Influence of rare-earth ion doping on magnetotransport behavior of potassium doped manganites

Influence of rare-earth ion doping on magnetotransport behavior of potassium doped manganites

Materials Chemistry and Physics 143 (2014) 983e990 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.e...

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Materials Chemistry and Physics 143 (2014) 983e990

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Influence of rare-earth ion doping on magnetotransport behavior of potassium doped manganites Y. Kalyana Lakshmi a, S. Manjunathrao b, P. Venugopal Reddy a, * a b

Department of Physics, University College of Science, Osmania University, Hyderabad 500 007, India Central Instruments Laboratory, University of Hyderabad, Hyderabad 500046, India

h i g h l i g h t s  The  The  The  The

magnetotransport properties of Ln0.67K0.33MnO3 manganites were investigated. lanthanum doped sample exhibit an appreciable MR% at room temperature. magnitude of thermopower increases with decrease of average cationic radii. small polaron hopping mechanism explains high temperature conductivity behavior.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 June 2013 Received in revised form 21 September 2013 Accepted 29 October 2013

The effect of A-site cationic radius on structural, electronic and magnetic properties of the perovskite manganites Ln0.67K0.33MnO3 (Ln ¼ La, Pr & Nd) have been investigated. The X-ray diffraction studies show a structural transformation from rhombohedral to orthorhombic with decreasing . The ferro to paramagnetic transition decreases from 280 to 110 K and the temperature dependent resistivity shows a metal to insulating transition at room temperature for La0.67K0.33MnO3 while insulating behavior was observed in the case of Pr and Nd substituted samples. Further, La0.67K0.33MnO3 sample exhibits room temperature magnetoresistance, and may be exploited for device applications. The sign of the thermopower is negative at 300 K and its magnitude increases with decreasing . The lanthanum based sample shows an upturn in the resistivity data at 30 K and the observed behavior may be attributed to the combined effect of weak localization, electroneelectron and electronephonon scattering. The paramagnetic insulating part of resistivity and thermopower data was analyzed using small polaron hopping mechanism. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Magnetic materials Crystal structure Rietveld analysis Electrical properties Phase transitions

1. Introduction Doped manganites with compositional formula, Ln1xAexMnO3, where Ln is a trivalent rare-earth cation and Ae is a divalent alkaline earth element have been extensively studied in recent years and are continued to be a very fascinating family of compounds in material science due to their attractive properties such as competing magnetic orders, metaleinsulator transition and colossal magnetoresistance (CMR) behavior [1e4]. These materials offer a high degree of chemical flexibility leading to complex interplay between structural, electronic and magnetic properties. The nature of charge, orbital and spin ordering is strongly influenced by various perturbations such as disorder effects at the rare-earth and transition metal sites, hydrostatic pressure, magnetic and electric field, * Corresponding author. Tel.: þ91 40 27682242; fax: þ91 40 27090020. E-mail address: [email protected] (P. Venugopal Reddy). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.10.033

and by particle size etc., [1e5]. The magnetic and electrical properties have traditionally been explained within the framework of double-exchange mechanism (DE), which considers the magnetic coupling between the neighboring Mn3þ and Mn4þ ions [6]. However, several studies have shown that Jahn-Teller (JT) distortion and grain boundary (GB) effects also play an important role in the magnetotransport properties [7]. On the other hand, doping Asite with cations of different sizes may produce a JT distortion of MnO6 octahedron, and hence the transport properties. For example, by changing the ionic radius of A-site, (keeping the holedoping x as a constant) of doped LnMnO3, MneOeMn bond angles may be changed. In view of this, one may understand that the average A-site radius is the key parameter and affects the magnetotransport properties of manganites. Several studies on the influence of lanthanum substitution by a divalent and trivalent element have been widely reported during the last decade [8e10]. In contrast, substitution of monovalent

Y. Kalyana Lakshmi et al. / Materials Chemistry and Physics 143 (2014) 983e990

3. Results and discussion 3.1. Structural The phase purity, structural and lattice parameters of the samples of present investigation were determined by powder XRD

2θ (Degree) 70

80

(240)

(224) 402) (240)

