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Grain boundary effects on the electrical and magnetic properties ofPr2/3Ba1/3MnO3 and La2/3Ca1/3MnO3 manganites

Neeraj Panwara,b, Vikram Sen a, D.K. Pandya b, S.K. Agarwal a,

a Superconductivity & Cryogenics Division, National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi-110012, Indiab Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi-110016, India

Received 22 January 2007; accepted 17 March 2007Available online 23 March 2007

Abstract

Electrical and magnetic properties of orthorhombic Pr2/3Ba1/3MnO3 (PBMO) and La2/3Ca1/3MnO3 (LCMO) manganites with considerabledifference in variance factors (2) are reported here. PBMO with higher variance exhibits distinct intrinsic (due to grains) and extrinsic (due tograin boundaries) transitions in the resistivity behaviour. Extrinsic effects, however, are not observed in the lower2 LCMO system. Low fieldmagnetoresistivity (LFMR) data also substantiate these results. Increase in the density of states obtained through Mott's 3-D variable rangehopping mechanism in the paramagnetic insulating regime indicates the suppression of magnetic domain scattering with applied magnetic field.Ferromagnetic metallic regime below the extrinsic transition in PBMO seems to emanate from the electron magnon scattering process. LFMR at77 K also points towards the higher canting of spins in the vicinity of grain boundary regions in PBMO compared to that in LCMO. 2007 Elsevier B.V. All rights reserved.

PACS: 75.47.Gk; 71.30.+hKeywords: Manganites; Grain boundary effects; Low field magnetoresistivity (LFMR); Variable range hopping (VRH); Electronmagnon scattering

1. Introduction

Perovskite manganites with the general formula L1xAxMnO3(whereL is a trivalentrare-earth ionlike La+3, Pr+3, Nd+3 etc.andA is the divalent alkaline earth ion like Ca+2, Sr+2, Ba+2 etc.) havetriggered the attention worldwide due to the occurrence ofcolossal magnetoresistive (CMR) effect in the vicinity ofinsulatormetal (Tp) and paramagneticferromagnetic (TC)

transitions [1

3]. Although such magnetoresistivity can bequantitatively explained by the Zener's theory of double ex-change [4], more explanatory mechanisms involving the

polaronic effects [5] and intrinsically inhomogeneous states [6],have been suggested to explain the observed high magnitude ofmagnetoresistivity.

In these perovskites the physical properties have been noticedto strongly depend on both the electronic doping x and the rare-

earth site mean ionic radiusbrAN [7,8] (where brAN is calculatedfor the nine-fold co-ordination from values tabulated in Ref. [9]).Higher CMR effects have generally been observed for x =0.33and for small values ofbrAN. StrongbrAN dependence of Tp andTC has been observed in some manganites, however, it has also

been established that additional parameters such as the A-sitecation size mismatch 2 (also called variance or cationicdisorder and defined by 2=ixiri

2brAN2, where xi is the

fractional substitution level of the i

th

rare-earth site species withionic radius ri and brAN=ixiri) and oxygen stoichiometry alsoexert a strong influence [1015]. In some cases larger2 affectsthe grain boundary properties as well. In this paper we report theeffect of2 on the grain boundary properties and the ensuingimpact on various physical properties of Pr2/3Ba1/3MnO3(PBMO) and La2/3Ca1/3MnO3 (LCMO) manganites where theformer is having higher brAN (1.27 >

2) and higher 2

(0.0187 2) and the later has relatively lower brAN (1.204 )and 2 (0.000287 2) respectively. Significantly, PBMOmaterial is also important from the fact that it does not followthe criteria ofbrAN vs. TC as proposed by Hwang et al. [16].

Materials Letters 61 (2007) 48794883www.elsevier.com/locate/matlet

Corresponding author. Tel.: +91 11 25742610 12x2239 2276; fax: +91 1125852678.

E-mail address: [email protected] (S.K. Agarwal).

