Magnetoelectric behavior of sodium doped lanthanum ...

JOURNAL OF APPLIED PHYSICS 106, 023707 共2009兲

Magnetoelectric behavior of sodium doped lanthanum manganites Y. Kalyana Lakshmi,1 G. Venkataiah,1,2 and P. Venugopal Reddy1,a兲 1

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

2

共Received 12 March 2009; accepted 11 June 2009; published online 21 July 2009兲 Nanocrystalline samples of sodium doped manganites with compositional formula La1−xNaxMnO3 共0.025艋 x 艋 0.25兲 were prepared by polyvinyl alcohol assisted precursor method. After characterizing the samples by x-ray diffraction and transmission electron microscopy a systematic investigation of electrical, magnetic, and thermopower properties has been undertaken. The resistivity data were analyzed using effective medium approximation. From the analysis it has been found 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 are slowly transformed from colossal magnetoresistance behavior to charge ordering behavior. Thermoelectric power data at low temperatures were analyzed by considering the magnon drag concept, while the high temperature data were explained by small polaron conduction mechanism. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3173285兴 I. INTRODUCTION

In the past decade, numerous efforts have been made to study the colossal magnetoresistance 共CMR兲 effect in perovskite manganites, Ln1−xAxMnO3 共where Ln= trivalent rare earth element and A = divalent alkaline element兲 due to their potential applications in magnetic field sensors, infrared detectors, microwave active components, etc., The profound influence of defect chemistry and the crystal structure on the physical properties of these mixed valence compounds is exemplified in the rich magnetic and electronic phase diagrams. Among the many interesting properties exhibited by these materials, the simultaneous occurrence of paramagnetic 共PM兲-ferromagnetic 共FM兲 and metal-insulator transitions 共MITs兲 are the most exotic and fundamental one and this behavior was explained within the frame work of the double exchange 共DE兲 model.1 However, there are many other instabilities that are competitive with the DE interaction, such as the anti-FM 共AFM兲, superexchange 共SE兲, Jahn-Teller 共JT兲, orbital-ordering, and charge-ordering 共CO兲 interactions. Millis et al. argued that DE alone is not sufficient to explain the experimental data and that strong electron-lattice coupling is to be taken into account.2–4 Moreover, the average sizes of the trivalent and divalent site cations, the mismatch effect, the vacancies in trivalent and Mn sites, and the oxygen stoichiometry also play an important role.5–8 Besides, the divalent doped manganites are well studied in the light of above mentioned interactions, in more recent years interest has been directed toward monovalent alkalimetal doped systems. Since the valence state of the alkali metal ions is +1, their substitution affects the ratio of Mn3+ and Mn4+ ions, which in turn is expected to influence various physical properties.9 Among the various monovalent doped manganites, sodium ion substituted ones are interesting due a兲

Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel.: ⫹91-40-27682242. FAX: ⫹9140-27090020.

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to high value of magnetoresistance 共MR兲 closer to room temperatures. Further, the ionic radius of sodium is close to the lanthanum so that the tolerance factor 共␶兲 is unchanged by the substitution; this reflects in a lower cation disorder induced by the doping. Moreover, compared to the divalent doped manganites, it is possible to achieve an equal amount of hole doping with a lower cation substitution, because for the same amount of monovalent dopant the hole density is twice that of divalent doping. In view of all these facts, a systematic investigation of magnetotransport properties of these materials has been undertaken and the results of such an investigation are presented here. II. EXPERIMENTAL DETAILS

