Materials Letters 64 (2010) 2638–2640

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Materials Letters

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Low field magnetoresistance, temperature coefficient of resistance andmagnetocaloric effect in Pr2/3Ba1/3MnO3:PdO composites

Neeraj Panwar a,⁎, Indrani Coondoo b, S.K. Agarwal b,⁎a Department of Physics, University of Puerto Rico, San Juan 00931, PR, USAb National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India

⁎ Corresponding authors. Tel.: +91 11 4560 8276; faE-mail addresses: neeraj.panwar@gmail.com (N. Pan

(S.K. Agarwal).

0167-577X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.matlet.2010.08.051

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 June 2010Accepted 18 August 2010Available online 23 August 2010

Keywords:MagnetoresistanceTCRMagnetocaloric effect

Electrical resistivity, magnetoresistance (MR), temperature coefficient of resistance (TCR) and magnetoca-loric effect of (1−x) Pr2/3Ba1/3MnO3:x PdO (x=0–30 mol% PdO) composite manganites are reported here.Pristine sample Pr2/3Ba1/3MnO3 (PBMO) shows two insulator–metal like transitions (TP1~194 K andTP2~160 K) in the electrical resistivity behavior. With PdO, TP1 becomes sharper whereas TP2 disappearsbeyond 10 mol% PdO addition. The intrinsic MR gets enhanced from 22% for the pristine sample to ~42% for27% PdO sample. However, the extrinsic MR is found to decrease in the composites. The TCR also increasesfrom a negligible value for PBMO to 8% for 25 mol% PdO sample. These features have been explained on thebasis of opening of new conducting channels and decrease in spin dependent scattering and the overalldecrease in electrical resistivity. The magnetic entropy change and relative cooling power (RCP) for thePBMO sample are 5.3418 J.Kg-K and 304.5428 J/Kg respectively. However, these values decrease in thecomposites.

x: +91 11 4560 9310.war), prof.agarwal@gmail.com

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Manganite perovskites (ABO3) are not only important because ofthe observation of colossal magnetoresistance (CMR) effect to be usedin magnetic field sensors, but they are also quite useful for otherapplications like infrared bolometers, magnetocaloric effect andelectrodes for solid-state oxide fuel cells [1–9]. While higher MR atlow magnetic field is the requirement of any magnetic device fromthese materials, at the same time, low electrical resistivity with sharptransition and higher relative cooling power are also needed forcircuit impedance matching, for better temperature coefficient ofresistance (TCR) and large magnetocaloric effect respectively. Re-cently, a great deal of experiments indicate that the metal or metaloxide doping/addition in manganites is helpful in observing improvedelectrical resistivity (TP), enhanced magnetoresistance, TCR andmagnetocaloric effect [10–17]. Yuan et al. [11,12] reported the highestvalue of MR at room temperature in LCBMO/Pd composite system(prepared by sol–gel method) under the application of 1 T magneticfield.We reported improved electrical,magnetotransport,magnetic andthermoelectric power (TEP) properties of (1−x) Pr2/3Ba1/3MnO3+x PdO composite system [13]. We also observed enhanced TCR close toroom temperature in La0.7Ca0.3−xBaxMnO3+ Agy (x=0.10, 0.15 and0≤y≤1.0) composite manganites [5]. Kamilov et al. [6] observed a RCP

value of 100 J Kg−1 between 273 K and room temperature at a fieldchange of 2.6 T for the metallic silver doped LaMnO3 system, whichis about half the RCP of gadolinium. In our previous studies [13] onPBMO+PdO composites, the magnetoresistance was observed 92%(maximum can be 100% in that case) in 5 T magnetic field but asmentioned earlier, higherMR should occur at a lowmagneticfield value.Therefore, in this paper we report the low field magnetoresistance(LFMR)behavior of PBMO+PdOcomposites alongwith the calculationsof TCR and RCP. Though the temperature range where all theseproperties are observed lies near 200 K, with proper substitutionthese properties may be enhanced to higher temperatures.

