Solid State Communications 139 (2006) 310–314www.elsevier.com/locate/ssc

Magnetoresistance behaviour of La0.7Sr0.3MnO3/NiO composites

Anurag Gaur, G.D. Varma∗

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

Received 21 April 2006; received in revised form 16 May 2006; accepted 17 May 2006 by E.V. SampathkumaranAvailable online 6 June 2006

Abstract

(La0.7Sr0.3MnO3)1−x/(NiO)x (x ∼ 0, 0.05, 0.10, 0.15 and 0.20) composites were prepared via a solid state reaction process. Detailed studiesof magnetic and magnetotransport properties of the composite samples have been performed. X-ray diffraction and scanning electron microscopyobservations reveal that there is no evidence of reaction between the La0.7Sr0.3MnO3 (LSMO) and NiO grains. It has been observed that theincorporation of NiO phase into the La0.7Sr0.3MnO3 matrix lowers the metal–insulator transition temperature (Tp). Magnetic measurementreveals that the ferromagnetic order of LSMO is weakened by addition of NiO. An enhancement in magnetoresistance (MR) is observed at lowtemperature, below 120 K, for the composites with x = 0.05, x = 0.10 and x = 0.15 and at high temperature (near room temperature) for allthe composites at applied magnetic field H ∼ 3 kOe. In the present work, it is suggested that the large MR at low temperature (below 120 K) islow field magnetoresistance (LFMR) caused by spin polarized tunneling, whereas at around room temperature it is high field magnetoresistance(HFMR) caused by magnetic disordering.c© 2006 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

The discovery of colossal magnetoresistance (CMR)in doped manganites and their potential applications inmagnetoresistive transducers and sensors triggered increasingattention [1,2]. One advantage of manganites is the nearly 100%spin polarization [3] of Mn ions that may boost spin dependenttransport and induce a large spin dependent magnetoresistance(MR) effect. However, the exact mechanism governing thelarge MR is not yet very well understood. Moreover, in mostof cases the large MR only occurs with a relatively highsaturation magnetic field, typically of the order of a fewteslas, making it questionable whether this class of materialswould be useful for low field sensors. Accordingly, manyresearch groups focused on the extrinsic magnetoresistanceeffects found in various magnetic oxides, since they showeda large MR in low magnetic fields. The extrinsic MR inthese manganites usually falls into three broad classes, namely

∗ Corresponding author. Tel.: +91 1332 285353(O); fax: +91 01332 273560.E-mail address: gdvarfph@iitr.ernet.in (G.D. Varma).

0038-1098/$ - see front matter c© 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2006.05.018

grain boundary MR, spin polarized transport in ferromagnetictunneling junction MR and domain wall MR [4]. Gupta et al. [5]investigated the effects of magnetic domain boundary pinningby polycrystalline grain boundaries and an enhanced MR wasobserved in a low temperature region. These extrinsic effectsusually rely on the existence of an insulating tunneling barrierseparating the ferromagnetic grains. Hwang et al. [3] were thefirst to propose a model for explaining the LFMR in both highand low temperature regions. They assign the LFMR to spinpolarized tunneling from one grain to another through highlyresistive grain boundaries.

In recent years, some attention has been paid to compositesystems which are granular manganite/insulator mixtures, suchas LCMO/STO [6], LSMO/CeO2 [7], LSMO/PSMO [8],LCMO/Al2O3 [9], LSMO/glass [10], LBMO/YSZ [11] andbilayers of LCMO with Fe3O4 [12]. In addition to this,manganite–polymer composites have also been studied [13].Enhancement of the magnetoresistance, especially near thepercolation threshold, in a mixture of metallic ferromagneticmanganite and an insulator has been reported.

The present work aims to study the magnetotrans-port properties of composites designed with ferromagnetic

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La0.7Sr0.3MnO3 (LSMO) and insulating NiO. NiO is a verysuitable candidate for being the second-phase material becauseit is an insulator with a high melting point, which is suitablefor construction of a percolating system. In this paper, we havestudied the magnetotransport properties of this LSMO/NiO sys-tem. It has been found that incorporation of the impurity phase(NiO) increases the zero-field resistivity of composites andthere is a concomitant shift in the value of the metal–insulatortransition temperature (Tp) to lower values. The enhanced mag-netoresistance has been observed in the composites within thelow (<120 K) and high temperature (around room temperature)regions.