The polycrystalline materials with the compositional formula La1−xNaxMnO3, 共x = 0.025, 0.05, 0.10, 0.15, 0.20, and 0.25兲 were prepared by a novel chemical method using reactive polymer matrix of polyvinyl alcohol 共PVA兲. In this method, highly pure La2O3 共99.99%兲, NaNO3 共99.9%兲, and freshly prepared MnCO3 共99.9%兲 taken in stoichiometric ratio were used as the starting materials. Later, all these materials, after converting them as nitrates, were mixed thoroughly by maintaining pH at 1.0. To this, an aqueous polymer solution of PVA was added, which upon heating yields a fluffy porous mass. On further heating, the precursor mass decomposes in air and finally gives nanocrystalline powders. The powder after calcining at 600 ° C was pressed into pellets and sintered at 1000 ° C in air for 3 h. The structural characterization of the samples was carried out by powder x-ray diffraction 共XRD兲 using Bruker AXS D8 Advance diffractometer, while the valence states of Mn ion and oxygen stoichiometry were determined using the redox titration technique. The dc magnetization studies were undertaken over temperature, 10– 350 K under zero field cooled 共ZFC兲 and field cooled 共FC兲 modes, while the M-H data were taken between −15 and +15 kOe field using vibrating sample magnetometer 共PPMS兲 共M/s. Quantum Design兲.

106, 023707-1

© 2009 American Institute of Physics

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023707-2

J. Appl. Phys. 106, 023707 共2009兲

Kalyana Lakshmi, Venkataiah, and Reddy

FIG. 1. 共Color online兲 共a兲 Room-temperature XRD patterns for the La1−xNaxMnO3 共x = 0.025, 0.05, 0.10, 0.15, 0.20, and 0.25兲 samples. The inset shows the variation of peak shape with doping for the most intense peak. 共b兲 Rietveld refined pattern of the LNM-3 sample.

The electrical resistivity and MR measurements were carried out using an Oxford superconducting magnetic system in the fields of 1, 3, and 5 T over a temperature range of 10– 310 K using four probe method. Finally, thermoelectric power 共TEP兲 studies were also carried out by differential method over a temperature range of 80– 300 K. The measurements were carried out in nitrogen 共exchange gas兲 atmosphere in heating mode. The absolute Seebeck coefficient 共S兲 values of the samples were obtained by subtracting the Seebeck coefficient values of the electrode 共copper兲 material. III. RESULTS AND DISCUSSION A. Structural properties

The phase purity, structural, and lattice parameters of all the samples are determined by power XRD and patterns are shown in Fig. 1共a兲. It can be seen from the figure that all samples are single phase with rhombohedral perovskite ¯ c space group in which La/ Na atoms are structure and R3

located at 6a共0 , 0 , 1 / 4兲, Mn at 6b共0 , 0 , 1 / 2兲, and O at 18e共x , 0 , 1 / 4兲 Wyckoff positions. It is interesting to note that the most intense peak exhibits a gradual change in the shape of the double peak with increasing sodium content 关inset of Fig. 1共a兲兴, signifying the onset of orthorhombic phase. A similar observation was also reported by Roy et al.9 The cell parameters of all the samples are obtained by refining the experimental data using a standard Rietveld refinement technique and are given in Table I. A typical plot of XRD pattern of LNM-3 along with its Rietveld refined one, including the difference between observed and calculated patterns are shown in Fig. 1共b兲. It can be seen from the table that after an initial increase, the lattice parameters are found to decrease with increasing sodium concentration. The average crystallite sizes of the materials have been evaluated using peak broadening technique and Scherrer’s formula, 具D典 = K␭ / ␤ cos ␪, where 具D典 average particle size in nanometers, K is a constant 共shape factor of 0.89兲, ␭ is Cu K␣ wavelength, and ␤ is corrected full width at half maxima of XRD peak of the sample. LaB6 was used to correct the intrinsic width associated with the equipment. The crystallite sizes are found to be in the range of 45– 55 nm 共Table I兲. The particle sizes have been determined by transmission electron microscope 共TEM兲 analysis 共Fig. 2兲. The average particle size of the aggregated nanocrystalline samples is estimated by considering the average of large number of particles and is found to be in close agreement with those obtained from XRD. The oxygen content 共␦兲 and Mn3+ / Mn4+ ratio of the samples of the present investigation have been estimated from the idometric titrations10 and the results are included in Table I. The results show that the oxygen content is overstoichiometric up to LNM-5, while it is less than the stoichiometric limit in LNM-6. Further, the Mn4+ is not found to vary systematically with increasing Na content. According to Malavasi et al.,5–8 the excess in oxygen leads to an increase in Mn4+. Therefore, it has been concluded that Mn4+ content might be more in the case of first five samples 共upto LNM-5兲 of the present investigation. B. Magnetic properties