0167-577X/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.03.062
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2. Experimental techniques

Both Pr2/3Ba1/3MnO3 and La2/3Ca1/3MnO3 materials havebeen synthesized in the polycrystalline form using the conven-tional solid state reaction route of taking stoichiometric ratios,grinding and calcining the powders (for homogeneity) at different

temperatures between 900 C and 1100 C for 12 h withintermediate grindings. Finally, the powders were pressed in

pellet form and sintered at 1260 C for 15 h. This sinteringtemperature of 1260 C was optimized for the single-phaseformation [13]. X-ray diffractometer (Rigaku, CuK, =1.54 )was used for thedetermination of the phase purity of the materials.Four-probe method was employed for the measurement oftemperature variation of the resistivity. Air drying silver epoxywas used for making the electrical contacts. Magnetic measure-ments were carried out in an AC susceptometer(Lakeshore ModelACS 7000) in the temperature range 300 K77 K at a fixedfrequency of 111.1 Hz and an ac field of 80 A/m under the zero

field cooled configuration. Scanning electron micrographs of thesamples were taken using LEO SEM 440 operating at 5 kV.Magnetoresistivity (MR) data was obtained at 0.6 T magneticfield in the temperature range of 300 K77 K. MR has beendefined using the relation: MR= [{R(0)R(H)} /R(0)]*100,where R(H) and R(0) are the resistances of the sample with andwithout magnetic field respectively.

3. Results and discussion

The single-phase nature of the synthesized PBMO and LCMOmaterials was revealed through their X-ray diffraction measurements.Both the samples possess the orthorhombic structure. The tolerancefactor (t) also confirms the structure to be orthorhombic (as it fallswithin the specified limits (0.89 t0.96). The temperature depen-dence of resistivity of Pr2/3Ba1/3MnO3 and La2/3Ca1/3MnO3 samplesare shown in Fig. 1. The sample Pr2/3Ba1/3MnO3 shows a sharptransition (TP1) at194 K followed by a broad transition like hump(TP2) at 160 K. However, La2/3Ca1/3MnO3 depicts only onetransition at 265 K. In the polycrystalline sample the contribution tothe resistivity originates from the two regions: grain and the grainboundary. Since the grain boundary is more chaotic than the core orthe grain, the contribution of the grain boundary to the totalresistivity in a polycrystalline sample therefore, always exceeds tothat of the grain. The effects on the physical properties due to grainare termed as the intrinsic effects and those arising from the grainboundary as the extrinsic effects. In the ferromagnetic state,

perovskites in general, behave like a metal in the electricalproperties. In this sense, a polycrystalline sample or granularperovskite is a granular ferromagnet similar to granular transitionmetals. However, according to the low temperature transportproperties observed the formation of the intergrain barrier may bea little different from that in granular transition metals. Since nomagnetic material, which can be the potential barrier between theferromagnetic grains exists in granular perovskites, the interface orthe grain boundary between neighbouring grains should be taken intoaccount as a barrier. In polycrystalline sample where the variance 2

is low e.g. LCMO (2 = 0.000287 2) insulatormetal (IM) tran-sition is single and broader because of the grain and the grainboundary effects occurring simultaneously. But in samples where 2

is larger, due to the larger ionic size mismatch between ions presentat the rare-earth site e.g. Pr2/3Ba1/3MnO3 (Pr