2. Experimental procedure

The La0.7Sr0.3MnO3 (LSMO) samples were prepared via asolid state reaction method by using La2O3, SrCO3 and MnO2powders. These powders were mixed in the requisite ratio,ground and then calcined at 900 ◦C for 12 h. The calcinedmixture was reground and mixed properly, and pressed intothe form of pellets and sintered at 1300 ◦C for 20 h withintermediate grindings. The LSMO powders obtained, withsingle-phase perovskite structure, were completely mixed witha commercial NiO powder according to the nominal ratio of(LSMO)1−x/(NiO)x with x = 0, 0.05, 0.10, 0.15 and 0.20.The resulting (LSMO)1−x/(NiO)x powders were pelletized ata pressure of 5 MPa cm−2 and then sintered at 1000 ◦C for 2 h.The low sintering temperature for a small duration was chosento avoid inter-diffusion of LSMO and NiO. With this procedure,the NiO introduced as the second phase of the compositesmainly segregated at the grain boundaries and on the surfaces ofthe LSMO phase. The structural characterization was examinedby using X-ray diffraction (Bruker AXS C-8 advance, Cu Kα

radiation) at room temperature and the surface morphologywas investigated by scanning electron microscopy (SEM ModelLEO 435-VP operating at 15 kV). Resistivity as a function oftemperature was measured by a standard four-probe methodusing with Keithley instruments without or with magnetic fields(0–3 kOe). The DC magnetization measurements were doneusing a vibrating sample magnetometer (VSM Model 155,Princeton Applied Research).

3. Results and discussion

The room temperature X-ray diffraction (XRD) patterns ofthe composite (LSMO)1−x/(NiO)x samples are shown in Fig. 1.The patterns show two different sets of peaks: one for theperovskite LSMO phase and another for cubic NiO. No otherphases were detected in the samples by means of XRD. TheBragg reflections of the LSMO and NiO phases do not shift,indicating that the cell parameters of LSMO and NiO do notchange upon sintering. This also indicates that there is noreaction between LSMO and NiO and it is expected that NiOintroduced as a second phase of the composite will mainlysegregate at the grain boundaries and on the surfaces of theLSMO grains.

The SEM micrographs of (LSMO)1−x/(NiO)x compositesfor x = 0 and x = 0.10 are shown in Fig. 2(a) and

Fig. 1. X-ray diffraction patterns of the composites (La0.7Sr0.3MnO3)1−x/(NiO)x.

2(b), respectively. A clear grain boundary is observed in pureLSMO as shown in Fig. 2(a). However, on addition of NiOthe grain boundaries of (LSMO)1−x/(NiO)x composite becomeambiguous as shown in Fig. 2(b). From the SEM image(Fig. 2(b)), it is also clear that doped NiO segregated mainly atthe grain boundaries and on the surfaces of the LSMO grains.These results also demonstrate that LSMO/NiO compositesare the mixture of two phases. In Fig. 2(b), LSMO and NiOgrains are indicated by white and black arrows respectively, todistinguish them.

The temperature dependence of the resistivity for all thesamples measured by the four-probe technique at zero fieldis shown in Fig. 3. As expected, metal-like conductivity isfound in pure LSMO with a metallic transition at Tp ∼

315 K. With increasing NiO concentration, x , the zero-fieldresistivity of the LSMO–NiO composites increases and themetal–insulator transition shifts successively towards lowertemperatures. The metal–insulator transition points (Tp) aremeasured at ∼227, 212, 198 and 156 K for x = 0.05, 0.10,0.15 and 0.20, respectively and these values are much lowerthan the value of Tp ∼ 315 K observed for pure LSMO. Thedifferent electrical transport behaviours and high resistivity ofthe composites should be related to the effect of dopant NiOon the electron transport channel in the composites. In pureLSMO, the electrical transport is achieved through a directcontact between LSMO grains. However, in doped composites,there are two kinds of conduction channels connected parallelto each other [14,15]. One is related to LSMO grains, anddetermines the transport properties of the system through directcontact between LSMO grains. The other is related to dopantsNiO. Since the dopant was mainly segregated at the grainboundaries and on the surface of LSMO grains producingenergy barriers to electrical transport process, due to this higherresistivity was observed for the doped composites.

The temperature dependence of the magnetization has beenmeasured at 5 kOe and is shown in Fig. 4. The values of themagnetization (M) at 80 K are 81.38 emu/gm, 80.10 emu/gm,77.70 emu/gm, 73.66 emu/gm and 68.70 emu/gm for

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Fig. 2. Scanning electron micrographs of (La0.7Sr0.3MnO3)1−x/(NiO)x: (a) x = 0 and (b) x = 0.10. LSMO and NiO grains are indicated by white and blackarrows respectively.