(114) (131) (204) (124)

(402)

(a) (224)

60

(220) (221) (114) (131) (204)

(202) (006)

50

(022) (220) (221)

PKMO

(121)

LKMO

40

(110) (104)

30

(112)

20

(021)

The polycrystalline Ln0.67K0.33MnO3 samples (Ln ¼ La, Pr and Nd) were prepared by using the solegel method. The analytical reagents of La2O3, Nd2O3, Pr6O11, KNO3 and freshly prepared MnCO3were used as starting materials. The constituent compounds were dissolved in dilute HNO3 solution and a suitable amount of citric acid was added as coordinate agent to generate a complete homogeneous transparent solution. On heating the solution, a gel was developed and it was thermally treated at 873 K to decompose the organic precursor. Finally, the powders were pressed into circular pellets and sintered at 1273 K in air for 3 h. Phase identification was performed using an X-ray powder diffractometer (XRD) (Rigaku Rotaflex RTC 300RC) with CuKa radiation. The data were analyzed by Rietveld method using FULLPROF program. Scanning electron microscopy (SEM) studies combined with energy dispersive X-ray spectroscopy (EDS) (HR FESEM: JEOL JSM-6700) were undertaken to investigate the morphology as well as the elemental analysis of the samples. Iodometric titrations using sodium thiosulphate and potassium iodide were performed to estimate Mn3þ/Mn4þ ratio and oxygen stoichiometry. Magnetization (M) vs. temperature (T) and applied magnetic field (H) were performed on a Vibrating Sample Magnetometer (Lake Shore model no 7460). The electrical resistivity and thermopower (S) measurements were performed by using two different equipments. The electrical resistivity and magnetoresistance measurements were carried out by a superconducting magnetic system of OXFORD in the fields between 0 and 8 T over a temperature range, 5e400 K using four-point probe method. Indigenous thermopower equipment with dynamic two probe differential method was employed for the measurement [15]. In this method, the samples were painted with silver paint and were kept between the two copper electrodes. The temperature of the samples was measured with copper-constantan thermocouple. The assembly was placed in a closed liquid nitrogen cryostat. Nitrogen atmosphere was used as an exchange gas mainly to maintain uniform temperature and to avoid condensation of moisture. The measurements were carried out in the heating mode. The absolute values of S were obtained by subtracting the experimental values with those of electrode material (copper). Lanthanum, Praseodymium and Neodymium doped samples are hereafter designated as LKMO, PKMO & NKMO respectively.

(012)

2. Experimental details

method and the diffraction patterns are shown in Fig. 1(a). A standard Rietveld refinement programme was employed to refine the crystal structure. It has been found that the structure of LKMO is rhombohedral with R-3c space group while PKMO and NKMO are having orthorhombic symmetry with Pnma space group. A typical plot of PKMO sample along with its Rietveld refined one, including the difference between the observed and calculated patterns is shown in Fig. 1(b). Excellent agreement between the calculated and the observed data clearly confirms the absence of secondary phase. The cell parameters obtained from the refinement are given in Table 1. The lattice parameters of the two orthorhombic samples are found to decrease and the observed behavior is expected because the substitution of a smaller cation at Ln site leads to decrease in the average A-site radius reducing the cell parameters. The average crystallite size () values were determined using the XRD peak width and Scherrer’s formula modified by Williamson and Hall [16]. The average crystallite size values are found to be in micrometer range. The representative SEM micrographs of all the samples are shown in Fig. 2. The grain sizes of the samples were estimated by considering the average of large number of grains and are found to be in close agreement with those obtained from the XRD. The quantitative analysis of constituent elements in the materials over many grains was examined by energy dispersive X-ray analysis and it has been found that the average concentrations of La, Pr, Nd, K and Mn elements are close to the nominal compositions. In order to determine the ratio of Mn3þ/Mn4þ concentration and oxygen content in the samples, iodometric titrations were carried out and the values of average Mn valency (Av) and oxygen stoichiometry (d)