The variation of magnetization with temperature has been studied both in ZFC and field cooled FC processes at 500 Oe and the behavior is shown in Fig. 3. The ZFC magnetization shows that all samples undergo a sharp FM to PM transition 共TC兲, which is determined by the minimum of dM / dT versus temperature curves and the values are tabulated in Table II. It can be seen from the table that TC values after an initial increase 共x = 0.10兲 are found to remain constant with increasing dopant concentration. The observed behavior may be explained by considering the fact that with increasing Na doping the ratio of Mn4+ / Mn3+ increases favoring the DE interaction. In the process, the number of ferromagnetically aligned domains supersedes paramagnetically aligned ones so that the percolation threshold is attained and the compound becomes FM, thereby enhancing the TC values initially in the high doping regime. As the concentration of of Mn4+ exceeds Mn3+, the SE contribution dominates over DE

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023707-3

J. Appl. Phys. 106, 023707 共2009兲

Kalyana Lakshmi, Venkataiah, and Reddy

TABLE I. Rietveld refined parameters of La1−xNaxMnO3 共x = 0.025, 0.05, 0.10, 0.15, 0.20, and 0.25兲 manganite ¯ c space group兲 at room temperature. system 共with R3 x Sample code a共Å兲 = b共Å兲 c 共Å兲 Volume 共Å3兲 O 共x兲 Mn–O–Mn 共deg兲 Mn–O 共Å兲 Mn3+% Mn4+% Mn valence 共Average valence兲

␦ Crystallite size D 共nm兲 R-factors RP RWP RF RBragg

0.025 LNM-1 5.510共5兲 13.338共8兲 350.72 0.462共6兲 168.0共7兲 1.951共9兲 69.7 30.3 3.303

0.05 LNM-2 5.518共8兲 13.358共4兲 352.36 0.450共7兲 163.5共7兲 1.964共7兲 57.5 42.5 3.425

0.10 LNM-3 5.498共7兲 13.334共3兲 349.08 0.454共5兲 165.4共6兲 1.953共8兲 73.6 26.4 3.264

0.15 LNM-4 5.484共8兲 13.319共4兲 346.88 0.456共8兲 166.1共6兲 1.948共8兲 58.6 41.8 3.430

0.20 LNM-5 5.486共6兲 13.318共3兲 347.11 0.458共4兲 166.4共5兲 1.948共7兲 55.0 45.0 3.450

0.25 LNM-6 5.484共8兲 13.317共4兲 346.94 0.452共1兲 164.49共9兲 1.951共2兲 53.6 46.4 3.464

0.126 54.5

0.162 45.1

0.032 53.5

0.059 49.5

0.025 55.2

−0.018 52.5

13.0 16.8 3.74 4.37

12.3 18.0 4.61 4.80

12.2 16.1 2.78 3.12

14.8 18.3 1.48 1.58

13.4 18.0 2.53 2.07

16.1 20.6 3.92 3.50

resulting in a small change in TC. In fact, a similar observation was reported earlier.11,12 Further, a competition between rhomohedral and orthorhombic phases also influences the transition temperatures. The rhombohedral perovskite structure is characterized by a weak Hund’s coupling favoring an increase in TC.4,13 In an orthorhombic symmetry transfer of eg electron is slow when compared with rhombohedral symmetry. The structural change from the rhombohedral to orthorhombic with higher doping may stimulate the corresponding effect thereby resulting constant TC values. It is also evident from the Fig. 3 that ZFC and FC curves do not coincide at low temperatures indicating the magnetic anisotropy13 and the separation increases with increasing sodium concentration. The observed behavior may be explained qualitatively as follows. The response of the spin to the external magnetic field depends on the competition between magnetocrystalline anisotropic energy and applied magnetic field strength. At low measuring fields, all the spins will not be oriented in the direction of the applied magnetic

