Magnetic, electric and thermoelectric behavior of electron-doped La1−xSbxMnO3 (x = 0.05, 0.10 and 0.15) manganites

Journal of Alloys and Compounds 562 (2013) 128–133

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Magnetic, electric and thermoelectric behavior of electron-doped La1xSbxMnO3 (x = 0.05, 0.10 and 0.15) manganites G. Venkataiah a, J.C.A. Huang b, P. Venugopal Reddy a,⇑ a b

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

a r t i c l e

i n f o

Article history: Received 11 December 2012 Received in revised form 28 January 2013 Accepted 29 January 2013 Available online 16 February 2013 Keywords: Manganite Electron-doped Magnon Small polaron Thermoelectric power

a b s t r a c t A systematic investigation of magnetic, electric and thermoelectric properties of sol–gel prepared electron-doped La1xSbxMnO3 (x = 0.05, 0.10 and 0.15) was undertaken to understand the magnetotransport behavior. The X-ray diffraction and X-ray photoelectron spectroscopy measurements confirm that the samples with x = 0.05 and 0.10 are having single phase, while x = 0.15 shows a secondary phase. The second phase observed in the sample is found to have considerable influence on the magnetic and electrical transport behavior. The magnetic and electrical transport properties in low temperature ferromagnetic region may be explained in terms of spin waves and phonons, while in the paramagnetic region the adiabatic small polaron model is invoked. These materials are found to exhibit magnetoresistance values ranging from 30% to 40% and 55% to 65% at 3 and 7 T magnetic fields respectively. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Manganite perovskites with general formula, R1xAxMnO3 (R = trivalent rare-earth ion and A = divalent alkaline earth ion) have been the subject of intense research in materials science for the last one and half decades due to their promising future applications in spintronic devices [1]. The parent compound, RMnO3 is an anti-ferromagnetic insulator. When the divalent ions are doped at R site, an equal amount of Mn3+ ions convert into Mn4+ thereby creating a hole in the unit cell and materials are referred as holedoped manganites [2]. The hole-doped manganites possess an inherent property of colossal magnetoresistance (CMR) in the vicinity of metal–insulator/ferro-paramagnetic transition temperature (TMI/TC) [3]. On the other hand, partial substitution of tetravalent ion such as Ce4+, Te4+ at R site in RMnO3 results in an electrondoped manganite by converting an equal number Mn3+ ions into Mn2+ by introducing an electron per unit cell [4–9]. In the same way, new class of electron-doped manganites are realized by doping with a pentavalent ion at R site and as a result double the number of Mn3+ ions convert into Mn2+, and may be considered as counterparts of alkali ion doped RMnO3 manganites [10–12]. In a manner similar to Mn3+ and Mn4+ ions in hole-doped manganites, Mn3+ and Mn2+ ions play a vital role in electron-doped manganites [4]. Therefore, the study of the electron-doped manganites is an ⇑ Corresponding author. Tel.: +91 40 27682242; fax: +91 40 27009002. E-mail addresses: [email protected] (G. Venkataiah), [email protected] (P. Venugopal Reddy). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.01.201

essential and important so as to use them for possible applications in spintronics. In the present investigation, an attempt has been made to address the magnetic, electrical and thermoelectric properties of sol–gel synthesized Sb doped LaMnO3 manganite system. To avoid the ambiguity of phase purity and faulty homogeneity, sol–gel synthesis method has been adopted. In order to understand the mechanism responsible for the magnetotransport behavior of these materials, the magnetization and electrical resistivity data were analyzed both in the ferro and paramagnetic regions by spin wave and adiabatic small polaron mechanisms respectively. It is known that thermoelectric power (TEP) is a useful and sensitive property to estimate the contributions from various scattering mechanism so as to understand the transport mechanism in polycrystalline materials. In fact, TEP is sensitive to even the slightest variations in the magnetic and electrical properties, which are not easily observable in magnetization and resistivity studies [13]. Therefore, a systematic investigation of magnetic, electrical and TEP data of Sb doped LaMnO3 manganites have been undertaken and the results are presented.