(110)

elements such as Li, Na, K and Ag at rare-earth site results in converting Mn3þ ions into Mn4þ ions leading to the exhibition of metal to insulator and ferro to paramagnetic transitions at low dopant concentrations with minimum lattice distortions [11e13]. Further, it is known that thermoelectric power (TEP) is a useful and sensitive property to estimate the contributions from various scattering mechanism. In fact, TEP is sensitive to even slightest variations in the magnetic and electrical properties, which are not easily observable in magnetization and resistivity studies [14]. Therefore, it is thought that it would be interesting to study the influence of rare-earth substitution on magnetotransport behavior of monovalent doped manganites by keeping x constant. In view of this, a systematic investigation of magnetic, electrical and thermopower data of Ln0.67K0.33MnO3 (Ln ¼ La, Pr and Nd) manganites have been undertaken and the results are presented here.

Intensity (a.u.)

984

NKMO

(b) Y obs Y cal Y obs - Y cal Bragg position

PKMO

20

30

40

50

60

70

80

2θ (Degree) Fig. 1. (a) The X-ray diffraction patterns of Ln0.67K0.33MnO3 (Ln ¼ La, Pr and Nd) manganite system at room temperature (b) Rietveld refined pattern of Pr0.67K0.33MnO3 sample.

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Table 1 Rietveld refined parameters and the experimental data of Ln0.67K0.33MnO3 (Ln ¼ La, Pr and Nd) manganite system. Sample composition

La0.67K0.33MnO3

Pr0.67K0.33MnO3

Nd0.67K0.33MnO3

Sample code Structure Space group a ( A) b ( A) c ( A) Volume ( A3) ( A)

LKMO Rhombohedral R-3c 5.525 5.525 13.380 353.7 1.326 0.979 0.0252 173.66 e 1.949 e 0.13 3.60 303 280 2.89

PKMO Orthorhombic Pnma 5.466 7.733 5.488 232.0 1.301 0.970 0.0304 149.87 164.60 2.234 1.886 0.11 3.56 e 130 2.14

NKMO Orthorhombic Pnma 5.461 7.718 5.474 231.2 1.290 0.966 0.0330 147.85 158.16 2.008 2.029 0.13 3.60 e 110 1.82

s s2 ( A2)

MneO1eMn (o) MneO2eMn (o) MneO1 ( A) MneO2 ( A)

d Mn Valence (Av) TP (K) TC (K) m (mB/Mn)

are included in Table 1. It can be seen from the table that all the sample are found to be oxygen deficient and the average Mn valency is approximately constant. Generally, the oxygen deficiency is compensated by an equivalent change in Mn valence resulting in decrease of number of Mn4þ sites [17]. It is clear from the table that d is same for all the samples and this inturn results in maintaining the Mn3þ/Mn4þ constant. Therefore, in the samples of the present investigation the average Mn valence remain almost constant. 3.2. Magnetic properties Plots of magnetization (M) vs. temperature (T) of all the three samples measured in the field cooled (FC) mode in an applied magnetic field of 500 Oe are shown in Fig. 3(a). It is clear from the plots that the magnetization values are found to decrease with increasing temperature exhibiting a sudden change of slope at their respective para to ferromagnetic transition temperatures (TC). TC values (Table 1) are found to decrease from 280 K to 110 K with the substitution of La, Pr and Nd ions and the observed behavior may be explained as outlined below. It is well known that the magnetic and electrical properties of manganites are governed by various factors such as average A-site ionic radii (), charge density and size variance parameter (s2). In the present investigation as the charge density remain constant the parameters which influence the magnetic and electric behavior might be and s2. With the substitution of a trivalent element with smaller ionic radii at Ln site, the average A-site cation radius decreases thereby enhancing its size variance parameter (Table 1) and hence affects the MneOeMn angle and average MneO distance (Table 1). The A-site cation size disorder may result in random displacement of oxygen ions from their average crystallographic positions causing local distortion in MnO6 octahedra. The increasing value of s2 with decreasing causes localization of eg electrons, which in turn prevents in longrange ferromagnetic ordering leading to decrease in TC values. In fact, the observed decrease of TC with s2 is consistent with earlier reports [18,19]. The isothermal MeH curves at T ¼ 80 K after cooling the sample in zero field are shown in Fig. 3(b) and it is clear that all the samples of the series exhibit a soft ferromagnetic behavior. The magnetic moments (m) values were calculated at H ¼ 1.5 T and are given in Table 2. One can notice from the table that the m values decrease with decreasing and this suggest the enhancement of