field, giving a hint that the magnetic anisotropic field dominates over the applied magnetic field. The magnitude of M ZFC at low temperature below TC will depend on the anisotropy. Therefore, it may be concluded that the samples become highly anisotropic with increasing sodium concentration. The field dependent magnetization curves at 10 K are shown in Fig. 4. The saturation magnetization M S increases continuously up to x = 0.1 and decreases thereafter 共inset of Fig. 4兲. The observed behavior may be explained as follows: With increasing Na concentration the percentage of Mn4+ increases favoring AFM interactions within the FM clusters.11,14 Washburn et al.15 have analyzed such a magnetic behavior using two lattice models, one with FM interactions and the other one with AFM interactions. In the present investigation, in addition to Mn3+ – O – Mn4+ DE interaction 共leading to FM behavior兲, the Mn4+ – O – Mn4+ SE interaction 共AFM behavior, which brings down the FM interaction兲 is also present. Therefore, the observed decrease of M S beyond the samples x = 0.1 共LNM-3兲 is expected. C. Electrical and magnetoresistive properties

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

The variation of electrical resistivity with temperature in zero and 5 T fields is shown in Fig. 5 and the values of MIT temperature 共T P兲 are given in Table II. It is clear from the table that T P values after an initial increase are found to decrease thereafter with increasing doping concentration. Generally, the transport properties of manganites depend on the particle size, doping level, ratio of Mn3+ / Mn4+, oxygen content, etc., The observed increase in T P values in the case of first three samples may be explained by considering the fact that for every addition of Na+ ions, double the number of Mn4+ ions may create and contribute to the hopping process.16 Thus even for small amount of Na dopant concentration, enough number of holes are created in the eg band

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023707-4

J. Appl. Phys. 106, 023707 共2009兲

Kalyana Lakshmi, Venkataiah, and Reddy

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

and will contribute to the conductivity thereby increasing T P. With further increase in sodium concentration 共beyond LNM-3兲 the ratio of Mn3+ / Mn4+ reaches a stage wherein DE weakens slowly, which in turn enhances SE leading to continuous decrease of T P values. It can be observed from Fig. 5 that LNM-1 exhibits a sharp MIT, and with increasing sodium concentration the sharpness of the MIT peak is found to disappear gradually

and becomes broad as it reaches to LNM-5 sample. It is interesting to note that on decreasing the temperature, the samples LNM-4 and 5 are found to exhibit AFM behavior below 50 K, whereas in the case of LNM-6 sample, FM and AFM are coexisting. However, under the influence of magnetic field 共5 T兲 a broad MIT is observed. From this observation, it has been concluded that the sodium doped samples

TABLE II. Transition temperatures and the fraction 共m兲 of metallic regions obtained from the fit of EMA model 关Eq. 共1兲兴 and the best fit parameters obtained for the low temperature region using the Eq. 共5兲. Sample code LNM-1 LNM-2 LNM-3 LNM-4 LNM-5 LNM-6

TP 共K兲

TC 共K兲

m

␳0 共⍀ cm兲

␳1 共⍀ cm K−1储2兲

␳2 共 ⫻ 10−16兲 共⍀ cm K−2兲

␳5 共⍀ cm K−5兲

212 259 290 231 212 202

236 276 332 325 327 327

0.520 0.660 0.780 0.505 0.505 ¯

0.8319 0.0622 0.2360 0.1862 129.79 ¯

0.021 36 0.001 16 0.000 47 0.003 21 3.428 63 ¯

40.0 1.536 0.833 4.468 3440 ¯

3.4831⫻ 10−12 2.2707⫻ 10−12 4.5281⫻ 10−13 9.6872⫻ 10−12 2.8887⫻ 10−10 ¯

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023707-5

Kalyana Lakshmi, Venkataiah, and Reddy

J. Appl. Phys. 106, 023707 共2009兲

FIG. 4. 共Color online兲 Field dependence of the magnetization of La1−xNaxMnO3 共x = 0.025– 0.25兲 manganites measured at 10 K. The inset shows the variation of M S with doping.

are slowly transformed from CMR to CO behavior. A similar observation was reported earlier in the case of Nd1−xNaxMnO3 manganites.17,18 The application of magnetic field reduces the resistivity of the manganites, exhibiting interesting phenomena called magnetoresistance 共MR兲. The MR measurements of all the samples have been undertaken and the variation of MR with temperature is shown in Fig. 6. It is clear from the figure that

FIG. 6. 共Color online兲 The percentage of MR vs temperature of La1−xNaxMnO3 共x = 0.025– 0.25兲 manganites. The inset shows for the LNM-1, 2, and 3 samples.