Fig. 3. Temperature dependence of the normalized resistivity at zero field forthe composites (La0.7Sr0.3MnO3)1−x/(NiO)x.

x = 0.0, 0.05, 0.10, 0.15 and 0.20, respectively. Thesesuccessive decreases in M with increasing NiO concentrationare expected and are due to a decrease in the volumefraction of the ferromagnetic LSMO phase and an increaseof the antiferromagnetic NiO phase in the composites. Allthe composites have almost the same behaviour of themagnetization as a function of temperature and there is ashift in the paramagnetic (PM) to ferromagnetic (FM) phasetransition temperature (Tc), which moves towards lower valueswith increase in the NiO content. The transition temperaturesdetermined from the peak in the dM/dT –T curve are found tobe ∼343 K, 342 K, 338 K, 325 K and 318 K for samples withx = 0.0, 0.05, 0.10, 0.15 and 0.20, respectively. As observedby SEM, NiO segregates into the grain boundaries/interfacialregions and this interfacial/grain boundary segregation of NiOaffects the double-exchange (DE) mechanism; this is morepronounced in the FM domain walls [16]. This consequentlyleads to the suppression of the PM–FM transition temperaturetowards lower values with increase in the NiO concentration.In fact, the reduction in the magnetization also suggestssuppression of the DE mechanism resulting from suppressionof the ferromagnetic alignment of Mn ions. This indicates

Fig. 4. Temperature dependence of the magnetization at 5 kOe for thecomposites (La0.7Sr0.3MnO3)1−x/(NiO)x.

magnetic spin disorder induced by the grain boundaries inthe composites. The magnetic hysteresis loops measured atroom temperature for the LSMO/NiO samples with x = 0.0,0.05, 0.10, 0.15 and 0.20 are displayed in Fig. 5. The M–Hcurve indicates that the magnetization of the samples increasesrapidly at low fields and then tends to saturate at higherfield. The value of the magnetization of the composites againdecreases with x . This demonstrates that the ferromagneticorder is weakened and magnetic disorder is increased withthe NiO content. Thus inclusion of antiferromagnetic NiOdilutes the magnetization of composite samples even at roomtemperature.

The magnetic field dependence of the MR for all the samplesstudied at a fixed temperature 80 K measured in the magneticfield range of 0–12 kOe is shown in Fig. 6. The MR ratio isdefined as MR (%) = [ρ(0, T ) − ρ(H, T )]/ρ(H, T ) × 100%,where ρ(0, T ) and ρ(H, T ) are the resistivity values for zeroand applied fields, respectively. Fig. 6 indicates that the changein MR is greater in the doped composites as compared to pureLSMO, which indicates that the enhancement in MR basicallycomes through the formation of composites. This suggeststhat spin polarized tunneling across the FM grains may be

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Fig. 5. Magnetic hysteresis loops at room temperature for the composites(La0.7Sr0.3MnO3)1−x/(NiO)x.

Fig. 6. Magnetic field dependence of the magnetoresistance (MR) in a mag-netic field (0–12 kOe) at 80 K for the composites (La0.7Sr0.3MnO3)1−x/(NiO)x.

the transport phenomenon in the doped composite samples forenhancement in the MR [3]. Moreover, the change in MR isgreater at low field (up to 3 kOe) for x = 0.05, 0.10 and x =

0.15 samples as compared to the virgin LSMO sample and themaximum MR change is obtained at H ∼ 3 kOe for the samplewith x = 0.15. The values of the MR at 80 K are ∼15.68%,∼16.94% and 19.84% for x = 0.05, 0.10 and 0.15 respectivelywhile they are ∼14.34% for x = 0 at 3 kOe. On the addition ofinsulating antiferromagnetic NiO, the spin disorder is producedthrough the tunneling process at the grain boundaries and whena magnetic field is applied, the spin disorder is suppressed,resulting in a high MR, especially at the low field ∼3 kOe.On the other hand, extra magnetic disorder at the interfacesalso increases the magnetic domain scattering in the boundaryregions [17,18]. In the pure LSMO, the conduction electronsare almost polarized inside a magnetic domain and are easilytransferred between pairs of Mn3+ and Mn4+ ions. However,when these electrons travel across grains, strong spin dependent

Fig. 7. Magnetic field dependence of the magnetoresistance (MR) in amagnetic field (0–12 kOe) at 300 K for the composites (La0.7Sr0.3MnO3)1−x/(NiO)x.