Fig. 2. SEM micrographs of Ln0.67K0.33MnO3 system (a) LKMO (b) PKMO and (c) NKMO.

antiferromagnetic (AFM) interaction in the system. It is clear that increasing s2 results in the random distribution of hopping of conduction electrons as well as an exchange between localized spins, thus introducing a random disorder in the magnetic lattice of the system. As a result the ferromagnetic interaction decreases and antiferromagnetic interaction increase between the Mn ions thereby causing m values to decrease [20]. 3.3. Electric behavior and magnetoresistance The variation of electrical resistivity (r) with temperature of the samples in different magnetic fields (H ¼ 0e8 T) is shown in Fig. 4(aec). It is clear from figure that the resistivity of LKMO increases with increasing temperature exhibiting a metal to insulator transition (TP) at 303 K. On the other hand, Pr and Nd doped samples are found to exhibit a large value of resistivity of about 106 U-cm exhibiting only insulating behavior in the entire temperature range of measurements (100e300 K). Moreover, both these samples remain insulating even after the application of 8 T

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and resistivity decreases which inturn results in exhibiting magnetoresistance (MR) [22]. The MR% values of all the samples were calculated using the relation,

T (K) 50 25

100

150

200

250

300

350

LKMO PKMO NKMO

20

MR% ¼ ðr0  rH =r0 Þ  100; where the rH and r0 are the resistivity measured in the presence and absence of magnetic fields respectively. The variation of percentage of magnetoresistance with temperature of the samples of present investigation is shown in Fig. 4(d, e). It can be seen from the figure that LKMO sample exhibits maximum MR not only in the vicinity of TP but also at a temperature much below TP, whereas PKMO and NKMO samples show significant MR% at low temperatures. The maximum MR observed in the vicinity of TP may be attributed to the suppression of spin fluctuations in a domain by aligning them in the field direction. However, the maximum MR observed in the low temperature ferromagnetic region, may be attributed to the inter-grain spin polarized tunneling (ISPT) of charge carriers across the grain boundaries [23]. From a practical view-point, the room temperature MR exhibited by LKMO may be exploited in magnetoelectronic devices.

15 10

M (emu/g)

5 0 100

LKMO PKMO NKMO

80 60 40 20

3.4. Thermopower studies

0 -20 -40 -60

T = 80K

-80 -100

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

H (T) Fig. 3. The plots of (a) Temperature dependence of Magnetization in an applied magnetic field of 500 Oe (b) MeH behavior at T ¼ 80 K of Ln0.67K0.33MnO3 (Ln ¼ La, Pr and Nd) manganites.

magnetic field. The observed behavior may be explained on the basis of lattice effects. When rare-earth elements with smaller ionic radii such as Pr (1.179  A) and Nd (1.163  A) are substituted at Ln site, the value of decreases resulting in moving oxygen towards the center of the cube, thereby distorting the lattice and deviating MneOeMn bond angle (q). This decrease in q provides a local trap for eg electrons causing possible phase or domain separation. The effective charge transfer (tij ¼ tij cos(q/2)) between the neighboring Mn sites of the local t2g spins decreases with decreasing q. Consequently, the charge localization increases due to reduction in the mobility of charge carriers and as a result, the samples exhibit a high resistivity along with insulting behavior [21]. One can also notice from reT plots that the resistivity of LKMO is found to decrease with increasing magnetic field and TP shifts to high temperature side. The decrease of resistivity may be due to delocalization of charge carriers and ordering of magnetic spins in the presence of external magnetic field. Due to this ordering, the ferromagnetic metallic (FMM) state might have suppressed the paramagnetic insulating (PMI) regime. On the application of magnetic field the conduction electrons are completely polarized inside the magnetic domains and are easily transferred between Mn3þ and Mn4þ ions thereby shifting TP towards higher temperature side