2. Experimental Polycrystalline materials with the compositional formula, La1xSbxMnO3 (x = 0.05, 0.10 and 0.15) were prepared by citrate based sol–gel method [14]. The synthesized powders were calcined at 1000 °C followed by sintering at 1200 °C in air for 4 h. The X-ray diffraction (XRD) measurements were carried out on a Philips (x’pert) diffractometer with Cu Ka radiation at room temperature and the data were analyzed using DBWS Rietveld refinement technique [15]. The grain size and sur-

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G. Venkataiah et al. / Journal of Alloys and Compounds 562 (2013) 128–133 face morphology of the samples were estimated using field emission scanning electron microscope (JSM 6700F) equipped with energy dispersive X-ray analysis. X-ray photoelectron spectroscopy measurements were carried out by using ESCA scanning microprobe (PHI 5000 Versa Probe) to analyze the valence state of Sb. The electrical resistivity data were collected in the temperature range 60–300 K at different magnetic fields (0–7 T). The magnetic properties were measured by a SQUID magnetometer over a temperature range, 5–300 K. Thermoelectric power measurements were carried out by a two-probe differential method over a temperature range, 77–300 K. The oxygen stoichiometry present in the samples was estimated from iodometric titration technique [16].

3. Results and discussion 3.1. Structural studies

Intensity (arb. units)

Intensity

Fig. 1 shows the XRD patterns of La1xSbxMnO3 (x = 0.05, 0.10 and 0.15) samples. It is clear from the patterns that x = 0.05 and 0.10 samples are showing single phase, while the sample with x = 0.15 shows a minute impurity phase and the impurity peak is located at 2h = 29.7°(shown in inset Fig. 1), which coincides with the crystalline phase of Sb2O4 (with ICDD code: 00-032-0042). It indicates that the concentration of Sb in x = 0.15 sample might be exceeding the solubility limit of Sb2O3 in LaMnO3 manganite and in fact similar type of problems arose earlier in other electron-doped manganites [4,6,7]. Further, XRD data of all the samples were analyzed by Rietveld refinement program and the refined cell parameters are given in Table 1. The refinement was done by assuming rhombohedral structure with R-3c space group. The cell parameters are found to decrease with increasing Sb doping concentration and the observed behavior may be attributed to the substitution of smaller Sb ion (0.76 Å) replacing a larger ion La (1.032 Å) by constricting the unit cell [17]. However, the change in lattice parameters for x = 0.15 is minimal compared to that observed in the case of samples with x = 0.05 and x = 0.10. The observed behavior may be attributed to the inadequacy of LaMnO3 lattice to accommodate excess Sb beyond its solubility

x = 0.05

limit, resulting in the formation of an energetically favorable impurity phase of Sb2O4. The surface morphological investigations were carried out by scanning electron microscope (SEM) and Fig. 2 shows micrographs of x = 0.10 and 0.15 samples. One may observe from the figures that the grain size and surface morphology are reasonably uniform in the case of x = 0.10 sample, while in the case of x = 0.15 they are irregularly coagulated with different sizes. The irregularity in the grain sizes might be due to Sb2O4 phase and the present results are in agreement with Sb doped rutile TiO2 materials in which the effect of second phase was studied by Morita et al. [18]. The quantitative analysis of the constituent elements was undertaken by energy dispersive X-ray analysis and the elements present in the composition are in agreement with those in nominal compositions. The oxygen stoichiometry (d) of the samples determined from iodometric titrations are given in Table 1. It can be seen from the table that the samples with x = 0.05 and 0.10 are in agreement with those of nominal compositions (within the experimental error, ±0.05), while the sample with x = 0.15 is exhibiting oxygen over stoichiometry. According to Malavasi [19], the excess oxygen

∗

28

30

32 2θ (Degree)

x = 0.10

x = 0.15

20

40 60 2θ (Degree)

80

Fig. 1. X-ray diffraction patterns of La1xSbxMnO3 (x = 0.05, 0.10 and 0.15) samples. The inset figure shows enlarged image.

Fig. 2. Scanning electron microscopy pictures of La1xSbxMnO3 with x = 0.10 and 0.15.

