Magnetoelectric behavior of sodium doped lanthanum manganitesY. Kalyana Lakshmi, G. Venkataiah, and P. Venugopal Reddy Citation: J. Appl. Phys. 106, 023707 (2009); doi: 10.1063/1.3173285 View online: http://dx.doi.org/10.1063/1.3173285 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v106/i2 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Magnetoelectric behavior of sodium doped lanthanum manganitesY. Kalyana Lakshmi,1 G. Venkataiah,1,2 and P. Venugopal Reddy1,a�

1Department of Physics, Osmania University, Hyderabad, Andhra Pradesh 500 007, India2Department of Physics, National Cheng Kung University, Tainan, Taiwan 701, Republic of China

�Received 12 March 2009; accepted 11 June 2009; published online 21 July 2009�

Nanocrystalline samples of sodium doped manganites with compositional formula La1−xNaxMnO3

�0.025�x�0.25� were prepared by polyvinyl alcohol assisted precursor method. Aftercharacterizing the samples by x-ray diffraction and transmission electron microscopy a systematicinvestigation of electrical, magnetic, and thermopower properties has been undertaken. Theresistivity data were analyzed using effective medium approximation. From the analysis it has beenfound that the metallic fraction is increasing up to x=0.10 and remains constant with further doping.A close examination of the resistivity data clearly indicates that the sodium doped samples areslowly transformed from colossal magnetoresistance behavior to charge ordering behavior.Thermoelectric power data at low temperatures were analyzed by considering the magnon dragconcept, while the high temperature data were explained by small polaron conduction mechanism.© 2009 American Institute of Physics. �DOI: 10.1063/1.3173285�

I. INTRODUCTION

In the past decade, numerous efforts have been made tostudy the colossal magnetoresistance �CMR� effect in perov-skite manganites, Ln1−xAxMnO3 �where Ln=trivalent rareearth element and A=divalent alkaline element� due to theirpotential applications in magnetic field sensors, infrared de-tectors, microwave active components, etc., The profoundinfluence of defect chemistry and the crystal structure on thephysical properties of these mixed valence compounds is ex-emplified in the rich magnetic and electronic phase diagrams.Among the many interesting properties exhibited by thesematerials, the simultaneous occurrence of paramagnetic�PM�-ferromagnetic �FM� and metal-insulator transitions�MITs� are the most exotic and fundamental one and thisbehavior was explained within the frame work of the doubleexchange �DE� model.1 However, there are many other insta-bilities that are competitive with the DE interaction, such asthe anti-FM �AFM�, superexchange �SE�, Jahn-Teller �JT�,orbital-ordering, and charge-ordering �CO� interactions. Mil-lis et al. argued that DE alone is not sufficient to explain theexperimental data and that strong electron-lattice coupling isto be taken into account.2–4 Moreover, the average sizes ofthe trivalent and divalent site cations, the mismatch effect,the vacancies in trivalent and Mn sites, and the oxygen sto-ichiometry also play an important role.5–8

Besides, the divalent doped manganites are well studiedin the light of above mentioned interactions, in more recentyears interest has been directed toward monovalent alkali-metal doped systems. Since the valence state of the alkalimetal ions is +1, their substitution affects the ratio of Mn3+

and Mn4+ ions, which in turn is expected to influence variousphysical properties.9 Among the various monovalent dopedmanganites, sodium ion substituted ones are interesting due

to high value of magnetoresistance �MR� closer to room tem-peratures. Further, the ionic radius of sodium is close to thelanthanum so that the tolerance factor ��� is unchanged bythe substitution; this reflects in a lower cation disorder in-duced by the doping. Moreover, compared to the divalentdoped manganites, it is possible to achieve an equal amountof hole doping with a lower cation substitution, because forthe same amount of monovalent dopant the hole density istwice that of divalent doping. In view of all these facts, asystematic investigation of magnetotransport properties ofthese materials has been undertaken and the results of suchan investigation are presented here.