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

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

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023707-6

tunneling across the grain boundaries.19 This contribution is more significant at low temperatures and low magnetic fields and produces a continuous increase in MR values as the temperature is lowered. The other contribution comes from the suppression of magnetic fluctuation as the field is increased. This process takes place within the volume of the grains and MR is dominant in the vicinity of the transition temperature. As the grain sizes of the samples of present investigation are found to be around 50 nm, surface and volume effects need to be taken into account. The highest MR over a wide range of temperature 共in the entire FM region兲 observed in the sample LNM-1 might be due to the combined effect of spin tunneling and the suppression of magnetic fluctuations. Such a large MR was reported in the case of Li doped LaMnO3 共Ref. 21兲 and Ca doped LaMnO3.22 From a practical viewpoint, the large MR over a wide temperature range observed in the sample LNM-1 is beneficial for the application in magnetoelectronic devices fabricated by using the CMR manganites. It has been concluded that the contribution from the spin fluctuations slowly decreases, and finally the mechanism due to spin tunneling across the grain boundary dominates with increasing Na concentration. Effective medium approximation method. Several authors23–25 attempted to explain the thermal variation of the electrical resistivity including the transition temperature region of CMR manganites in terms of phase separation models. Monte Carlo simulations of the DE model with JT coupling also suggest the phase segregation of FM metal 共FMM兲

␳=

J. Appl. Phys. 106, 023707 共2009兲

Kalyana Lakshmi, Venkataiah, and Reddy

from PM insulator regimes in doped manganites. As the CMR effect in doped manganites is proved to be larger involving both DE and strong coupling to local lattice deformations2,26 and as the polaronic distortion of PM state is thought to play over some temperature range in the FM phase. Jaime et al.25 proposed a phenomenological model by considering an effective medium approach 共EMA兲, where the total resistivity of doped perovskite manganite is supposed to be both due to band electrons and polarons assuming it as a two component model. In order to understand this apparent decoupling of the MI and PM-FM transitions, the ␳共T兲 has been analyzed using EMA. The EMA was originally conceived to explain the electrical conduction in composite materials.27,28 The classical limits of this theory were later invoked to study the electronic conduction in disordered materials, where hopping among localized states well below the mobility edge was the main interest.29 The basic premise of this approximation is that the material under consideration is assumed to be homogeneous, random mixture of two types of bonds with different conductances, and the conduction in the composite is essentially studied as a percolation problem. The conduction in such a bond percolation scenario is indeed decided by the strength of these two individual bonds as well as the ratio of these two bond fractions. For a typical three-dimensional case, the effective conductivity of the composite system is given by the expression,27,28

4 ␳ 1␳ 2 , 共3m − 1兲␳1 + 共2 − 3m兲␳2 + 兵关共3m − 1兲␳1 + 共2 − 3m兲␳2兴2 + 8␳1␳2其1/2

where ␳1 and ␳2 are the resistivity contributions of the two types of bonds and m is metallic fraction of the material.28 For instance, if we assume that ␳1 and ␳2 are metallic and nonmetallic bond contributions in a random mixture of such bonds, respectively, the EMA model predicts a transition between metallic and nonmetallic states across a percolation threshold pC of the metallic bonds. This value depends sensitively on the dimensionality of the system and for metallic conduction in a three-dimensional matrix, pC = 1 / 3. Around this value, the effective resistivity ␳ is crucial of the ratio 共␳1 / ␳2兲. Investigations of continuous random resistor network by Kirkpatrick,28 Eggarter and Cohen,29 and Frisch et al.30 have proved success of this EMA model in different contexts. A similar model has been adopted by Rao et al. to explain the resistivity data in La1−xCaxMnO3 manganites.31 In general, the CMR materials exhibit a distinct metallic behavior at low temperatures, while insulting behavior dominates at high temperatures with crossover at intermediate temperatures resulting in a MIT. To model the resistivity data of the samples using EMA, it is necessary to know the analytical expression for the temperature dependence of ␳1 and

