Structural, transport, magnetic, and dielectric properties of La1−x Te x MnO3 (x = 0.10 and 0.15) Irshad Bhat, Shahid Husain, Wasi Khan & S. I. Patil
Journal of Materials Science Full Set - Includes `Journal of Materials Science Letters' ISSN 0022-2461 J Mater Sci DOI 10.1007/s10853-012-7112-9
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Author's personal copy J Mater Sci DOI 10.1007/s10853-012-7112-9
Structural, transport, magnetic, and dielectric properties of La12xTexMnO3 (x 5 0.10 and 0.15) Irshad Bhat • Shahid Husain • Wasi Khan S. I. Patil
•
Received: 3 October 2012 / Accepted: 22 December 2012 Ó Springer Science+Business Media New York 2013
Abstract In this study, we have investigated the structure, temperature-dependent resistivity, magnetization, and dielectric properties of La1-xTexMnO3±d (x = 0.10 and 0.15). X-ray diffraction analysis confirms the rhombohedral crystal symmetry with space group R3c. For both the samples, the temperature dependence of magnetization plots show paramagnetic-to-ferromagnetic phase transition. The Curie temperature (Tc) and magnitude of magnetization increase with the Te concentration. Field-dependent magnetization produces the asymmetric hysteresis loop that has been attributed to the magneto crystalline anisotropy induced by lattice distortion and the rare earth spin coupling at room temperature. Temperature-dependent resistivity plots exhibit metal–insulator transition (MIT) and charge-ordering state. These plots have been fitted using variable range hopping model, and the density of states [N(EF)] has been estimated. Magnetoresistance is measured as a function of temperature in the field of 1T, 5T, and 8T. The dielectric constant shows an anomaly near MIT. The dielectric constant exhibits a peaking behavior with the applied frequency and the temperature dependence of dielectric constant attains colossal values at high temperatures.
I. Bhat (&) S. Husain Department of Physics, Aligarh Muslim University, Aligarh 202002, India e-mail:
[email protected] W. Khan Department of Applied Physics, Z.H. College of Engg. & Technology, Aligarh Muslim University, Aligrah 202002, India S. I. Patil Department of Physics, University of Pune, Ganeshkhind 411007, Pune, India
Introduction Perovskite manganites Ln1-xKxMnO3 (Ln = rare earth ion; K = divalent or tetravalent ions) have been a subject of vivid interest in recent years because of their exotic electronic and magnetic properties [1–5]. Most interesting properties of the manganese perovskite arise from the competition between ferromagnetic (FM) double exchange (DE) and antiferromagnetic (AFM) super exchange, with the ratios of these competing interactions being determined by intrinsic parameters such as doping level, average ‘‘A’’ size cation radius hrAi, cation disorder, and oxygen stoichiometry. These materials exhibit high magnetoresistance for certain composition ranges [5]. Besides the colossal magnetoresistance (CMR), charge-ordering (CO) phenomena [6] has also attracted a lot of interest. Earlier studies show that CO state and the concomitant spin and/or orbital-ordering (OO) are favored when the long range coulomb interaction and/or a strong lattice interaction due to Jahn–Teller (J–T) distortion overcomes the kinetic energy of the electron [7–9]. Wider ‘‘eg’’ bandwidth, i.e., larger average A site radius (rA) favors lower TCO because the mobility of the itinerant electrons through the lattice is higher, while narrower bandwidth induces an opposite trend. Such, CO phenomenon has been observed when the concentration of the charge carriers takes the rational values of the periodicity of the crystal lattice [10–12]. In the picture of Millis et al. [13] the Jahn–Teller effect leads to the opening of a gap at the Fermi level, which together with the DE gives at least a qualitative description of the experimental observation. The strong electron–phonon interaction arising from the Jahn–Teller splitting of the ‘‘Mn’’ d level plays a crucial role. Although optical, magnetic, thermoelectric power, dc conduction measurement studies on these materials suggest a high possibility