scattering at the boundaries will lead to a high resistivity.Application of a moderately low field can align the domainsinto a parallel configuration causing a drop in the resistivity andenhancement in the LFMR. Thus the overall change in LFMRat low temperature is due to enhanced spin disorder in theintergranular regions which leads to enhanced spin polarizedtunneling. The spin polarized tunneling is controlled by thethickness of the insulating NiO layer. The reduction in MR forthe composite with x = 0.20 at low field may occur becauseof grain boundaries becoming too thick for electron tunneling.However, the magnetic field dependence of the MR at roomtemperature is different from that at 80 K as shown in Fig. 7.The magnetic field dependence MR curve at 80 K (as shownin Fig. 6) clearly indicates that a sharp enhancement in MRis observed at low fields and it saturates at higher values ofthe fields, while at room temperature the MR increases withfield almost linearly (as shown in Fig. 7) over the whole fieldrange (0–12 kOe), which is HFMR behaviour. This HFMRoriginates from the ordering of grain boundary spins andprovides information about the magnetic behaviour of grainboundary regions [19]. In our case, this HFMR increases withincreasing NiO concentration in the composite samples and ismaximum for the composite with x = 0.20. The value of theMR at 300 K is ∼13.34% for the composite with x = 0.20while it is ∼4.59% for pure LSMO at 12 kOe.

The temperature dependence of the magnetoresistance (MR)in a field of 3 kOe for the composite samples is shown inFig. 8. The values of the MR are large for the composites withx = 0.05, 0.10 and 0.15 and less for that with x = 0.20as compared to pure LSMO (x = 0) at temperatures below120 K. The MR ratios at 80 K are ∼15.87%, 16.64%, 20.14%and 12.19% for the composites with x = 0.05, 0.10, 0.15 and0.20, respectively, while they are ∼14.28% for pure LSMOat 3 kOe. This indicates that the MR ratio for the composites(up to x = 0.15) is larger than that for pure LSMO. Thisenhanced MR effect obviously arises through the formation ofcomposites. The increase in MR at low temperature seems to

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Fig. 8. Temperature dependence of the magnetoresistance (MR) in a field of3 kOe for the composites (La0.7Sr0.3MnO3)1−x/(NiO)x.

be due to enhanced tunneling across LSMO grain boundariesand additional grain boundary effects introduced by NiO grains.Usually this low temperature MR results mainly from a grainboundary effect as proposed by Hwang et al. [3]. Accordingto this, the spin polarized tunneling between neighbouringgrains of manganites is responsible for the magnetoresistanceeffect in polycrystalline samples. This tunneling takes placeacross the grain boundaries or interfaces especially at lowtemperatures T < Tc. However, the decrease in MR for x =

0.20 in the low temperature region (below 120 K) may occurbecause of the grain boundaries becoming too thick for thespin preserving electron tunneling. An enhancement in the MRis also observed at high temperature (near room temperature)for all the composites. The value of this high temperatureMR increases with NiO dopant. The maximum MR change isobserved for the composites with x = 0.20. The value of MRat 285 K is ∼6.34% for the composite with x = 0.20 while itis ∼0.80% for pure LSMO at 3 kOe. This large enhancementin the MR in the high temperature region is related to high fieldmagnetoresistance phenomena. As is clear from Fig. 7, the fielddependence of the MR at room temperature follows HFMRbehaviour and the value of the MR increases almost linearlywith field for all NiO concentrations. So, the enhancementin MR at around room temperature for all the composites isrelated to HFMR. This HFMR originates from the magneticdisorder produced by the antiferromagnetic coupling of thegrain boundaries [19]. As the room temperature M–H curvealso shows, the magnetization of the composites decreases withthe NiO content which demonstrates that the ferromagneticorder of LSMO is weakened and magnetic disorder is produced,which suppresses the ferromagnetic alignment of the Mn ionsand this leads to enhanced MR around room temperature [20].

4. Conclusions

In conclusion, we have synthesized LSMO/NiO compositesvia a solid state reaction process and investigated theirmicrostructural, magnetic and magnetotransport properties.These studies show that NiO does not enter into the LSMO

lattice and is found to remain in grain boundary regionsand on the surfaces of LSMO grains, without disturbingthe stoichiometry of the LSMO phase within the grain.With increase of the NiO content, Tp shifts towards lowertemperature and the magnetization decreases. The enhancementin the MR is observed at low temperature, below 120 K, forthe composites with x = 0.05, x = 0.10 and x = 0.15and at high temperature (near room temperature) for all thecomposites at applied magnetic fields H ∼ 3 kOe. It is arguedthat at around room temperature the HFMR produced throughmagnetic disorder is responsible for the enhanced MR in thecomposites. However, at low temperature, the enhanced LFMRis related to spin polarized tunneling.

Acknowledgements

The authors are grateful to Prof. O.N. Srivastava, Prof. D.Pandey (B.H.U.) and Dr. H.K. Singh (N.P.L.) for helpfuldiscussions and encouragement. One of the authors (AG) isgrateful to the University Grant Commission (UGC), NewDelhi, for the award of a Senior Research Fellowship.

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