共1兲

␳2. The conduction via metallic regions is assumed to be due to electron-magnon interaction, and their corresponding resistivity is given by ␳1 = ␳0 + ␳2.5T2.5 ,

共2兲

where term ␳0 arises due to the grain or domain boundary and term ␳2.5 represents the resistivity due to magnon scattering process. Similarly, the conduction via nonmetallic regions is assumed to be represented by the polaron hopping model,

␳2 = ␳0T exp共E P/kBT兲,

共3兲

where ␳0 = 关kB / ␯phNe2R2C共1 − C兲兴exp共2␣R兲, N is the number of ion sites per unit volume 共obtained from density data兲, R is the average intersite spacing, C is the fraction of sites occupied by polaron, ␣ is the electron wave function decay constant, ␯ph is the optical phonon frequency 共estimated from the relation h␯ph = kB␪D, ␪D is the Debye temperature兲, and E P is the sum of the activation energy required for the creation of carriers and activating the hopping of the carriers.

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023707-7

J. Appl. Phys. 106, 023707 共2009兲

Kalyana Lakshmi, Venkataiah, and Reddy

The resistivity data are fitted to Eq. 共1兲 for LNM-1 to LNM-5 samples 共Fig. 5兲, and the solid lines in the figure clearly indicate that the equation fits well. The value of m obtained from the fitting are listed in Table II. It is evident from the table that the metallic fraction value m increases up to LNM-3 and remains constant thereafter. Further, under the influence of external magnetic field T P shifts to high temperature and causes a decrease in resistivity. It means that the application of magnetic field melts a part of insulating fraction and converts into FM state, liberating its position of carriers and leading to the large enhancement of conductivity and as an evident CMR effect will appear. This prediction is quite reasonable, since the metallic component triggered by FM is also sensitive to the magnetic field, and the sizes of FM clusters grow as magnetic field is applied especially near the transition temperature. The parameters, ␳0, ␳2.5, and Eg are found to decrease with increasing magnetic field. This can be understood by considering the fact that under the influence of external magnetic field the PI regions change into FM regions more easily and suppresses the formation of polarons and spin-disorder scattering, leading to the monotonous decrease in Eg and also increasing m. A distinct low temperature minimum is observed in the electrical resistivity below 60 K. The depth of the minima is found to increase with increasing doping concentration, and the observed behavior may be explained by fitting the experimental data to an equation which results from the combined effect of weak localization, electron-electron and electronphonon scattering mechanisms, and given by

␳共T兲 = 兵1/共a + bT1/2兲其 + ␳2T2 + ␳5T5 ,

共4兲

where the term in the parentheses arises due to the weak localization effect,32 a is a temperature independent residual conductivity, and b is the diffusion constant. The other two terms, namely, ␳2T2 and ␳5T5, arise due to electron-electron and electron-phonon scattering, respectively.33 By expanding Eq. 共4兲 binomially,

␳共T兲 = ␳0 − ␳1T1/2 + ␳2T2 + ␳5T5 ,

共5兲

where ␳0 = 1 / a and ␳1 = b / a2 are constants. ␳共T兲 data up to LNM-5 fits well to Eq. 共5兲 共Fig. 7兲 both in the presence and absence of magnetic field, and the best fit parameters are given in Table II. In fact, Neeraj et al.34 have also used this model to explain the low temperature resistivity data of Pr2/3Ba1/3MnO3Ag2O composites. The fitting parameters 共Table II兲 increase with increasing dopant concentration and decrease with the application of magnetic field. This indicates that the weak localization, electron-electron and electron-phonon scattering process increase with Na concentration. D. Thermoelectric power 1. Low temperature behavior

The variation of thermoelectric power 共Seebeck coefficient S兲 with temperature in the temperature range of 80– 300 K is shown in Fig. 8共a兲. One may see from the figure that in the case of samples, LNM 1–3, S value remains positive throughout the temperature range of investigation,