The variation of thermopower with temperature is shown in Fig. 5. It can be seen from the figure that the samples PKMO and NKMO have a similar behavior. At T ¼ 300 K, the magnitude of S is found to be small and negative and increases with decreasing temperature and reaches a maximum, value hereafter designated as TS (Table 2) and decreases thereafter on further lowering the temperature. In fact a similar behavior was reported in the case of Pr doped divalent manganites [15]. The TS values are close to their respective magnetic transition temperature (TC), On the other hand, in the case of LKMO apart from exhibiting a peak at TS, a broad peak at T z 120 K was also observed. The increase in S value at TS reflects sudden change of spin entropy due to enhancement of spin polarization caused by magnetic transition [24]. It is also clear from S vs. T plots that the sign is negative at room temperature and as the temperature is decreased it becomes positive, indicating that both the electrons and holes contribute to thermopower. Further, the magnitude of S is found to increase with decreasing and the observed variation is consistent with the resistivity data of these samples. In fact, such large S values were also observed in SmeSre MnO3 manganites [20]. The observed variation in thermopower data with decreasing suggests that the strong electronic modifications induced by the size effect which are correlated to overlapping of atomic MneO orbitals. Further, it has also been reported that the effects of the Coulumbian localization play an important role in determining the magnitude of thermopower values [20]. In view of this, one may conclude that thermopower is sensitive to cationic size of the samples. 3.5. Conduction mechanism 3.5.1. Resistivity and thermoelectric power in the low temperature region (T < TC) As discussed in Section 3.3, PKMO and NKMO samples are not only found to exhibit insulating behavior but also remain as

Table 2 The best fit parameters obtained from thermopower data of Ln0.67K0.33MnO3 (Ln ¼ La, Pr and Nd) manganites. Sample code LKMO PKMO NKMO

S0 (mVK1) 13.888 435.455 7079.12

S1 (mVK2) 0.4463 29.2890 407.4560

S3/2 (mVK5/2) 0.0341 3.2890 42.3798

S3 (mVK4)

S4 (mVK5) 6

3.91  10 11.5  103 11.8  103

9

6.07  10 3.38  106 30.0  106

TS (K)

Er (meV)

ES (meV)

a0

270 150 130

123.25 167.60 180.40

2.24 45.02 81.80

0.1398 2.8459 5.1788

Y. Kalyana Lakshmi et al. / Materials Chemistry and Physics 143 (2014) 983e990

987

Temperature (K) 0

100

200

0.08 H= = = =

0.06

300

400 LKMO

0

100

200

300

400

0T 3T 5T 8T

50 40 30

0.04

20

(a)

10

(d)

0 80

6

10

PKMO

5

10

H=0T =4T =8T

Resistivity (Ω cm)

4

10

3

10

60 40

2

10

1

20

10

0

10

(b)

(e)

0

-1

10 7 10

100

200

300

100

200

300

NKMO

6

10

5

60

H=0T =4T =8T

10

4

10

3

10

Percentage of magnetoresitance

0.02

60

40 20

2

10

1

10

(c)

(f)

0

0

10

100

200

300

100

200

300

Temperature (K) Fig. 4. (aec) The temperature dependence of resistivity at different magnetic fields for Ln0.67K0.33MnO3 (Ln ¼ La, Pr and Nd) samples. (def) Percentage of magnetoresistance vs. temperature plots of LKMO, PKMO and NKMO samples with different magnetic fields upto 8 T.

insulators even after the application of 8 T magnetic field. In contrast, LKMO sample show a clear metal to insulator transition at 303 K followed by a resistivity minima (rmin) in the low temperature ferromagnetic region. Further, one may also notice that the resistivity of this sample decreases and the resistivity minimum shifts towards low temperature side with increasing magnetic field indicating that it is sensitive to the external field. Considerable efforts were made by many research groups to explain the resistivity minima by considering weak localization effects of electrons, electroneelectron interactions, inelastic scattering effects and uncontrolled magnetic impurities, etc. The conductivity data above low temperature minima (T < TP, metallic region) is dominated by electroneelectron scattering (T2 dependency) and electron-magnon scattering (T4.5). In order to understand the conduction mechanism in the metallic regime (below TP including resistivity minima) exhibited by LKMO sample, the electrical resistivity data were analyzed using the following equation,