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of conduction due to a hopping process of small polarons [14–16]. In general, doping of LaMnO3 with divalent or tetravalent ions derives the manganese ions into a mixed valence state. The effect of tetravalent ion doping at La site of the LaMnO3 like Ce4?, Zr4?, Sn4?, and Te4? has been reported earlier [17–24]. These reports include x-ray photoemission spectroscopy (XPS) and Hall effect measurements have revealed that Te ions are in the tetravalent state and the manganese ions exist in a mixed valence state of Mn2? and Mn3? in La1-xTexMnO3 (x = 0.10 and 0.15) [19–21]. The introduction of Te drives manganese Mn3? ions of LaMnO3 into a Mn2? ions, which is equivalent to introducing electron into the eg band. The mixed phase of Mn3?/Mn2? is the key component to understand the CMR, charge-ordering effects, and the transition from the FM metal to paramagnetic (PM) semiconductor in such systems. The transition metal oxides (manganites) of the type ABO3 where (A = rare earth ion and B = transition metal ion) also show ferroelectric behavior [25–27]. Large dielectric constants are expected for ferroelectrics in a narrow temperature range close to transition temperature or in systems where we observe dielectric response based on the mechanism of hopping of charge carriers that diverge toward low frequencies. Recently, a new class of materials exhibiting a colossal dielectric constant (CDC) (e0 [ 103) has also gained considerable attention because of its application in high-e0 electronic materials, such as random access memories. Fundamental interest was initiated by the observation of CDC behavior in some high Tc compounds [28–30]. During the last decade, similar observations of CDC behavior have been reported in an increasing number of materials, such as transition-metal oxides [31–35]. Several theoretical and experimental results previously reported have claimed that internal barrier layer capacitor (IBLC) [36–39] and surface barrier layer capacitor (SBLC) [40, 41] are the two special influential factors giving rise to the CDC behavior. It is believed that such barrier layers may get developed at grain boundaries or at the planar defects and at the interface between the metallic electrode and the bulk sample, giving rise to the formation of Schottky barriers. The role of grain boundaries is expected to be drastically enhanced in compounds with a mixed-phase character, not only owing to their inherent character, but also because they can act as accumulative pinning centers for structural defects. Earlier studies [42, 43] on low-frequency dielectric properties of doped manganite reveal that these properties are affected by barrier-layer capacitor microstructure, created because of the oxidation of grain boundaries, which forms an insulating layer. Various theories have excluded the possibility of the intrinsic origin of high dielectric
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constant [44, 45]. These studies conclude that the internal inhomogeneity plays the major role. It is suspected that such inhomogeneity arises from crystal twinning or some internal domain boundaries. In view of the above, we have focused on the problem to study the structural, transport, magnetic, and dielectric properties of Te-doped LaMnO3. It is one of the lesser studied materials as far as the dielectric properties are concerned.
Experimental The standard solid-state reaction route is used to prepare the samples of La1-xTexMnO3 (x = 0.10 and 0.15). Stoichiometric amounts of La2O3, TeO2, and MnO2 are mixed, ground, and heated at 1050 °C for 20 h. After first heat treatment, these samples are ground again and sintered at 1100 °C, with two intermediate grindings. The powder x-ray diffraction pattern is recorded using a x-ray diffrac˚ ) radiation at room tometer with Cu Ka (k = 1.5406 A temperature. Magnetization versus temperature measurements and the magnetic hysteresis measurements were carried out using vibrating sample magnetometer. The resistivity as a function of temperature is measured with four-probe technique. The dielectric constant measurement is performed on the Novo control, Alfa-A high-performance frequency analyzer. The samples were palletized in disk shape at a pressure of 6 ton/cm2, and silver paste was sputtered on their surfaces to ensure good electrical contact with the electrode capacitor.