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

whereas for the other samples it is found to change from positive to negative, with increasing temperature. The change in sign in the S共T兲 data of the samples indicates the coexistence of two types of carriers. The negative S at high temperature is attributed to the electrons which are excited from the valence band 共VB兲 into the conduction band 共CB兲. Because of the higher mobility of electrons within the CB, S is negative. At low temperatures, the electrons in the VB band are excited into the impurity band which generates holelike carriers, which is responsible for a positive S.35 The magnitude of S increases with increasing Na doping except in the case of LNM-1, and the observed behavior may be due to the fact that for every ion of Na doping, double the hole centers, which are localized and causes narrowing of eg band, distorting the Fermi surface.36 It is also interesting to note that all samples exhibit a peak in the low temperature region 共100– 120 K兲, in addition to the one generally observed at T P. Further, the second peak observed for samples ⬎LNM-4 might be due to the MIT. It was reported earlier36 that phonon drag 共Sg兲 and magnon drag 共Sm兲 contributions are present in the low temperature region. In the low temperature FM phase, a magnon drag effect is produced due to the presence of electron-magnon interaction, while the phonon drag is due to electron-phonon interaction. In general, the variation of S共T兲 of transitional metal oxides is analyzed by a relation of the form S = S0 + S3/2T3/2 + S4T4 ,

共6兲

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

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023707-8

J. Appl. Phys. 106, 023707 共2009兲

Kalyana Lakshmi, Venkataiah, and Reddy

FIG. 9. 共Color online兲 共a兲 Variation of magnon drag component with T3/2 and 共b兲 variation of phonon drag component with T3. The arrow marks represent the deviation of linear fit to experimental curve.

FIG. 8. 共Color online兲 共a兲 The temperature dependence of TEP from 80 to 300 K. 共b兲. The solid lines represents the fitting with Eq. 共7兲.

FM phase.36 The above equation was fitted to FMM part of the thermopower data and is found to fit well only below ⬃110 K. Therefore, it is clear that Eq. 共6兲 may not be suitable to account the entire FMM part of TEP. In view of these facts, Eq. 共6兲 has been modified by adding two more 共viz., phonon drag and diffusion兲 terms and the resulting equation is given by37 S = S0 + S1T + S3/2T3/2 + S3T3 + S4T4 .

共7兲

The solid curve in Fig. 8共b兲 represents the best fit of the experimental data up to 200 K and the best fit parameters are given in Table III.

In order to explain the origin of peak at low temperatures, the phonon and magnon drag contributions to the TEP were investigated using Eq. 共7兲. The variation of phonon drag component with T3 for all the samples is shown in Fig. 9共a兲. It is clear from the figure that the phonon drag contribution 共S3T3兲 is found to vary linearly up to a certain temperature and deviates below 250 K 共except in the case of LNM-6兲. As the phonon drag contribution deviates at 250 K, it has been concluded that the origin of the low temperature peak might not be due to the phonon drag effect. Therefore, the magnon drag component was calculated from Eq. 共7兲 and is shown in Fig. 9共b兲. It is interesting to note that the magnon drag component fits well below 200 K, indicating that the low temperature peak might have arisen due to magnon drag effect. 2. High temperature behavior

The charge carriers in the insulating region are not itinerant and the transport properties are governed by thermally