II. EXPERIMENTAL DETAILS

The polycrystalline materials with the compositional for-mula La1−xNaxMnO3, �x=0.025, 0.05, 0.10, 0.15, 0.20, and0.25� were prepared by a novel chemical method using reac-tive polymer matrix of polyvinyl alcohol �PVA�. In thismethod, highly pure La2O3 �99.99%�, NaNO3 �99.9%�, andfreshly prepared MnCO3 �99.9%� taken in stoichiometric ra-tio were used as the starting materials. Later, all these mate-rials, after converting them as nitrates, were mixed thor-oughly by maintaining pH at 1.0. To this, an aqueouspolymer solution of PVA was added, which upon heatingyields a fluffy porous mass. On further heating, the precursormass decomposes in air and finally gives nanocrystallinepowders. The powder after calcining at 600 °C was pressedinto pellets and sintered at 1000 °C in air for 3 h.

The structural characterization of the samples was car-ried out by powder x-ray diffraction �XRD� using BrukerAXS D8 Advance diffractometer, while the valence states ofMn ion and oxygen stoichiometry were determined using theredox titration technique. The dc magnetization studies wereundertaken over temperature, 10–350 K under zero fieldcooled �ZFC� and field cooled �FC� modes, while the M-Hdata were taken between −15 and +15 kOe field using vibrat-ing sample magnetometer �PPMS� �M/s. Quantum Design�.

a�Author to whom correspondence should be addressed. Electronic mail:paduruvenugopalreddy@gmail.com. Tel.: �91-40-27682242. FAX: �91-40-27090020.

JOURNAL OF APPLIED PHYSICS 106, 023707 �2009�

0021-8979/2009/106�2�/023707/10/$25.00 © 2009 American Institute of Physics106, 023707-1

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The electrical resistivity and MR measurements were carriedout using an Oxford superconducting magnetic system in thefields of 1, 3, and 5 T over a temperature range of 10–310 Kusing four probe method. Finally, thermoelectric power�TEP� studies were also carried out by differential methodover a temperature range of 80–300 K. The measurementswere carried out in nitrogen �exchange gas� atmosphere inheating mode. The absolute Seebeck coefficient �S� values ofthe samples were obtained by subtracting the Seebeck coef-ficient values of the electrode �copper� material.

III. RESULTS AND DISCUSSION

A. Structural properties

The phase purity, structural, and lattice parameters of allthe samples are determined by power XRD and patterns areshown in Fig. 1�a�. It can be seen from the figure that allsamples are single phase with rhombohedral perovskite

structure and R3̄c space group in which La /Na atoms are

located at 6a�0,0 ,1 /4�, Mn at 6b�0,0 ,1 /2�, and O at18e�x ,0 ,1 /4� Wyckoff positions. It is interesting to note thatthe most intense peak exhibits a gradual change in the shapeof the double peak with increasing sodium content �inset ofFig. 1�a��, signifying the onset of orthorhombic phase. Asimilar observation was also reported by Roy et al.9 The cellparameters of all the samples are obtained by refining theexperimental data using a standard Rietveld refinement tech-nique and are given in Table I. A typical plot of XRD patternof LNM-3 along with its Rietveld refined one, including thedifference between observed and calculated patterns areshown in Fig. 1�b�. It can be seen from the table that after aninitial increase, the lattice parameters are found to decreasewith increasing sodium concentration.

The average crystallite sizes of the materials have beenevaluated using peak broadening technique and Scherrer’sformula, �D�=K� /� cos �, where �D� average particle sizein nanometers, K is a constant �shape factor of 0.89�, � isCu K� wavelength, and � is corrected full width at halfmaxima of XRD peak of the sample. LaB6 was used to cor-rect the intrinsic width associated with the equipment. Thecrystallite sizes are found to be in the range of 45–55 nm�Table I�. The particle sizes have been determined by trans-mission electron microscope �TEM� analysis �Fig. 2�. Theaverage particle size of the aggregated nanocrystallinesamples is estimated by considering the average of largenumber of particles and is found to be in close agreementwith those obtained from XRD.