Table 1 Experimental data of La1xSbxMnO3 (x = 0.05, 0.10 and 0.15) manganites. Sample

x = 0.05 x = 0.10 x = 0.15

Unit cell parameters a = b (Å)

c (Å)

V (Å3)

5.5377 5.5283 5.5251

13.3742 13.3675 13.3608

355.19 353.85 353.22

d

TC (K)

TMI (K)

TS (K)

Eq (meV)

ES (meV)

WH (meV)

3.00 3.05 3.19

161 226 216

210 252 227

210 252 227

198.45 166.32 169.60

2.11 0.64 1.44

196.34 165.68 168.16

G. Venkataiah et al. / Journal of Alloys and Compounds 562 (2013) 128–133

may cause the lattice defects by creating cation vacancies in the sample. Therefore, the over oxygen stoichiometry present in x = 0.15 sample may cause the lattice defects by forming the secondary phase. Fig. 3 shows Sb 3d core level spectra of all the three samples. The spectra consist of two peaks, Sb3d5/2 and Sb3d3/2 located around 530 and 539 eV respectively. As the core level spectrum of oxygen and Sb3d5/2 are overlapped, it is very difficult to approximate the valence state of Sb. However, by analyzing 3d3/2 core level spectrum the valance state of Sb has been evaluated. In the case of x = 0.05 and 0.10 samples, 3d3/2 peak is located at 539.7 eV, while that in the case of x = 0.15, is at 539.1 eV. According to the reference tables [20], the positions of Sb2O5 and Sb2O3 in 3d3/2 are at 539.8 and 539 eV respectively. In the present investigation, the XPS spectra of 3d3/2 for x = 0.05 and 0.10 samples are in agreement with 3d3/2 of Sb2O5, while for x = 0.15 it is close to spectra of Sb2O3 within the experimental error (<± 0.1 eV). This analysis indicates that valence state of Sb for the samples with x = 0.05 and 0.10 is 5+, while it may be 3+ or the average of 3+ and 5+(i.e., 4+) in the case of x = 0.15 sample. Indeed this result supports the secondary phase observed in XRD and SEM studies. 3.2. Magnetic behavior Fig. 4a shows the ferromagnetic hysteresis (M–H) loops of La1(x = 0.05, 0.10 and 0.15) samples measured at 5 K. The coercivity (HC) values are found to be 50, 20, and 161 Oe for x = 0.05, 0.10 and 0.15 samples respectively (shown in inset of Fig. 4a). It is interesting to note that HC value observed in the case xSbxMnO3

x = 0.05 Intensity (arb.units)

x = 0.10 x = 0.15

525

530 535 540 Binding energy (eV)

545

Fig. 3. X-ray photoelectron spectrum of La1xSbxMnO3 (x = 0.05, 0.10 and 0.15) samples.

75 50

(b)

x = 0.05 x = 0.10 x = 0.15

25 0 -25 -50 -75

M (emu/g)

Magnetization (emu/g)

(a)

of x = 0.15 samples is relatively very high when compared with the other two samples, indicating the presence of large potential barrier between the magnetic domains to align them in the field direction. It means that the domain walls can be either pinned or strained by impurity material surfaces, which arises due to magnetoelastic coupling [21]. The magnetic moment per Mn ion values have been computed at 5 K and shown in Fig. 4b. It is interesting again to notice from the figure that the magnetic moment per Mn values for x = 0.15 sample are small compared to other two samples and are found to increase with increasing magnetic field without attaining saturation. This indicates that the non-magnetic clusters present in the domain boundary might have influenced the magnetic behavior of x = 0.15 sample. Further, in order to compare the experimentally determined magnetic moment per Mn ion values with theoretical values, the magnetic moment per Mn ion values were estimated by l = g[S(S + 1)]1/2lB with g = 2. Here, Mn2+ and Mn3+ were considered to evaluate the spin moment by assuming that the orbital magnetic moment contribution is quenched. The calculated magnetic moment values for x = 0.05, 0.10 and 0.15 samples are 5.0, 5.1 and 5.2 lB respectively. The experimentally determined values are slightly lower than those of calculated ones in the case of x = 0.05 and 0.10 samples, while in the case of x = 0.15 sample the difference is large. The plausible reason for observed difference in experimental and calculated magnetic moments might be due to non-collinear magnetic domains in the case of x = 0.05 and 0.10 samples and for x = 0.15 sample it may be due to non-magnetic Sb-oxide secondary phase. In order to understand more clearly about the magnetic behavior of the samples, zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements were also carried out over a temperature range 5–300 K and the plots are shown in Fig. 5. The TC values were determined from the inflection points of dM/dT versus temperature of ZFC curves and are given in Table 1. The TC values are found to be higher than those of TMI and the observation is quite common in sol–gel prepared manganites [13,14,22,23]. It can be seen from the figure that all the samples are found to show bifurcation between FC and ZFC curves in the ferromagnetic region. In the case of x = 0.05 and 0.15 samples bifurcation is large while the sample, x = 0.15 exhibits a cusp at 122 K in ZFC magnetization curve. All the three samples are found to exhibit thermo-magnetic irreversibility close to ferro-paramagnetic transition temperature (TC). The large difference observed between FC and ZFC in the case of x = 0.05 might be due to the short range ferromagnetic clusters due to the presence of low Sb ionic concentration at La site. In the case of x = 0.15 sample, Sb oxide (Sb2O4) impurity phase responsible for clusters like behavior, is clearly indicated by the cusp in the ZFC curve. The non vanishing cusp like signatures in ZFC curve shows the non-magnetic nature of the impurity phase.