+3 and Ba+2 sizes in

their nine-fold co-ordination are 1.179 and 1.47 respectively,2 = 0.0187 2), the grain boundary effects are larger and separateout from the grain effects which is clear from T data of Fig. 1.Therefore, PBMO shows one sharp transition at194 K (TP1) andthe broader one at 1 6 0 K (TP2). The reason for larger grainboundary effects in PBMO sample is that due to the larger ionic sizedifference between Pr+3 and Ba+2 the lattice within the grainexperiences a good deal of strain. The lattice seemingly unloads thisstrain to the grain surface or to the grain boundary. Consequently, thelattices at the grain boundary or in its vicinity are more distorted andwould weaken the electron transfer probability from one Mn-site tothe other. This results in the separation of the two transitions inPBMO. However, in LCMO the ionic size difference between the

ions La

+3

(1.216 ) and Ca

+2

(1.18 ) is smaller resulting in thenon-separation of the two transitions. Now the question arises whythe broader transition TP2 occurs below TP1 in PBMO. The answerto this emerges from the Heisenberg theory of ferromagnetism [17].The ferromagnetic transition can be expressed as TC= 2qJ/kB, whereq is the co-ordination number of the ion, J is the exchange integralbetween the neighbouring atoms and kB is the Boltzmann constant.On one hand, the average co-ordination number (q) is lower at thegrain boundary due to the presence of the dangling bonds and on theother hand, the overlapping between the neighbouring ions is lowerdue to the co-ordination number being lower and so is the exchangeintegral J. Thus TC or TP in the grain surface would certainly belower to that within the core. In the PBMO sample due to the largerstrain at the grain boundary these two effects (grain/grain boundary)

separate out but not in LCMO where ionic size difference is notlarger. Due to the reason that in the grains Mn+3/Mn+4 ions areparallel to each other and thus the double exchange mechanism isstronger in grain and hence larger TP but at the grain surface thoseions are in chaotic order so TP for grain boundary is lower. It wouldbe worth mentioning here that for the pristine sample PBMOincrease in the sintering temperature leads only to the decrease in thepeak resistivity with the transition at TP2 (160 K) remainingpractically unchanged. Such a behaviour can be explained on thebasis of the increase in the grain size with the increase in thesintering temperature resulting in the overall decrease in the disorderpresent there. Barnabe et al. [18] have used higher sinteringtemperature (1500 C) and reported that electrical resistivity at TP2is lower than that at TP1, whereas it is higher in the present study.However, the value of TP2 is same in both cases. The higher

Fig. 1. Resistivity-temperature variation of Pr2/3Ba1/3MnO3 and La2/3Ca1/3MnO3.

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temperature transition TP1 is also slightly higher (205 K) in case ofBarnabe et al. [18]. The higher insulatormetal (IM) transition isrelated to the larger grain size. Further, it is observed that there isappearance of a re-entrant insulating behaviour below 35 K whichhas been ascribed to the localization of the carriers. Such localizationcould be due to the spin polarized tunneling in these samples but in

PBMO due to the larger variance

2

an extra term is also added tothe resistivity. Tunneling is made possible between the ferromagneticgrains via the grain boundary that is rather paramagnetic [19]. If thetunneling is of the spin polarized electrons between two grains ofanti-parallel spins then to reverse it one has to add an extra term inthe resistivity. Due to the strain in PBMO system the carriers areunable to overcome this strain (apparently due to insufficient energy)and get localized leading to the observed re-entrant insulatingbehaviour at low temperatures.

Resistivity data (with and without the application of the magneticfield, Fig. 2) above TP (from room temperature up to TP) fits the 3-Dvariable range hopping model (VRH) =0exp(T0 /T)

1/4 as proposedby Mott[20] where T0 is a constant (16a

3 /KBN(EF), is the inverse ofthe localization length, N(EF) being the density of states at the Fermi

level). From the VRH fit (between 295 K and corresponding TP) wehave calculated T0 and N(EF) (Table 1) both in the presence andabsence of the magnetic field.