2. Experimental details

Polycrystalline composites (1−x) PBMO+x PdO (x=0, 5 10, 1520, 25, 27 and 30 mol%) were synthesized using the conventionalsolid-state reactionmethod. First, the polycrystalline powder of PBMOwas obtained through several calcinations with intermediate grind-ings and final sintering at 1260 °C for 25 h. Later, PdO was mixed withcalcined PBMO in different mole ratio and the grinded mixture wasagain treated at 1260 °C for about 2 h to avoid any diffusion. Electricalresistivity with andwithout magnetic field (0.6 T) was recorded usingfour-probe method from 300–77 K with field applied parallel to thecurrent direction and with a precision of 0.01% at 2 readings persecond. Heat capacity measurements have been carried out on PPMSfrom 300 K to 5 K with an absolute accuracy of 0.5% for temperatureup to 100 K and 1% for higher temperature.

Fig. 2. Magnetoresistance variation with temperature of (1−x) PBMO+x PdO (x=0–30 mol%) composites. The inset shows the MR variation with magnetic field at 77 K.

2639N. Panwar et al. / Materials Letters 64 (2010) 2638–2640

3. Results and discussion

Fig. 1 shows the electrical resistivity variation with temperature ofthe composite system. The parent compound PBMO shows twoinsulator–metal (I–M) transitions. Higher temperature transition(TP1) is at 194 K followed by a broader transition (TP2) at ~160 K.The mechanism of the occurrence of two insulator–metal liketransitions due to grain boundary effects has already been describedelsewhere [13]. During the sintering process oxygen liberated fromthe dissociation of PdO is expected to go to the grain and compensatethe oxygen deficiency (if any) since the bond energy of Pd–O (about~381 kJ mol−1) is smaller than that ofMn–O (~403 kJ mol−1) andO–O(~498 kJ mol−1) [11]. With PdO addition, TP1 remains almostunchanged with the peak getting sharper whereas TP2 is greatlyaffected and even disappears beyond 10 mol% PdO addition [13].Electrical resistivity in the whole temperature range also decreaseswith PdO addition when compared with that of the pristine PBMOsample. However, the electrical resistivity of the 30 mol% PdO sampleis more than that of 25 and 27 mol% samples, this may be due tovolatilization of Pd from the grain boundary regions and thus creatingsome defects. To take advantage of the reduction in electricalresistivity we further calculated the TCR for the composite systemwhich is shown in the inset of Fig. 1. The TCR has been defined as:

TCR %ð Þ = 1 = ρð Þ dρ= dTð Þ × 100:

From the inset of Fig. 1, it can be noticed that the TCR value for thepristine sample is almost negligible which gets enhanced up to ~8% at192 K for the 25 mol% PdO added sample. The observed enhancementof TCR is attributed to the grain growth andopening of newconductingchannels and decrease in electrical resistivity in the composites [5].These results also prove that PdO addition may be useful in observingthe enhanced value of TCR at/near room temperature.

To investigate the effect of PdO addition on the magnetoresistancebehavior at low magnetic field we carried out the magnetoresistancemeasurements of the composite systems in 0.6 T magnetic field andthe results are shown in Fig. 2. It is evident from that the intrinsic MR(related to grain) gets enhanced from 22% for pristine PBMO to 42%for the 27% PdO added sample. In the 30% PdO sample, peak MRdecreases slightly as compared to the 27% PdO sample. Suchdifference has also earlier been published [11,12]. This may beattributed to partial volatilization of Pd in the sample with 30% PdO. It

Fig. 1. Resistivity-temperature variation of (1−x) PBMO+x PdO (x=0–30 mol%)composites. The inset shows the temperature coefficient of resistance variation of (1−x)PBMO+x PdO (x=0–30 mol%) composites.

is worthwhile to note that extrinsic MR (MR related to grainboundary) decreases in composites (as shown in the inset of Fig. 2).This behavior can be understood from the fact that spin dependentscattering decreases with Pd addition at the grain boundary, which isresponsible for the extrinsic MR in polycrystalline manganites.