3 0 -3 -150 0 150

H (Oe)

-5000 -2500 0 2500 Magnetic Field (Oe)

5000

Magnetic moment (μB/Mn)

130

3

2 x = 0.05 1

x = 0.10 x = 0.15

0

0

2500 Magnetic Field (Oe)

5000

Fig. 4. (a) Ferromagnetic hysteresis loops at 5 K and (b) Variation of magnetic moment per Mn with applied field of La1xSbxMnO3 (x = 0.05, 0.10 and 0.15) samples. The inset of figure (a) shows enlarged image.

(a)

20

Magnetization (emu/g)

G. Venkataiah et al. / Journal of Alloys and Compounds 562 (2013) 128–133

15

one may conclude that the ferromagnetic behavior of La1xSbxMnO3 manganites may be due to spin waves. 3.3. Electrical behavior

10 5

FC 0

Magnetization (emu/g)

100 200 Temperature (K)

20

10 x = 0.10 ZFC FC 0

Magnetization (emu/g)

300

30

0

(c)

Fig. 6 shows the variation of electrical resistivity (q) with temperature in different magnetic fields. The metal–insulator transition temperatures, TMI obtained from the zero field resistivity curves are given in Table 1. The samples with x = 0.10 and 0.15 are found to exhibit metal–insulator transition with metallic behavior (dq/dT > 0) below TMI and insulating nature (dq/dT < 0) above TMI, while x = 0.05 sample exhibits an insulating behavior throughout the temperature range of investigation. It is believed that in the case of x = 0.05 due to minute doping, the ratio of Mn2+/Mn3+ ions may not be sufficient to generate the metallic behavior in the material. Further, x = 0.10 sample exhibits the metallic nature with reasonable transition temperature and the behavior is similar to hole doped manganites. Although, x = 0.15 sample exhibits metal–insulator transition, the value of TMI is low and resistivity is high when compared with that of x = 0.10 sample. As discussed in the previous sections, the impurity phase present in the material might have reduced Mn2+/Mn3+ ratio leading to low TMI and TC and also large resistivity values. In fact, the accumulation of impurity phase in the grain boundary region as evidenced from the SEM figures (Fig. 2) also strengthens the argument. The impurity phase present in the grain boundary acts as a potential barrier for the conduction electrons. Further, TMI values are found to increase with increasing magnetic field, while the peak resistivity values are decreasing for all the samples. The observed behavior is due to the fact that applied magnetic field partially induces the local ordering of the spins by delocalizing the charge carriers, which in turn enhances the ferromagnetic nature by suppressing the paramagnetic behavior. This further increases the net polarization of magnetic domains and causes easy transfer of conduction electrons between the neighboring sites [14,25]. The ferromagnetic metallic part of q(T) for x = 0.10 and 0.15 has been explained in terms of grain/domain boundary, electron–electron, and magnon scattering mechanisms given by the following equation [14] and best fit curves are shown in Fig. 6.

x = 0.05 ZFC

0

(b)

131

100 200 Temperature (K)

300

10

5

x = 0.15 ZFC

0

FC 0

100 200 Temperature (K)