It is observed that N(EF) increases with the application of themagnetic field. However, the increase is not significant in PBMO incomparison with LCMO because of the lesser effect of magnetic field

on PBMO above TP. To estimate N(EF) we used the value of= 2.22 nm1 calculated by Viret et al. [21]. It is noticed that the valueof N(EF) increases (or corresponding T0 value decreases) on theapplication of the magnetic field which may be due to the suppressionof the magnetic domain scattering by the magnetic field. In order toanalyze the data in the metallic regime (below 265 K in LCMO and160 K in PBMO), the following equations were fitted

q q0 q1T2 1

q q0 q2T2:5 2

q q0 q1T2 q3T

4:5 3

where the temperature independent part 0 is the resistivity due todomain, grain boundary and other temperature independent scatteringmechanisms. 1T

2 term in Eqs. (1) and (3) represents the electricalresistivity due to the electronelectron scattering process and isgenerally dominant up to 100 K. 2T

2.5 is the term arising due toelectronmagnon scattering process. On the other hand, the term 3T

4.5

is a combination of electronelectron, electronmagnon and electronphonon scattering processes. We find that in the metallic regime,conductivity data for both the samples best fit the equation =0+2T

2.5

both in the presence and absence of the magnetic field. Thereforemetallic regime can be attributed to the electronmagnon scatteringprocesses, which further demonstrates that the metallic regime is in theferromagnetic phase. However, 2 is much larger for PBMO than thatof LCMO which further implies the prominent role of the grainboundary in PBMO by making the sample lesser ferromagnetic ascompared with LCMO after the transition atTC. The best-fit parameters(the linear correlation coefficient R2 is maximum for2T

2.5) obtainedfrom the fitting of the low temperature metallic regime of the resistivitydata with Eq. (2) are shown in Table 2.

It is observed that0 decreases significantly with magnetic field,but the influence of the field on term 2 is small. As the magnetic fieldincreases, the size of the domain boundary decreases and 0 becomessmaller. The slight decrease of 2 with field may be due to thesuppression of spin fluctuations in the field. The electronmagnonscattering process (=0+2T

2.5) has also been invoked earlier to fitthe electrical resistivity data in the metallic region of the manganites[22]. The MR behaviour (Fig. 2) shows sharp peaks at 194 K and 262 K

for the samples PBMO and LCMO respectively and below thesetransitions it again starts increasing reflecting the role of grainboundaries. The susceptibility (T) measurements (Fig. 3a) forboth PBMO and LCMO samples show transitio ns from theparamagneticferromagnetic (PMFM) state at 194 K and 267 Krespectively.

Fig. 2. Resistivity-temperature variation with and without magnetic field andMR behaviour of Pr2/3Ba1/3MnO3 (a) and La2/3Ca1/3MnO3 (b).

Table 1

T0 (H=0 T)(106 K)

T0 (H=0.6 T)(106 K)

N(EF)[H=0 T](1020 eV1 cm3)

N(EF)[H=0.6 T](1020 eV1 cm3)

LCMO 1.51 1.24 13.4 16.4PBMO 1.96 1.94 10.4 10.5

Table 2

(=0+2T2.5) Pr2/3Ba1/3MnO3 La2/3Ca1/3MnO3

H=0 H=0.6 T H= 0 H=0.6 T

R2 0.999 0.998 0.999 0.9990 ( cm) 0.06062 0.05251 3.478 10

3 2.823103

2 ( cm K2.5) 3.1 106 2.51106 2.04108 1.98108

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These transitions being close to IM transitions confirm thatelectrical and magnetic properties are indeed coupled in manganites[23]. The magnetic transitions have been calculated by differentiatingthe T curves and measuring the temperature where d / dT isminimum. AfterTC, for the sample LCMO is almost constant whichshows that it is a long-range ordered ferromagnetic sample. However, decreases drastically for PBMO sample indicating the cluster glassbehaviour and short-range ordered ferromagnet. The imaginary part(Fig. 3b) of the susceptibility (=+ i ) also peaks near the IMtransitions for both the samples and the signal of is about two ordersof magnitude smaller than as have been reported earlier also [24].Since is a measure of the heat loss in the sample so it shouldcorrespond to the electrical transition which is the case here. This also