Results and discussion Structural analysis The room temperature x-ray diffraction (XRD) patterns confirmed single-phase nature of the samples. The compound La1-xTexMnO3 (x = 0.10 and 0.15) has a rhombohedral crystal structure with space group R3c. The crystallite sizes of the samples calculated using Debye–Scherrer’s ˚ , for x = 0.10 and formula are found to be 346 and 377 A 0.15, respectively. The structural parameters were refined by the standard Rietveld technique [46]. The experimental and calculated XRD patterns for La0.9Te0.1MnO3 and La0.75Te0.15MnO3 with conventional Rietveld parameters are shown in Fig. 1. The lattice parameters and the unit cell volume of both the samples are tabulated in Table 1. The unit cell volume increases with Te concentration. This is consistent with the earlier reports [19–21, 47] since it is known that the radius of Te4? is about 0.097 nm and that of the La3? is 0.1216 nm. The part of substitution of Te4? ion for La3? ions should cause the reduction in the
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Fig. 2 FTIR spectra for La1-xTexMnO3 (x = 0.10 and 0.15) samples
Fig. 1 X-ray diffraction patterns of La0.9Te0.1MnO3 and La0.8Te0.2MnO3. The experimental data points are indicated by stars, and the calculated profile by solid traces. The lowest curve shows the differences between the experimental and the calculated data. The vertical bars indicate the expected reflection positions for rhombohedral structure
Table 1 Lattice parameters and unit cell volumes (V) of La1-xTexMnO3 (x = 0.10 and 0.15) at room temperature ´˚ 3 Concentration (x) Lattice parameters V (A ) ´˚ ´˚ a (A) c (A) x = 0.10
5.511
13.321
350.36
x = 0.15
5.514
13.325
350.84
ionic size hrAi of the rare earth ion, which will cause the cell constriction and the distortion in the MnO6 octahedron, resulting in the decrease of Mn–O–Mn bond angle from 180°. However, we have found that an increase in the Mn– O–Mn bond angle from 160.7° to 163.4° and small changes in Mn–O bond lengths (dMn–O * 1.96 for x = 0.10 and dMn–O * 1.95 for x = 0.15) with the increasing Te concentration result in the increase in volume. Another reason for this increase may be the oxygen stoichiometry (d) in La1-xTexMnO3±d, and consequently the respective valence states of ions present at A and B sites.
Figure 2 shows the Fourier transform infrared (FTIR) spectrum for La1-xTexMnO3 (x = 0.10 and 0.15) at room temperature. Since rare earth manganites of type (ABO3), such as LaMnO3, are known to have a distorted crystal structure with Mn (central atom) octahedral surrounded by its nearest neighbor six O ions. It is believed that MnO6, in its ideal form has six vibrating modes, two of which are reported to be IR active [48–50]. The irrational bands present around 606 and 418 cm-1 for La0.9Te0.1MnO3 and the bands 607 and 409 cm-1 for La0.75Te0.15MnO3, are the characteristics of the perovskite structure [48]. The bands at 606 and 607 cm-1 are attributed to the Mn–O stretching vibrations mode (ts), and the bands at 418 and 409 cm-1 are corresponding to the Mn–O–Mn deformation (bending) vibrations mode (tb). We have observed a shift in the tb with the increase in Te doping. The tb mode shifts slightly toward the lower wavenumber (cm-1) region, and a very small shift of 1 cm-1 toward higher wavenumber is observed in ts. Since both the bands are sensitive to the octahedral distortion in MnO6, the lowering of symmetry arising from the Jahn–teller effect results in Mn–O–Mn bond length to vary which produces the slight shift in bending vibration mode tb. The relative intensity of two vibrating modes is influenced not only by the distortion surrounding the MnO6 octahedron, but also by the local fluctuation of the electron density at the A (La) site. Magnetization Fig. 3 shows the temperature dependence of magnetization plots for Te-doped LaMnO3. The Curie temperatures (Tc) are found to be 210.15 and 211.43 K for x = 0.10 and x = 0.15 concentrations, respectively. The increase in Tc with the increasing Te content may be attributed to the strengthening of the interaction of the lattice constriction and bond angle increase, which strengthens the FM coupling between Mn
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Fig. 3 Magnetization versus temperature plots for a La0.9Te0.1MnO3 and b La0.85Te0.15MnO3 samples
ions. The values of metal–insulator transition (MIT) temperature Tp for both the compositions obtained from the resistivity measurement are lower than the values of corresponding Tc; this could be due to the increase of carrier density with the increasing Te concentration and is a common characteristics of electron-doped compounds [17, 51]. Another reason could be the size effects at rare earth site (A site) and the variation of Mn–O–Mn bond angle, since the substitution of Te at rare earth site varies hrAi and causes the distortion in the MnO6 octahedron; consequently, the Mn–O–Mn bond angle increases, which may influence the FM-DE and AFM super-exchange interaction differently, and hence the increment in Tc. The magnetization has drastically increased with Te concentration, and can be attributed to the competition between the DE and the core spin interaction. This