Now, we discuss about the heat capacity (specific heat)measurements of the composites (0, 10, 20 and 30 mol% PdOadded samples) carried out with (5 T) and without the applicationof magnetic field. Fig. 3 shows the heat capacity measurementswithout magnetic field and the inset shows the data under 5 Tmagnetic field. The behavior of all the samples is similar and matcheswith earlier reported data [18–20]. A lambda-like peak is observed inall the samples near TP1, arising due to magnetic ordering in thesamples. No anomaly corresponding to TP2 is observed in CP vs Tmeasurements because thermal measurements are volume averagedmeasurements and thus grain boundary effects which otherwise areseen in electrical properties are masked in such measurements. Asseen from the inset 1 of Fig. 3, it is clear that the peak anomaly getssuppressed and the peak temperature shifts to higher temperatures onthe application of magnetic field. Using the two values, we calculatedthe magnetic entropy change ΔSM, for the pure and 30 mol% PdO

Fig. 3. Heat capacity variation with the temperature of pristine, 10, 20 and 30 mol% PdOadded sample. Inset (1) shows the variation under 5 Tmagnetic field. Inset (2) is for themagnetic entropy change for pristine and the 30 mol% PdO added sample.

2640 N. Panwar et al. / Materials Letters 64 (2010) 2638–2640

added sample as in anymaterial, the entropy is a sum of contributionsfrom the electronic, lattice and magnetic parts and near the heatcapacity peak magnetic entropy dominates. Here, ΔSM has beendefined using the following relation:

ΔSM T;ΔHð Þ = ∫T0C T ;Hð Þ−C T;0ð Þ

TdT :

The results are shown as inset (2) in Fig. 3. For the pristine sampleΔSM~5.3418 J/Kg-K at 194 Kwhereas for the 30%PdOadded sample itsvalue is 3.4523 J/Kg-K at the same temperature. The RCP of themagnetocaloric material can be, evaluated by considering themagnitude of ΔSM and its full-width at half maximum (δTFWHM), RCP(S) =−ΔSM (T ,H)×δTFWHM [21]. For the pristine and 30 mol% addedsamples, RCP values come out to be 304.5428 and 147.8762 J/Kg.Though these values are goodbut occur at low temperatures. However,the properties of manganites can be tailored by proper chemicalsubstitution or addition and the enhanced value can be observed nearroom temperature for utilization in devices.

4. Conclusions

Electrical resistivity, magnetoresistance (MR), temperature coef-ficient of resistance (TCR) and the magnetocaloric effect of (1−x)Pr2/3Ba1/3MnO3:x PdO (x=0–30 mol% PdO) composite manganiteshave been studied. Two insulator–metal like transitions (TP1~194 Kand TP2~160 K) have been observed in the electrical resistivitybehavior of the pristine PBMO sample. While TP1 becomes sharper,TP2 gets disappeared beyond 10 mol% PdO addition. The intrinsic MR(related to grain) gets enhanced from 22% for the pristine sample to~42% in the 27% PdO sample. On the other hand, extrinsic MR(related to grain boundary) decreases in the composites. TCR alsoincreases in the composites. The magnetic entropy change andrelative cooling power (RCP) are highest for the PBMO samples anddecrease in the composites.

Acknowledgements

The authors would like to thank Professor Ajay Gupta, CenterDirector and Dr V. Ganesan of the UGC-DAE Consortium for ScientificResearch, Indore, for allowing their facilities for heat capacitymeasurements. They are also thankful to their colleague Dr. VikramSen for help in magnetoresistance measurements. The authors (SKA &IC) would like to thank the Council of Scientific and IndustrialResearch (CSIR) for providing the financial grants under the EmeritusScientist and the Research Associateship schemes, respectively.

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