The oxygen content ��� and Mn3+ /Mn4+ ratio of thesamples of the present investigation have been estimatedfrom the idometric titrations10 and the results are included inTable I. The results show that the oxygen content is oversto-ichiometric up to LNM-5, while it is less than the stoichio-metric limit in LNM-6. Further, the Mn4+ is not found tovary systematically with increasing Na content. According toMalavasi et al.,5–8 the excess in oxygen leads to an increasein Mn4+. Therefore, it has been concluded that Mn4+ contentmight be more in the case of first five samples �upto LNM-5�of the present investigation.

B. Magnetic properties

The variation of magnetization with temperature hasbeen studied both in ZFC and field cooled FC processes at500 Oe and the behavior is shown in Fig. 3. The ZFC mag-netization shows that all samples undergo a sharp FM to PMtransition �TC�, which is determined by the minimum ofdM /dT versus temperature curves and the values are tabu-lated in Table II. It can be seen from the table that TC valuesafter an initial increase �x=0.10� are found to remain con-stant with increasing dopant concentration. The observed be-havior may be explained by considering the fact that withincreasing Na doping the ratio of Mn4+ /Mn3+ increases fa-voring the DE interaction. In the process, the number of fer-romagnetically aligned domains supersedes paramagneticallyaligned ones so that the percolation threshold is attained andthe compound becomes FM, thereby enhancing the TC valuesinitially in the high doping regime. As the concentration of ofMn4+ exceeds Mn3+, the SE contribution dominates over DE

FIG. 1. �Color online� �a� Room-temperature XRD patterns for theLa1−xNaxMnO3 �x=0.025, 0.05, 0.10, 0.15, 0.20, and 0.25� samples. Theinset shows the variation of peak shape with doping for the most intensepeak. �b� Rietveld refined pattern of the LNM-3 sample.

023707-2 Kalyana Lakshmi, Venkataiah, and Reddy J. Appl. Phys. 106, 023707 �2009�

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resulting in a small change in TC. In fact, a similar observa-tion was reported earlier.11,12 Further, a competition betweenrhomohedral and orthorhombic phases also influences thetransition temperatures. The rhombohedral perovskite struc-ture is characterized by a weak Hund’s coupling favoring anincrease in TC.4,13 In an orthorhombic symmetry transfer ofeg electron is slow when compared with rhombohedral sym-metry. The structural change from the rhombohedral toorthorhombic with higher doping may stimulate the corre-sponding effect thereby resulting constant TC values.

It is also evident from the Fig. 3 that ZFC and FC curvesdo not coincide at low temperatures indicating the magneticanisotropy13 and the separation increases with increasing so-dium concentration. The observed behavior may be ex-plained qualitatively as follows. The response of the spin tothe external magnetic field depends on the competition be-tween magnetocrystalline anisotropic energy and appliedmagnetic field strength. At low measuring fields, all the spinswill not be oriented in the direction of the applied magnetic

field, giving a hint that the magnetic anisotropic field domi-nates over the applied magnetic field. The magnitude ofMZFC at low temperature below TC will depend on the aniso-tropy. Therefore, it may be concluded that the samples be-come highly anisotropic with increasing sodium concentra-tion.

The field dependent magnetization curves at 10 K areshown in Fig. 4. The saturation magnetization MS increasescontinuously up to x=0.1 and decreases thereafter �inset ofFig. 4�. The observed behavior may be explained as follows:With increasing Na concentration the percentage of Mn4+

increases favoring AFM interactions within the FMclusters.11,14 Washburn et al.15 have analyzed such a mag-netic behavior using two lattice models, one with FM inter-actions and the other one with AFM interactions. In thepresent investigation, in addition to Mn3+–O–Mn4+ DE in-teraction �leading to FM behavior�, the Mn4+–O–Mn4+ SEinteraction �AFM behavior, which brings down the FM in-teraction� is also present. Therefore, the observed decrease ofMS beyond the samples x=0.1 �LNM-3� is expected.