competition leads to the parallel alignment of core spins as the doping level increases. The exchange interaction of the Mn–Mn depends on the bond angle and the bond length. The decrease of Mn–O length and increase of Mn–O–Mn bond angle will make Mn–Mn exchange interaction stronger, leading to a higher Tc and higher magnetization value. The decreases in magnetization at lower temperatures signifies the existence of AFM-insulating clusters along with the FM metallic clusters, which results in the magnetic inhomogeneity and domain wall [52, 53]. The hysteresis curves for magnetization versus magnetic field at room temperature are shown in Fig. 4. The asymmetric reversal magnetization has been noticed in our samples La1-xTexMnO3 (x = 0.10 and 0.15), which may indicate the presence of FM and AFM clusters, since such coexistence of FM and AFM domains is often the cause for the asymmetric magnetization hysteresis and their interaction can strongly influence the reversal mechanism of the
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Fig. 4 The magnetization versus field at room temperature for a La0.9Te0.1MnO3 and b La0.85Te0.15MnO3 samples. The insets show the enlarged view for the same
magnetization, modifying nucleation and annihilation of the vortex. Our results show a shift of the hysteresis loop toward negative fields. The magnetization reversal mechanism depends upon the orientation of the cooling field with respect to the twinned microstructure of the antiferromagnet, and on whether the applied field is increased to (or decreased from) a positive saturation field. The reversal magnetization occurs via either domain wall motion or magnetization rotation on opposite sides of the same hysteresis loop. We believe that the asymmetric magnetization in both the samples may be due to the magneto crystalline anisotropy [54–56] induced by lattice distortion and the rare earth spin coupling at room temperature. There are two aspects of lattice distortion in the context of the magnetic anisotropy of manganites: one is the global strain, which is a general feature for many manganites and determines the anisotropy of the uniform rotation of the spin magnetic moments; and the other one is the alternating local distortion of the magnetic anisotropy energy known as one of the common mechanisms responsible for the non-collinear magnetic order. If La spin coupling has a contribution to the magnetic anisotropy, then it is most likely due to the interaction of La, 4f electron spin with 3d-electron Mn spin, which weakens the magnetization as well as induces the magnetic anisotropy. The charge-ordering and magnetic anisotropy, which is induced by lattice distortion and the rare earth spin coupling at room temperature, could behave in an AFM-ordered or FM-insulating state. We have observed an enhancement in the magnetization with the increasing Te concentration.
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Transport Analysis The temperature-dependent resistivity q(T) plots for La1-xTexMnO3 (x = 0.10 and 0.15) are shown in Fig. 5. On decreasing the temperature, these compounds first undergo a PM (insulating) to FM (metallic) phase transition at transition temperatures Tp = 203 and 209 K, respectively, then resume the semiconducting behavior. A wide charge-ordered region centered between 70 and 140 K is observed in both the samples which could be due the coexistence of the AFM and FM spin clusters, and the competition between them yields charge-ordering characteristics. Such phase coexistence features are explained in the framework of electronic phase separation scenario, predicted for the manganites [57]. This behavior is attributed to the structural inhomogeneous characteristic of the strongly correlated electronic systems that govern the electric and magnetic properties. The insulating behavior results from the single ion J–T distortion as well as the blocking of hopping by strong Hund’s coupling to the paramagnetically disordered Mn spins. A shift in the MIT and charge-ordering transition temperature (TCO) is observed with the increasing Te doping amount. It is well documented in the literature [58, 59] that the strong increase of the resistivity with decreasing temperature can be described by the Mott’s variable range hopping (VRH) model [60–62]. In this model, it is assumed that the charge carriers move along a path described by the optimal pair hopping rate from one localized state to another. To investigate the electronic transport mechanism, we have fitted the resistivity q(T) data with thermally activated conduction (TAC) model, q(T) = q0(T) exp(Ea/kBT); Small polaron hopping (SPH) model, q(T) = q0(T) T
Fig. 5 lnq(T) versus T plots for La1-xTexMnO3 (x = 0.10 and 0.15) samples
exp(Ea/kBT); and three-dimensional Mott’s VRH model, q(T) = q0(T) exp(To/T)1/c—with c = 2 or 4—predicting the charge transport by tunneling of electrons or holes [63]; c = 4 has been considered as indicative for isotropic charge transport. Efros and Shklovski [64, 65] proposed an alternative explanation for c = 2 in Mott’s three-dimensional model where the coulomb interaction between the charges is taken into account. We have found that VRH model is much superior to other models in describing the carrier transport in our system. The fitted results are shown in Fig. 6 a, b. We have found VRH model responsible for the transport properties below (T [ TCO) as well as above (T \ TCO) chargeordered regions. The most convincing results (straight line) are observed for log q against 1/T1/4 below MIT in both the samples, which are typical for three-dimensional VRH