tells about the better homogeneity of the samples. The Tcurve alsoreflects only the higher temperature transitions in both the samples as thefield is lower andunable to make thesurface spinsparallel so no signatureof the second transition is seen. However, as T represents themagnetic behaviour of the sample below 160 K where grain boundaryeffects are prominent there is competition between the core Mn ions(ferromagnetically ordered) and surface Mn ions (which are ratherparamagnetic) and the result is destruction of the long range order inPBMO, making thesample cluster glass.The SEMpictures (Fig. 4)ofthesamples also show clear grains and the average grain sizes forPBMO andLCMO are 3 m and 5 m respectively. The grain sizes are smallerreflecting therole of grain boundariesin thetransport properties.The MRcurves (Fig. 2) also indicate the higher temperature transition as the fieldis low it only reduces the grain resistivity noticeably and not of the grainboundary because of chaos there. The MR vs. temperature curve (0.6 T)

shows higher and constant value of MR after TP1 for PBMO sample

while that of LCMO shows a lower MR and an increasing trendbelow the transition TP. MR vs. H variation at 77 K (Fig. 5) showsthe behaviour of low field magnetoresistivity (LFMR) due to spinpolarized tunneling for both the samples (with a sudden rise underlow magnetic field and then saturation at higher magnetic field value)[25]. But MR of PBMO remains unsaturated which means that itrequires a larger field to get the saturation for this particular sampleas compared to LCMO where MR gets saturated. This also indicatesthat canting of spins at the grain boundary region is higher in PBMO

Fig. 5. MR variation with applied magnetic field of Pr2/3Ba1/3 MnO3 and La2/3Ca1/3MnO3 at 77 K.

Fig. 4. Scanning electron micrographs of Pr2/3Ba1/3MnO3 and La2/3Ca1/3MnO3.Fig. 3. a. Plot forT of Pr2/3Ba1/3 MnO3 and La2/3Ca1/3MnO3. b. Plot forT of Pr2/3Ba1/3 MnO3 and La2/3Ca1/3MnO3.

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(larger strain) sample than LCMO (lesser strain). The role of oxygencontent is very crucial on the electrical as well as the magneticproperties. The reduced oxygen content will increase the Mn+3

concentration and can result in two transitions in the resistivity-temperature behaviour [26]. TP in our LCMO sample is 265 K andmatches well with others [2729] and it shows IM transition which

confirms that our sample is not oxygen deficient. Since both PBMOand LCMO samples were synthesized under the same conditions sothe second transition in PBMO due to the reduced oxygen content isruled out. Barnabe et al. [18] have synthesized PBMO and checkedthe oxygen content to be 3 and the material shows two IMtransitions.

4. Conclusions

Electrical and magnetic properties of orthorhombic (PBMO)and (LCMO) manganites are reported here. These materials have

been chosen because of the considerable difference in theirvariance (2) values. PBMO exhibits two types of transitions in

the resistivity-temperature behaviour, characteristic of bothintrinsic (due to grains) and extrinsic (due to grain boundaries)situations whereas only intrinsic effects are observed in LCMO.Low field magnetoresistivity (LFMR) data also substantiate theseresults. Increase in the density of states (and the decrease in theT0) obtained through Mott's 3-D variable range hoppingmechanism in the paramagnetic insulating regime (above intrinsictransition) is attributed to the suppression of magnetic domainscattering with applied field. Ferromagnetic metallic regime

below the extrinsic transition in PBMO seems to emanate fromthe electronmagnon scattering process. LFMR at 77 K also

points towards the higher canting of spins in the vicinity of grainboundary regions in PBMO compared to that in LCMO.

Acknowledgements

The authors express their gratitude to the Director, NPL forhis keen interest in the present work. Assistance from thescanning electron microscopy section, NPL, New Delhi isgratefully acknowledged. Two of us (NP and VS) are thankfulto the CSIR, New Delhi for the grant of Senior ResearchFellowships.

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