C. Electrical and magnetoresistive properties

The variation of electrical resistivity with temperature inzero and 5 T fields is shown in Fig. 5 and the values of MITtemperature �TP� are given in Table II. It is clear from thetable that TP values after an initial increase are found todecrease thereafter with increasing doping concentration.Generally, the transport properties of manganites depend onthe particle size, doping level, ratio of Mn3+ /Mn4+, oxygencontent, etc., The observed increase in TP values in the caseof first three samples may be explained by considering thefact that for every addition of Na+ ions, double the number ofMn4+ ions may create and contribute to the hoppingprocess.16 Thus even for small amount of Na dopant concen-tration, enough number of holes are created in the eg band

TABLE I. Rietveld refined parameters of La1−xNaxMnO3 �x=0.025, 0.05, 0.10, 0.15, 0.20, and 0.25� manganite

system �with R3̄c space group� at room temperature.

x 0.025 0.05 0.10 0.15 0.20 0.25Sample code LNM-1 LNM-2 LNM-3 LNM-4 LNM-5 LNM-6a�Å�=b�Å� 5.510�5� 5.518�8� 5.498�7� 5.484�8� 5.486�6� 5.484�8�c �Å� 13.338�8� 13.358�4� 13.334�3� 13.319�4� 13.318�3� 13.317�4�Volume �Å3� 350.72 352.36 349.08 346.88 347.11 346.94O �x� 0.462�6� 0.450�7� 0.454�5� 0.456�8� 0.458�4� 0.452�1�Mn–O–Mn �deg� 168.0�7� 163.5�7� 165.4�6� 166.1�6� 166.4�5� 164.49�9�Mn–O �Å� 1.951�9� 1.964�7� 1.953�8� 1.948�8� 1.948�7� 1.951�2�Mn3+% 69.7 57.5 73.6 58.6 55.0 53.6Mn4+% 30.3 42.5 26.4 41.8 45.0 46.4Mn valence 3.303 3.425 3.264 3.430 3.450 3.464�Average valence�� 0.126 0.162 0.032 0.059 0.025 −0.018Crystallite size D�nm�

54.5 45.1 53.5 49.5 55.2 52.5

R-factorsRP 13.0 12.3 12.2 14.8 13.4 16.1RWP 16.8 18.0 16.1 18.3 18.0 20.6RF 3.74 4.61 2.78 1.48 2.53 3.92RBragg 4.37 4.80 3.12 1.58 2.07 3.50

FIG. 2. TEM patterns of the La1−xNaxMnO3 �x=0.025–0.25� manganites.

023707-3 Kalyana Lakshmi, Venkataiah, and Reddy J. Appl. Phys. 106, 023707 �2009�

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and will contribute to the conductivity thereby increasing TP.With further increase in sodium concentration �beyondLNM-3� the ratio of Mn3+ /Mn4+ reaches a stage wherein DEweakens slowly, which in turn enhances SE leading to con-tinuous decrease of TP values.

It can be observed from Fig. 5 that LNM-1 exhibits asharp MIT, and with increasing sodium concentration thesharpness of the MIT peak is found to disappear gradually

and becomes broad as it reaches to LNM-5 sample. It isinteresting to note that on decreasing the temperature, thesamples LNM-4 and 5 are found to exhibit AFM behaviorbelow 50 K, whereas in the case of LNM-6 sample, FM andAFM are coexisting. However, under the influence of mag-netic field �5 T� a broad MIT is observed. From this obser-vation, it has been concluded that the sodium doped samples

FIG. 3. �Color online� Magnetization vs temperature curves of La1−xNaxMnO3 �x=0.025–0.25� measured under ZFC and FC conditions in a magnetic fieldof 500 Oe.

TABLE II. Transition temperatures and the fraction �m� of metallic regions obtained from the fit of EMA model�Eq. �1�� and the best fit parameters obtained for the low temperature region using the Eq. �5�.