Fig. 6 lnq(T) versus 1/T1/4 plots for a La0.9Te0.1MnO3 and b La0.85Te0.15MnO3 samples. Solid line shows Mott’s VRH model fitting, and the inset shows the VRH fitting below the transition temperature
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model. The value of resistivity increases above the arbitrary temperature (TA) with the temperature decreasing further. A small metallic region has been observed between 203 and 196 K for La0.90Te0.10MnO3 and between 209 and 198 K for La0.85Te0.15MnO3 before resuming semiconducting behavior with the temperature decreasing further. It is a known fact that VRH transport behavior does require the appearance of random potential; hence, due to the large anisotropic lattice distortion of MnO6 octahedron in La1-xTexMnO3 (x = 0.10 and 0.15) caused by charge ordering, it will trap the electrons below TCO, and hopping across the grain boundaries contributes toward transport process at low temperatures. The To (characteristic temperature) occurring in the VRH relation can be related to the carrier localization length, using the expression To = 24/(kBN(EF)pL3) [66]. The value of the localization length should be comparable to Mn–O bond distance for VRH type of conduction; in the present case, the mean Mn–O bond lengths for x = 0.10 and 0.15 calculated from ˚ and dMn–O = 1.95 A ˚, XRD data are dMn–O = 1.96 A respectively. The values of To and density of states N(EF) are tabulated in Table 2. It is clear from the Table 2 that the characteristic temperature (To) shows the minimum value in charge-ordered region for both the samples, which indicate less-disordered state and hence attains higher values of density of states near Fermi level N(EF). The Chisquare values indicate the quality of the fitted data. The magnetoresistance as a function of temperature is measured in the presence of the magnetic field 1T, 5T, and 8T as shown in Fig. 7. Sharp and broad peaks can be attributed to the coexistence of the AFM and FM clusters, contributing to the spatially metallic and insulating areas in the studied temperature range, which is believed to occur in materials where the electron density is commensurate to the number of lattice sites. Under the applied field, the resistivity gets suppressed, especially near the (I–M) transition as well as in the charge-ordered region. Under 8T magnetic field, the maximum MR ratios reached are about 70 and 67 % for x = 0.10 and 0.15 concentrations, respectively, in the vicinity of charge-ordered region.
Table 2 Values of characteristic temperature To, density of states at the Fermi energies N(EF), and Chi-square (v2) determined from fitting of resistivity temperature data for La1-xTexMnO3 (x = 0.10 and 0.15)
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Fig. 7 Temperature dependence of magnetoresistance (MR) with magnetic fields of 1T, 5T, and 8T for a La0.9Te0.1MnO3 and b La0.85Te0.15MnO3
Dielectric Properties Low-frequency dielectric properties of doped manganites have been the subject matter of earlier studies [67, 68]. We have studied the effects of frequency and temperature dependence on dielectric functions for both the samples.
Concentration (x)
Temperature ranges
To (K)
N (EF) (eV-1 cm3) 9 1020
v2
x = 0.10
Below Tp
560 9 105
2.09
0.9996
T \ Tco
485 9 104
24.2
x = 0.15
0.9959 7
Ta [ T [ Tco
7.86
1.49 9 10
T [Ta
437.71
2.68 9 105
0.9996
Below Tp
660 9 105
1.78
0.9996
T \ Tco
677 9 104
17.4
0.9952
Ta [ T [ Tco
7.7
1.52 9 107
0.9240
T [ Ta
1923.48
6.11 9 104
0.9965
0.9800
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Figs. 8 and 9 show the variation of dielectric constant with frequency and temperature, for La1-xTexMnO3 (x = 0.10 and 0.15). Dielectric constant e0 can be expressed as e0 = ea ? ed ? el, where ea is associated with the displacement of ionic charge distribution relative to their nuclei and contribution at higher frequencies; the lattice contribution (el) arises from displacement of ions and their charge distributions; and ed gives the dipolar contribution associated with the charge carrier hopping. Dielectric constant as a function of frequency shows a peak behavior; e0 first decreases for specific low-frequency region up to 2 kHz, above which a dispersion maxima starts to develop giving a broad peak between 400 kHz and 1.25 MHz and attains the colossal values of dielectric constant (CDC). The height of the dispersion peak shows a decrease with the decreasing temperature, the signature of dipolar relaxation. The interface and the grain boundaries control the