rðTÞ ¼ r0 þ re T 1=2  rs lnT þ rp T 5 þ r2 T 2 þ r4:5 T 4:5

(1)

where the term r0 represents resistivity due to grain/domain boundary effects [25e27], as the polycrystalline materials contain grains, grain boundaries and their contribution to the resistivity is

proved in microwave measurements and hence term r0 plays an important r role in the conduction process [28]. On the other hand, the second term arises due to contributions from the correlated electroneelectron (eee) interactions [28], the third may be attributed to the Kondo-like spin-dependent scattering [29], the fourth term is due to electronephonon (eep) interaction [30], the fifth term indicates the electroneelectron scattering [27] and the last term is attributed to the electron-magnon scattering process in the ferromagnetic phase [27]. The parameters obtained using the above equation both in the presence and absence of magnetic field are given in Table 3 and the corresponding plots are shown in Fig. 6(a). It can be seen from the figure that the theoretical curve (solid lines) coincides well with the experimental data indicating that the above equation explains the conduction mechanism in the low temperature region satisfactorily. Further, one may observe from the fitting parameters are found to decrease with increasing magnetic field. The behavior may be attributed to the fact that with increasing magnetic field, the domain becomes large thereby reducing the value of r0, while the reduction in the other parameters is due to decrease in the electron spin fluctuations in the presence of magnetic field. Intrinsically, the scattering effects are suppressed due to parallel configuration of the spins present in the domain leading to the decrease of all the contributing parameters [22,25].

988

Y. Kalyana Lakshmi et al. / Materials Chemistry and Physics 143 (2014) 983e990

-1

0.8

100

200

-1

T (K )

T(K) 300

0.004 0.006 0.008 0.010

(a)

LKMO

(d)

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0

100 PKMO

(b)

(e)

80

60

60

40

40

20

20

0

S (μV/K)

S (μV/K)

80

0

-20

100 160 NKMO

200

300

0.003

0.006

0.009

0.012

(c)

(f)

160

120

120

80

80

40

40

0

0 100

200

300

0.003

0.006 -1

T (K)

0.009

0.012

-1

T (K )

Fig. 5. (aec) The variation of thermopower (S) with temperature (T) from 80 to 300 K for Ln0.67K0.33MnO3(Ln ¼ La, Pr and Nd) samples. The arrow in the figure indicates their corresponding TS values. The solid line represent the fittings using an eqn. S ¼ S0 þ S1T þ S3/2T3/2 þ S3T3 þ S4T4. (def) The plots of S vs. T1and the solid lines represent the fitting curves with small polaron hopping mechanism.

An effort has also been made to analyze thermopower data in the ferromagnetic region using an empirical equation containing spin wave and lattice vibration terms [31],

S ¼ S0 þ S1 T þ S3=2 T 3=2 þ S3 T 3 þ S4 T 4

(2)

where S0 accounts for the problem of truncating the low temperature data, S1T corresponds to the diffusion term, S3/2T3/2 represents the spin wave (magnon drag) contribution, S3T3 corresponds to the phonon drag and S4T4 term represents spin fluctuations in ferromagnetic phase [32]. The experimental data were fitted to eq. (2) and it has been observed that the equation fits well with the data in the ferromagnetic region and is shown in Fig. 5(aec). One may also observe that S3/2 values are much larger than those of S3 suggesting that electron-magnon scattering might be dominating in the low temperature region.

3.5.2. Resistivity and thermoelectric power in the high temperature region (T > TC) The formation of lattice polarons in manganites due to strong electronephonon interaction is a well known phenomenon. In the case of adiabatic small polaron hopping mechanism (SPH), the transport is governed by thermally activated hopping of charge carriers. The r (T) behavior due to small polaron can be best fitted to an equation,



rðTÞ ¼ ra Texp Er =kB T ;

(3)

where Er is the activation energy, ra ¼ 2kB/3ne2a2v, here kB is Boltzmann’s constant, e is the electronic charge, a is site - to - site hopping distance and v is the longitudinal optical phonon frequency. The activation energy values (Er) calculated from the bestfit parameters are given in Table 2 and the corresponding best-fit