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

S0 共␮V K−1兲

S1 共␮V K−2兲

S3/2 共␮V K−5/2兲

S3 共␮V K−4兲

S4共 ⫻ 10−8兲 共␮V K−5兲

EP 共meV兲

ES 共meV兲

WH = E P − ES 共meV兲

␣

LNM-1 LNM-2 LNM-3 LNM-4 LNM-5 LNM-6

−79.695 −51.312 −22.213 −45.679 −85.888 −42.882

3.492 2.341 0.965 1.996 3.473 1.684

−0.3114 −0.2125 −0.0829 −0.1757 −0.2946 −0.1354

0.000 05 0.000 04 0.000 01 0.000 03 0.000 04 0.000 02

−10.70 −8.15 −2.24 −0.572 −8.07 −2.17

161.01 139.16 35.97 50.97 53.95 85.54

1.24 1.26 0.34 3.88 5.98 7.36

159.77 137.90 35.63 47.09 47.97 78.18

−0.047 −0.001 −0.005 −0.017 −0.027 −0.038

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023707-9

J. Appl. Phys. 106, 023707 共2009兲

Kalyana Lakshmi, Venkataiah, and Reddy

method. The resistivity data signify that the sodium doped samples are slowly transforming from CMR to CO behavior with increasing doping concentration. The low temperature upturn in the electrical resistivity was explained using the combined effect of weak localization, electron-electron, and electron-phonon scattering mechanisms. The broad hump observed in the low temperature TEP was attributed to magnon drag effect. Finally, it has also been concluded that, for a practical viewpoint, the large MR over a wide temperature range observed in the sample LNM-1 is beneficial for the application in magnetoelectronic devices. ACKNOWLEDGMENTS

FIG. 10. 共Color online兲 Variation of S vs 1 / T of LNM-4, 5, and 6 samples. The solid line represents the best fit to SPH model.

activated carriers because the effect of JT distortions in manganites results in strong electron-phonon coupling and hence the formation of polarons. Therefore, the thermoelectric power data of the present samples in the insulating regime are fitted to Mott’s polaron hopping equation, S共T兲 =

冋

册

kB ES +␣ , e k BT

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共8兲

where kB is the Boltzmann constant, e is the electronic charge, ES is the activation energy obtained from thermoelectric power data, and ␣ is a constant. In Eq. 共8兲, ␣ ⬍ 1 implies the applicability of small polaron hopping 共SPH兲 model, whereas ␣ ⬎ 2 indicates the large polaron hopping. The best fit curves of S versus 1 / T of the samples LNM-4, 5, and 7 are shown in Fig. 10, and the fitting parameters 共ES and ␣兲 are given in Table III. Using the activation energy values from ␳共T兲 plots 共E P兲 and those from S共T兲 plots 共ES兲, the polaron hopping energy values of all the samples have been calculated using the relation, WH = E P − ES, and are given in Table III. The E P values are found to be higher than those of ES. Such a large difference in the activation energy is the indication of the applicability of the SPH model in the insulating region.36 It can be observed from Table III that the hopping energy WH is increasing 共except in the case of LNM-3兲 with increasing Na concentration, thereby implying that polaronic radius might be decreasing. This is because polaron radius varies inversely as per the relation WH = e2/4␧共1/r P − 1/R兲,

The first author is grateful to the CSIR for providing fellowship. The DST, Government of India is acknowledged for funding 14T PPMS. The authors thank the Centre Director, Dr. A. Banerjee and Dr. R. Rawat of UGC-DAE CSR, Indore, India, for providing Low Temperature Magnetization and MR facilities. The authors thank Dr. G.V.N. Rao, ARCI, Hyderabad for providing XRD facilities. The authors also thank Dr. B. Sreedhar, IICT, Hyderabad, India for providing TEM facilities.

共9兲

where e is electronic charge, ␧ is the effective dielectric constant, r P is the polaron radius, and R is the mean spacing between the hopping sites, i.e., Mn3+ and Mn4+. Further, as the calculated values of ␣ are less than 1, it has been concluded that the SPH mechanism might be appropriate to explain the electrical resistivity as well as thermopower data in the high temperature regime. IV. CONCLUSION

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

2

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023707-10

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J. Appl. Phys. 106, 023707 共2009兲

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1967兲. P. Neeraj, D. K. Pandya, and S. K. Agarwal, J. Phys.: Condens. Matter 19, 456224 共2007兲. 35 J. Yang, Y. P. Sun, W. H. Song, and Y. P. Lee, J. Appl. Phys. 100, 123701 共2006兲. 36 S. Battacharya, S. Pal, A. Banerjee, H. D. Yang, and B. K. Chaudhuri, J. Chem. Phys. 119, 3972 共2003兲. 37 B. H. Kim, J. S. Kim, T. H. Park, D. S. Le, and Y. W. Park, J. Appl. Phys. 103, 113717 共2008兲. 34

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