Samplecode

TP

�K�TC

�K� m0

� cm�1

� cm K−1�2�2 ��10−16�� cm K−2�

5

� cm K−5�

LNM-1 212 236 0.520 0.8319 0.021 36 40.0 3.4831�10−12

LNM-2 259 276 0.660 0.0622 0.001 16 1.536 2.2707�10−12

LNM-3 290 332 0.780 0.2360 0.000 47 0.833 4.5281�10−13

LNM-4 231 325 0.505 0.1862 0.003 21 4.468 9.6872�10−12

LNM-5 212 327 0.505 129.79 3.428 63 3440 2.8887�10−10

LNM-6 202 327 ¯ ¯ ¯ ¯ ¯

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are slowly transformed from CMR to CO behavior. A similarobservation was reported earlier in the case ofNd1−xNaxMnO3 manganites.17,18

The application of magnetic field reduces the resistivityof the manganites, exhibiting interesting phenomena calledmagnetoresistance �MR�. The MR measurements of all thesamples have been undertaken and the variation of MR withtemperature is shown in Fig. 6. It is clear from the figure that

the sample LNM-1 exhibits a maximum and constant MRover a wide temperature range, whereas from LNM-3 theMR% is found to increase linearly with decreasing tempera-tures. The observed behavior may be explained on the basisof two different mechanisms.19,20 One of them is due to spin

FIG. 4. �Color online� Field dependence of the magnetization ofLa1−xNaxMnO3 �x=0.025–0.25� manganites measured at 10 K. The insetshows the variation of MS with doping.

FIG. 5. �Color online� Temperature dependence of resistivity measured under zero and at an applied magnetic field of 5 T. The solid line indicates the bestfit to the Eq. �1�.

FIG. 6. �Color online� The percentage of MR vs temperature ofLa1−xNaxMnO3 �x=0.025–0.25� manganites. The inset shows for theLNM-1, 2, and 3 samples.

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tunneling across the grain boundaries.19 This contribution ismore significant at low temperatures and low magnetic fieldsand produces a continuous increase in MR values as the tem-perature is lowered. The other contribution comes from thesuppression of magnetic fluctuation as the field is increased.This process takes place within the volume of the grains andMR is dominant in the vicinity of the transition temperature.As the grain sizes of the samples of present investigation arefound to be around 50 nm, surface and volume effects needto be taken into account. The highest MR over a wide rangeof temperature �in the entire FM region� observed in thesample LNM-1 might be due to the combined effect of spintunneling and the suppression of magnetic fluctuations. Sucha large MR was reported in the case of Li doped LaMnO3

�Ref. 21� and Ca doped LaMnO3.22 From a practical view-point, the large MR over a wide temperature range observedin the sample LNM-1 is beneficial for the application inmagnetoelectronic devices fabricated by using the CMRmanganites. It has been concluded that the contribution fromthe spin fluctuations slowly decreases, and finally the mecha-nism due to spin tunneling across the grain boundary domi-nates with increasing Na concentration.

Effective medium approximation method. Severalauthors23–25 attempted to explain the thermal variation of theelectrical resistivity including the transition temperature re-gion of CMR manganites in terms of phase separation mod-els. Monte Carlo simulations of the DE model with JT cou-pling also suggest the phase segregation of FM metal �FMM�

from PM insulator regimes in doped manganites. As theCMR effect in doped manganites is proved to be larger in-volving both DE and strong coupling to local latticedeformations2,26 and as the polaronic distortion of PM stateis thought to play over some temperature range in the FMphase. Jaime et al.25 proposed a phenomenological model byconsidering an effective medium approach �EMA�, where thetotal resistivity of doped perovskite manganite is supposed tobe both due to band electrons and polarons assuming it as atwo component model. In order to understand this apparentdecoupling of the MI and PM-FM transitions, the �T� hasbeen analyzed using EMA.

The EMA was originally conceived to explain the elec-trical conduction in composite materials.27,28 The classicallimits of this theory were later invoked to study the elec-tronic conduction in disordered materials, where hoppingamong localized states well below the mobility edge was themain interest.29 The basic premise of this approximation isthat the material under consideration is assumed to be homo-geneous, random mixture of two types of bonds with differ-ent conductances, and the conduction in the composite isessentially studied as a percolation problem. The conductionin such a bond percolation scenario is indeed decided by thestrength of these two individual bonds as well as the ratio ofthese two bond fractions. For a typical three-dimensionalcase, the effective conductivity of the composite system isgiven by the expression,27,28

=412

�3m − 1�1 + �2 − 3m�2 + ���3m − 1�1 + �2 − 3m�2�2 + 8121/2 , �1�

where 1 and 2 are the resistivity contributions of the twotypes of bonds and m is metallic fraction of the material.28

