an external magneticfield is applied, the spin disorder is suppressed and Mn spinswithin the disordered region realign along the field direction, asa consequence, MR improvement is obtained.

4. Conclusions

We have synthesized the (LCMO)1−x(PPS)x composites andinvestigated their magnetotransport properties. XRD and SEMresults show that PPS does not react with LCMO and is found toremain at grain boundary regions and on the surfaces of LCMOgrains without disturbing the stoichiometry of LCMO phasewithin the grain. With an increase in the PPS content, TIM shiftstowards lower temperature and Tc remains unchanged. The fairamount of enhancement in MR is observed for the compositeswith x = 0.10 and 0.20 at temperature below 175 K and atapplied magnetic field H ∼ 3 kOe. The value of enhanced MRwith respect to pure LCMO at 80 K is ∼7% and ∼17% for thecomposites with x = 0.10 and 0.20, respectively at appliedmagnetic field H ∼ 3 kOe. It is argued that this enhanced

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LFMR is related to enhanced spin-polarized tunneling, which ismanipulated by the spin disorder at the LCMO surfaces causedby PPS.

Acknowledgements

The authors are grateful to Prof. O.N. Srivastava, Prof. D.Pandey (B.H.U.), Prof. R. Nath (I.I.T.R.) and Dr. H.K. Singh(N.P.L.) for helpful discussions and encouragement. One of theauthors (AG) is grateful to the University Grant Commission(U.G.C.), New Delhi for the award of Senior ResearchFellowship.

References

[1] A.P. Ramirez, J. Phys.: Condens. Matter. 9 (1997) 8171.[2] P. Schiffer, A.P. Ramirez, W. Bao, S.W. Cheong, Phys. Rev. Lett. 75

(1995) 3336.[3] H.Y. Hwang, S.W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77

(1996) 2041.[4] C. Zener, Phys. Rev. 82 (1951) 403.[5] A. Gupta, J.Z. Sun, J. Magn. Magn. Mater. 200 (1999) 24.[6] S.P. Issac, N.D. Mathur, J.E. Evetts, M.G. Blamire, Appl. Phys. Lett. 72

(1998) 2038.[7] L.I. Balcells, A.E. Carrillo, B. Martinez, J. Fontcuberta, Appl. Phys. Lett.

74 (1999) 4014.[8] S. Gupta, R. Ranjit, C. Mitra, P. Raychaudhury, R. Pinto, Appl. Phys. Lett.

78 (2001) 362.[9] Z.C. Xia, S.L. Yuan, W. Feng, L.J. Zhang, G.H. Zhang, J. Tang, L. Liu,

S. Liu, G. Peng, D.W. Niu, L. Chen, Q.H. Zheng, Z.H. Fang, C.Q. Tang,Solid State Commun. 128 (2003) 291.

[10] Anurag Gaur, G.D. Varma, H.K. Singh, J. Phy. D: Appl. Phys. 39 (2006)3531.

[11] Anurag Gaur, G.D. Varma, Solid State Commun. 139 (2006) 310.[12] L.W. Lei, Z.Y. Fu, J.Y. Zhang, H. Wang, Mater. Sci. Eng. B 128 (2006)

70.[13] C.-H. Yan, F. Luo, Y.-H. Huang, X.-H. Li, Z.-M. Wang, C.-S. Liao, H.-W.

Zhao, B.-G. Shen, J. Appl. Phys. 91 (2002) 7406.[14] Y.-H. Huang, X. Chen, Z.-M. Wang, C.-S. Liao, C.-H. Yan, H.-W. Zhao,

B.-G. Shen, J. Appl. Phys. 91 (2002) 7773.[15] F. Luo, W. Song, Z.-M. Wang, C.-H. Yan, Appl. Phys. Lett. 84 (2004)

1719.[16] J. Kumar, R.K. Singh, P.K. Siwach, H.K. Singh, R. Singh, O.N.

Srivastava, J. Magn. Magn. Mater. 299 (2006) 155.[17] H.W. Hill, D.G. Brady, Polym. Eng. Sci. 16 (1976) 832.[18] A. de Andres, M. Garcia-Hernandez, J.L. Martinez, C. Prieto, Appl. Phys.

Lett. 74 (1999) 3884.[19] J. Mark Rubinstein, J. Appl. Phys. 87 (2000) 5019.