300

qðTÞ ¼ q0 þ q2 T 2 þ q4:5 T 4:5

Fig. 5. Temperature dependent zero-field-cooled and field-cooled (at 100 Oe) of La1xSbxMnO3 (x = 0.05, 0.10 and 0.15) samples. The red solid line represents the best fit to the equation, M(T) = M0 + M3/2T3/2 + M2T2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

As a matter fact, the non-magnetic Sb oxide clusters are equally efficient in inducing the strains in the material between the magnetic domains resulting in pinning the domain walls [18]. With a view to analyze magnetic behavior in ferromagnetic region, the temperature dependent field-cooled magnetization curves were analyzed by conventional spin wave theory. In correlated electron systems such as manganites; spin waves, their fluctuations and domain boundaries are quite common in influencing the ferromagnetism [1]. According to Lonzarich and Taillefer [24], the magnetization in manganites is governed by spin wave theory. As per this theory, magnetization varies as T3/2 (Bloch’s law) at low temperatures and as T2 over a wide range of temperatures, while close to TC, it varies as (1T4/3/TC4/3)1/2. In view of this, the magnetization data in the FM region has been fitted to an equation,

MðTÞ ¼ M 0 þ M 3=2 T 3=2 þ M2 T 2

ð1Þ

where M0 is the temperature independent spontaneous magnetization. The best fit curves are shown in Fig. 5a–c. From the fittings,

ð2Þ

where q0 is due to grain/domain boundary effect, q2 arises due to electron–electron scattering, and q4.5 is attributed to the two magnon scattering process. It can be seen that q0 values in the case of x = 0.15 is higher by an order when compared with that of the sample x = 0.10, thereby representing the large grain boundary contribution from secondary phase. The charge conduction at high temperatures (T > TC) is usually governed by thermally activated hopping following an exponential law. According to Emin and Holstein’s theory [26] for adiabatic polaron hopping, q(T) can be expressed as

qðTÞ ¼ qa TexpðEq =kB TÞ

ð3Þ

where qa is the resistivity coefficient, Eq is the activation energy and kB is the Boltzmann constant. Fig. 6d shows the best fit curves of the plots ln(q/T) versus 1/T of all the samples. The activation energies obtained from the curves are found to be in the range 165–200 meV (Table 1). The percentage of negative magnetoresistance (MR) of these materials at different magnetic fields has been evaluated by the well known relation,

MR% ¼ ½ðqðHÞ  qð0Þ Þ=qð0Þ   100

ð4Þ

where q(0) is resistivity in the absence of magnetic field, q(H) is resistivity in presence of magnetic field. The maximum MR% values are found to be 30–40% and 55–65% at 3 T and 7 T magnetic

G. Venkataiah et al. / Journal of Alloys and Compounds 562 (2013) 128–133

2x10

7

0T 3T 7T

1x10

0T 3T 7T

120 40

3T 7T 7

(b) 140

60

40

80

30

60

20

40 0 50

20 50

100 150 200 250 300 Temperature (K)

(c)

100 150 200 250 300 Temperature (K)

30

50

3T 7T

100 150 200 250 Temperature (K)

0.015

0 5 x = 0.05 x = 0.10 x = 0.15

-2

10

300

0

10

20

200

0.010

50 40

400

10

2

ln(ρ/T)

ρ (Ω cm)

600

60

MR%

0T 3T 7T

3T 7T

0.005

(d) 800

50

100

20

0

60

MR%

7

0

ln(ρ/T)

3x10

ρ (Ω cm)

ρ (Ω cm)

(a)

MR%

132

0.005

0

0.010 1/T (K-1)

ð5Þ

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

20

S(μV/K)

S(μV/K)

x = 0.05

(d)

10

(b)

(e)

x = 0.10

4

2

2

0

0 (c)

(f)

x = 0.15

10

10

5

5

S(μV/K)

0

4 x = 0.10

x = 0.15

A systematic investigation of TEP was carried out over a temperature range 77–300 K and the variation of Seebeck coefficient (S) with temperature is shown in Fig. 7a–c and the behavior is similar to that of resistivity exhibiting a maximum value in the vicinity of TC. The temperature at which the peak occurs is designated as TS (Table 1). It can be seen from the table that the TS values of all the three samples are higher than those of TMI and TC. The change of S from semiconducting (T > TS) to metallic (T < TS) behavior with decreasing temperature is attributed to the sudden change of spin entropy due to spin polarization caused by magnetic transition [7]. An effort has been made to analyze thermopower data in the ferromagnetic regime using an empirical equation containing spin wave and lattice vibration terms [13,28],