For instance, if we assume that 1 and 2 are metallic andnonmetallic bond contributions in a random mixture of suchbonds, respectively, the EMA model predicts a transition be-tween metallic and nonmetallic states across a percolationthreshold pC of the metallic bonds. This value depends sen-sitively on the dimensionality of the system and for metallicconduction in a three-dimensional matrix, pC=1 /3. Aroundthis value, the effective resistivity is crucial of the ratio�1 /2�. Investigations of continuous random resistor net-work by Kirkpatrick,28 Eggarter and Cohen,29 and Frisch etal.30 have proved success of this EMA model in differentcontexts. A similar model has been adopted by Rao et al. toexplain the resistivity data in La1−xCaxMnO3 manganites.31

In general, the CMR materials exhibit a distinct metallicbehavior at low temperatures, while insulting behavior domi-nates at high temperatures with crossover at intermediatetemperatures resulting in a MIT. To model the resistivity dataof the samples using EMA, it is necessary to know the ana-lytical expression for the temperature dependence of 1 and

2. The conduction via metallic regions is assumed to be dueto electron-magnon interaction, and their corresponding re-sistivity is given by

1 = 0 + 2.5T2.5, �2�

where term 0 arises due to the grain or domain boundaryand term 2.5 represents the resistivity due to magnon scat-tering process.

Similarly, the conduction via nonmetallic regions is as-sumed to be represented by the polaron hopping model,

2 = 0T exp�EP/kBT� , �3�

where 0= �kB /�phNe2R2C�1−C��exp�2�R�, N is the numberof ion sites per unit volume �obtained from density data�, Ris the average intersite spacing, C is the fraction of sitesoccupied by polaron, � is the electron wave function decayconstant, �ph is the optical phonon frequency �estimated fromthe relation h�ph=kB�D, �D is the Debye temperature�, andEP is the sum of the activation energy required for the cre-ation of carriers and activating the hopping of the carriers.

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The resistivity data are fitted to Eq. �1� for LNM-1 toLNM-5 samples �Fig. 5�, and the solid lines in the figureclearly indicate that the equation fits well. The value of mobtained from the fitting are listed in Table II. It is evidentfrom the table that the metallic fraction value m increases upto LNM-3 and remains constant thereafter. Further, under theinfluence of external magnetic field TP shifts to high tem-perature and causes a decrease in resistivity. It means that theapplication of magnetic field melts a part of insulating frac-tion and converts into FM state, liberating its position ofcarriers and leading to the large enhancement of conductivityand as an evident CMR effect will appear. This prediction isquite reasonable, since the metallic component triggered byFM is also sensitive to the magnetic field, and the sizes ofFM clusters grow as magnetic field is applied especially nearthe transition temperature. The parameters, 0, 2.5, and Eg

are found to decrease with increasing magnetic field. Thiscan be understood by considering the fact that under theinfluence of external magnetic field the PI regions changeinto FM regions more easily and suppresses the formation ofpolarons and spin-disorder scattering, leading to the monoto-nous decrease in Eg and also increasing m.

A distinct low temperature minimum is observed in theelectrical resistivity below 60 K. The depth of the minima isfound to increase with increasing doping concentration, andthe observed behavior may be explained by fitting the experi-mental data to an equation which results from the combinedeffect of weak localization, electron-electron and electron-phonon scattering mechanisms, and given by

�T� = �1/�a + bT1/2� + 2T2 + 5T5, �4�

where the term in the parentheses arises due to the weaklocalization effect,32 a is a temperature independent residualconductivity, and b is the diffusion constant. The other twoterms, namely, 2T2 and 5T5, arise due to electron-electronand electron-phonon scattering, respectively.33 By expandingEq. �4� binomially,

�T� = 0 − 1T1/2 + 2T2 + 5T5, �5�

where 0=1 /a and 1=b /a2 are constants. �T� data up toLNM-5 fits well to Eq. �5� �Fig. 7� both in the presence andabsence of magnetic field, and the best fit parameters aregiven in Table II. In fact, Neeraj et al.34 have also used thismodel to explain the low temperature resistivity data ofPr2/3Ba1/3MnO3Ag2O composites. The fitting parameters�Table II� increase with increasing dopant concentration anddecrease with the application of magnetic field. This indi-cates that the weak localization, electron-electron andelectron-phonon scattering process increase with Na concen-tration.