Table 3 Resistivity fitting parameters of La0.67K0.33MnO3 sample below TP at H ¼ 0, 3, 5 & 8 T. H (T)

0T

3T

5T

8T

r0 (U cm) re (U cm K1/2) rs (U cm) rp (U cm K5) r2 (U cm K2) r4.5 (U cm K4.5)

0.01531 570.00  106 0.00037 2.207  1015 7.551  107 1.847  1013

0.00973 30.00  106 0.00014 19.15  1015 5.374  107 2.401  1013

0.00903 20.00  106 0.00014 14.07  1015 4.934  107 1.487  1013

0.00818 7.82  106 0.00014 8.522  1015 4.357  107 5.349  1013

Y. Kalyana Lakshmi et al. / Materials Chemistry and Physics 143 (2014) 983e990

Nd ions at Ln site decreases the average A-site ionic radii resulting in bending of MneOeMn bond angle which inturn might have narrowed down the bandwidth enhancing the effective mass of the charge carrier. Due to this, the effective band gap increases with decreasing . Therefore, higher energies are needed for the charge carries to overcome this band gap. The difference in activation energies obtained from the resistivity and thermopower data reflects that the charge transport occurs due to the hopping of carriers. Further, it is also clear from the table that the calculated values of a0 are less than unity and the result strongly supports the validity of using small polaron hopping mechanism to explain the electrical resistivity as well as the thermopower data of these materials in the high temperature regime.

T (K) 0

100

200

300

400

0.08

(a)

LKMO

ρ (Ω cm)

0.06 0.04 0.02

H=0T H=8T

0.00

(b)

LKMO

-8.4

4. Conclusions The influence of average cationic radii () on the structural, magnetic and electric properties of Ln0.67K0.33MnO3 (Ln ¼ La, Pr and Nd) were investigated by maintaining the charge carrier density constant. The ferro to paramagnetic transition temperature shifts to low temperature side and the metal to insulator transition disappear exhibiting insulating behavior with the substitution of smaller rare earth ion at Ln site. The LKMO sample exhibit maximum MR% both at low temperature and in the vicinity of TP whereas PKMO and NKMO samples show significant MR% at low temperatures only. The room temperature MR% observed in the sample LKMO is beneficial for the magnetoelectronic devices. The thermopower vs. temperature behavior exhibits a maximum in the vicinity of their respective transition temperatures and its magnitude increase with the decrease of . S (T) data in the low temperature region may be explained by considering diffusion, phonon drag and magnon drag terms. The resistivity and thermopower data in the high temperature region are explained within the framework of small polaron hopping mechanism.

-8.8

H=0T H=8T

ln (ρ / T)

-9.2 0 .0 0 2 4

0.0 0 2 7

0 .00 3 0

0.0 0 3 3

0.00 3 6

(c)

PKMO NKMO

8 4 0 -4 -8

0.003

0.006

T

-1

0.009

(K

-1

Acknowledgments

0.012

)

Fig. 6. (a) The variation of electrical resistivity (r) vs. temperature (T) for La0.67K0.33MnO3 manganite at H ¼ 0 and 8 T in the metallic region (
plots (ln r/T vs. 1/T) are shown in Fig. 6(b). It is clear from the table that Er values are found to increase with decreasing . According to adiabatic SPH mechanism the thermopower can be expressed as,

S ¼ kB =e ½ES =kB T þ a0 

989

(4)

where, ES is the activation energy obtained from TEP data. a΄ is a constant of proportionality between the heat transfer and the kinetic energy of an electron and may be used to ascertain the type of polarons participating in conduction process. For example, if a0 < 1 the charge carriers responsible for thermopower might be due to small polarons, while if a0 > 2, they are large polarons [33,34]. From the linear fit of the curves the activation energies for thermopower (ES) were determined and are given in Table 2, while S vs. 1/T plots are shown in Fig. 5(d, e). It is clear from the table that ES and Er values are found to increase with the substitution of smaller rareearth ion. The enhancement in the values of both the activation energies may be attributed to the fact that due to doping of Pr and

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