(a)

10

3.4. Thermoelectric power studies

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

x = 0.05

S(μV/K)

20

0 S(μV/K)

fields respectively and the variation of MR% with temperature is shown in Fig. 6a–c. It can be seen from the figure that the MR values are not only high in the vicinity of TC but also significant at low temperatures (T < TC). The maximum MR observed in the vicinity of TC may be attributed to the suppression of spin fluctuations in a domain by aligning them in field direction, while in the low temperature region the inter-grain spin polarized tunneling (ISPT) of the charge carriers across the grain boundaries [1,23,27]. In fact, the MR arising from ISPT at low magnetic fields is useful for spintronic device applications.

S(μV/K)

Fig. 6. (a–c) Variation of electrical resistivity and MR% of La1xSbxMnO3 (x = 0.05, 0.10 and 0.15) manganites with temperature at different magnetic fields. The solid red line represents the best fit to the equation, q(T) = q0 + q2T2 + q4.5T4.5. (d) Variation of ln(q/T) with inverse temperature for La1xSbxMnO3 (x = 0.05, 0.10 and 0.15) samples. The solid red line represents the best fit to the small polaron hopping model. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

0

0 100 150 200 250 300 0.003 Temperature (K)

0.006

0.009

0.012

1/T (K-1)

Fig. 7. (a–c) Variation of S(lV/K) with temperature for La1xSbxMnO3 (x = 0.05, 0.10 and 0.15) samples and the solid red curve represents best fit to the equation, S = S0 + S1T + S3/2T3/2 + S3T3 + S4T4. (d–f) Variation of S(lV/K) with inverse temperature and the solid red line represents the best fit to the small polaron hopping model. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

phonon drag, and S4T4 term represents spin fluctuation in ferromagnetic phase [9]. The experimental data were fitted to Eq. (5) and it has been observed that the equation is found to fit well with the data in the entire ferromagnetic region. Therefore, it has been concluded that the low temperature ferromagnetic region is dominated

G. Venkataiah et al. / Journal of Alloys and Compounds 562 (2013) 128–133

by spin waves, while the phonon drag dominates the high temperature ferromagnetic part [13,28]. It is clear from the magnetization and electrical transport studies that the spin waves and phonon carrier interaction play vital role in the ferromagnetic part of the materials of present investigation. As mentioned in the previous section, the high temperature (T > TC) electrical conductivity is governed by thermally activated small polarons and the data at T > TS were fitted to the small polaron hopping model [26] given by

S ¼ kB =e½ES =kB T þ a0 

ð6Þ

where ES is the activation energy obtained from TEP data and a0 is a sample dependant constant. The best fit curves are shown in Fig. 7d–f and the calculated values of ES are given in Table 1. The polaron hopping energy (WH) values calculated using the relation, WH = Eq  ES are also given in Table 1. Here, Eq is the activation energy obtained from the resistivity data. The difference in the activation energies obtained from the resistivity and thermoelectric data clearly indicate that the charge transport occurs due to the hopping of carriers rather than semi-conducting like activated conduction. 4. Conclusions In summary, the magnetic and electrical transport properties of sol–gel prepared electron-doped La1xSbxMnO3 (x = 0.05, 0.10 and 0.15) manganites were investigated systematically. The samples with x = 0.05 and 0.10 exhibit single phase behavior, while x = 0.15 shows an impurity phase, indicating that the doping level might have exceeded the solubility limit. In the case of x = 0.15 sample, the transition temperatures are low due the influence of secondary phase, while x = 0.10 sample shows optimum magnetic and electrical properties like hole-doped counterparts. The thermoelectric power in the ferromagnetic region is governed by spin waves and phonons, while for the paramagnetic part, small polaron hopping model is invoked. The samples of the present investigation are found to exhibit magnetoresistance values of 30–40% at 3 T and 55–65% at 7 T and are promising materials for device applications in future. It is believed that the results of the present investigation not only trigger further experimental work but also accelerate the theoretical understanding of these electron-doped manganites.

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