D. Thermoelectric power

1. Low temperature behavior

The variation of thermoelectric power �Seebeck coeffi-cient S� with temperature in the temperature range of80–300 K is shown in Fig. 8�a�. One may see from the fig-ure that in the case of samples, LNM 1–3, S value remainspositive throughout the temperature range of investigation,

whereas for the other samples it is found to change frompositive to negative, with increasing temperature. Thechange in sign in the S�T� data of the samples indicates thecoexistence of two types of carriers. The negative S at hightemperature is attributed to the electrons which are excitedfrom the valence band �VB� into the conduction band �CB�.Because of the higher mobility of electrons within the CB, Sis negative. At low temperatures, the electrons in the VBband are excited into the impurity band which generatesholelike carriers, which is responsible for a positive S.35

The magnitude of S increases with increasing Na dopingexcept in the case of LNM-1, and the observed behavior maybe due to the fact that for every ion of Na doping, double thehole centers, which are localized and causes narrowing of eg

band, distorting the Fermi surface.36 It is also interesting tonote that all samples exhibit a peak in the low temperatureregion �100–120 K�, in addition to the one generally ob-served at TP. Further, the second peak observed for samples LNM-4 might be due to the MIT. It was reported earlier36

that phonon drag �Sg� and magnon drag �Sm� contributionsare present in the low temperature region. In the low tem-perature FM phase, a magnon drag effect is produced due tothe presence of electron-magnon interaction, while the pho-non drag is due to electron-phonon interaction. In general,the variation of S�T� of transitional metal oxides is analyzedby a relation of the form

S = S0 + S3/2T3/2 + S4T4, �6�

where S0 is a constant and accounts the low temperaturevariation of thermopower. The second term S3/2T3/2 is attrib-uted to the magnon scattering process, while the origin of thelast term S4T4 is related to the spin-wave fluctuations in the

FIG. 7. �Color online� Low temperature electrical resistivity fitting forLNM-1 and LNM-3 samples using the Eq. �5�.

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FM phase.36 The above equation was fitted to FMM part ofthe thermopower data and is found to fit well only below110 K. Therefore, it is clear that Eq. �6� may not be suit-able to account the entire FMM part of TEP. In view of thesefacts, Eq. �6� has been modified by adding two more �viz.,phonon drag and diffusion� terms and the resulting equationis given by37

S = S0 + S1T + S3/2T3/2 + S3T3 + S4T4. �7�

The solid curve in Fig. 8�b� represents the best fit of theexperimental data up to 200 K and the best fit parameters aregiven in Table III.

In order to explain the origin of peak at low tempera-tures, the phonon and magnon drag contributions to the TEPwere investigated using Eq. �7�. The variation of phonondrag component with T3 for all the samples is shown in Fig.9�a�. It is clear from the figure that the phonon drag contri-bution �S3T3� is found to vary linearly up to a certain tem-perature and deviates below 250 K �except in the case ofLNM-6�. As the phonon drag contribution deviates at 250 K,it has been concluded that the origin of the low temperaturepeak might not be due to the phonon drag effect. Therefore,the magnon drag component was calculated from Eq. �7� andis shown in Fig. 9�b�. It is interesting to note that the magnondrag component fits well below 200 K, indicating that thelow temperature peak might have arisen due to magnon drageffect.

2. High temperature behavior

The charge carriers in the insulating region are not itin-erant and the transport properties are governed by thermally

FIG. 8. �Color online� �a� The temperature dependence of TEP from80 to 300 K. �b�. The solid lines represents the fitting with Eq. �7�.

TABLE III. The best fit parameters obtained from TEP data of La1−xNaxMnO3 compound.

Samplecode

S0

��V K−1�S1

��V K−2�S3/2

��V K−5/2�S3

��V K−4�S4��10−8���V K−5�

EP

�meV�ES

�meV�WH=EP−ES

�meV� �

LNM-1 −79.695 3.492 −0.3114 0.000 05 −10.70 161.01 1.24 159.77 −0.047LNM-2 −51.312 2.341 −0.2125 0.000 04 −8.15 139.16 1.26 137.90 −0.001LNM-3 −22.213 0.965 −0.0829 0.000 01 −2.24 35.97 0.34 35.63 −0.005LNM-4 −45.679 1.996 −0.1757 0.000 03 −0.572 50.97 3.88 47.09 −0.017LNM-5 −85.888 3.473 −0.2946 0.000 04 −8.07 53.95 5.98 47.97 −0.027LNM-6 −42.882 1.684 −0.1354 0.000 02 −2.17 85.54 7.36 78.18 −0.038

FIG. 9. �Color online� �a� Variation of magnon drag component with T3/2

and �b� variation of phonon drag component with T3. The arrow marksrepresent the deviation of linear fit to experimental curve.

023707-8 Kalyana Lakshmi, Venkataiah, and Reddy J. Appl. Phys. 106, 023707 �2009�

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activated carriers because the effect of JT distortions in man-ganites results in strong electron-phonon coupling and hencethe formation of polarons. Therefore, the thermoelectricpower data of the present samples in the insulating regimeare fitted to Mott’s polaron hopping equation,

S�T� =kB

e� ES

kBT+ �� , �8�

where kB is the Boltzmann constant, e is the electroniccharge, ES is the activation energy obtained from thermoelec-tric power data, and � is a constant. In Eq. �8�, ��1 impliesthe applicability of small polaron hopping �SPH� model,whereas � 2 indicates the large polaron hopping. The bestfit curves of S versus 1 /T of the samples LNM-4, 5, and 7are shown in Fig. 10, and the fitting parameters �ES and ��are given in Table III. Using the activation energy valuesfrom �T� plots �EP� and those from S�T� plots �ES�, thepolaron hopping energy values of all the samples have beencalculated using the relation, WH=EP−ES, and are given inTable III. The EP values are found to be higher than those ofES. Such a large difference in the activation energy is theindication of the applicability of the SPH model in the insu-lating region.36 It can be observed from Table III that thehopping energy WH is increasing �except in the case ofLNM-3� with increasing Na concentration, thereby implyingthat polaronic radius might be decreasing. This is becausepolaron radius varies inversely as per the relation

WH = e2/4��1/rP − 1/R� , �9�

where e is electronic charge, � is the effective dielectric con-stant, rP is the polaron radius, and R is the mean spacingbetween the hopping sites, i.e., Mn3+ and Mn4+. Further, asthe calculated values of � are less than 1, it has been con-cluded that the SPH mechanism might be appropriate to ex-plain the electrical resistivity as well as thermopower data inthe high temperature regime.

IV. CONCLUSION

In conclusion, nanocrystalline sodium doped lanthanummanganites were synthesized using PVA assisted precursor

method. The resistivity data signify that the sodium dopedsamples are slowly transforming from CMR to CO behaviorwith increasing doping concentration. The low temperatureupturn in the electrical resistivity was explained using thecombined effect of weak localization, electron-electron, andelectron-phonon scattering mechanisms. The broad hump ob-served in the low temperature TEP was attributed to magnondrag effect. Finally, it has also been concluded that, for apractical viewpoint, the large MR over a wide temperaturerange observed in the sample LNM-1 is beneficial for theapplication in magnetoelectronic devices.

ACKNOWLEDGMENTS

The first author is grateful to the CSIR for providingfellowship. The DST, Government of India is acknowledgedfor funding 14T PPMS. The authors thank the Centre Direc-tor, Dr. A. Banerjee and Dr. R. Rawat of UGC-DAE CSR,Indore, India, for providing Low Temperature Magnetizationand MR facilities. The authors thank Dr. G.V.N. Rao, ARCI,Hyderabad for providing XRD facilities. The authors alsothank Dr. B. Sreedhar, IICT, Hyderabad, India for providingTEM facilities.

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