behavior of e0 at lower frequencies: the thinner the grain boundary layers, the higher the value of e0 . The peaking behavior of e0 with frequency can be explained using Rezlescu model [69]. According to this model, the peaks of e0 curves can be ascribed to the presence of collective contribution to the polarization from two different types of charge carriers—both intrinsic and extrinsic effects contribute to the overall dielectric response. Intrinsic effects are related to charge condensation that takes place in some systems with cations of variable valances such as Mn3?– Mn2?/Mn4? and Ni2?–Ni3? [70, 71]. External factors lead to the interfacial polarization produced in grain boundaries and contact sample electrodes [72]. An increase in dielectric constant has been observed with the increasing temperature as depicted in Figs. 8b and 9b, suggesting the thermal effects on charge carriers which orient themselves after getting liberated, thereby enhancing the space-charge
Fig. 8 a. Frequency dependence of dielectric constant (e0 ) at different temperatures for La0.9Te0.1MnO3. b Temperature dependence of the dielectric constant (e0 ) at different frequencies for La0.9Te0.1MnO3. Inset shows the enlarged view near transition region
Fig. 9 a Frequency dependence of dielectric constant (e0 ) at different temperatures for La0.85Te0.15MnO3. b Temperature dependence of the dielectric constant (e0 ) at different frequencies for La0.85Te0.15MnO3
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Fig. 10 Temperature dependence of loss tangent (tand) at different frequencies for a La0.9Te0.1MnO3; and b La0.85Te0.15MnO3 samples
polarization and hence the overall dielectric constant. We have observed anomaly in dielectric constant near the MIT temperature. A decrease in dielectric constant near MIT has been observed, and above which, it rises rapidly to higher values, upon decreasing the resistivity further, suggesting a correlation between the two phenomena. It is pertinent to mention that the higher values of the dielectric constant is attributed mainly in space-charge polarization mechanism, known to predominate in heterogeneous structures, in which a material is assumed to be composed of different regions (grain and grain boundaries). The conductivity of grains is considered relatively better than that of grain boundaries because the charge carriers encounter different resistances so that accumulation of charges at separating boundaries occurs, and hence, the dielectric constant values increase. Earlier reports interpret
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high dielectric responses like CDC as an artifact emerging from Schottky effect at the electrode contacts [69]. M. Sanchez-Andjar et al. [73] suggest the dielectric behavior as a consequence of the formation of polar entities at the temperature of the charge condensation, due to an asymmetric charge distribution, intermediate between sitecentered and bond-centered types as described by Efremov et al. [74] for real half-doped manganites. Loss tangent Figure 10 shows the temperature dependence of loss tangent at different frequencies. We have observed high dielectric loss, with relaxation dispersion peaks centered at different temperatures for both the samples, which is the direct consequence of the semiconducting nature of these
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Fig. 10 continued
materials, having high dc resistivity. These loss peaks can be discussed within the framework of the Maxwell– Wagner model which refers to interfacial polarization occurring in electrically inhomogeneous systems [75]. The presence of oxygen non-stoichiometry in our samples can be thought of as a possible source of inhomogeneity. It is known that when an electric current passes through interfaces between two different dielectric media, because of their different conductivities, surface charges pile up at the interface [76], and the system shows a Debye-like relaxation process under an external alternating field [75]. Double-dissipation peak behavior can be seen at the frequencies of 1.25 MHz, 625 kHz, 312 kHz, and 156 kHz for La0.90Te0.10MnO3, and can be interpreted by the hopping of localized carriers and the thermally excited relaxation processes. We have noticed only single peak at lowfrequency toward low-temperature region, but with the
increase in frequency of the applied field above 7800 Hz, a secondary dissipation peak starts to develop toward the higher-temperature region, thereby minimizing the characteristic dissipation peak formed at low-temperature region. While, in the case of La0.85Te0.15MnO3, a different scenario has been observed: the characteristic height of the peak starts to develop with the decrease in frequency, and a complete phase transition can be observed at 155 kHz near MIT in La0.85Te0.15MnO3. The dissipation energy (loss) decreases with the increasing frequency of the applied field. The decrease in loss tangent with the increasing frequency can be attributed to the dc conductivity. The loss tangent increases with the increasing concentration as depicted in Fig. 10. It should be noted that the temperature at which the dielectric constant begins to increase rapidly is approximately equal to that at which the dielectric loss dispersion starts.
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Conclusion We have systematically investigated the structural, magnetization, electronic transport, and dielectric properties for La1-xTexMnO3 (x = 0.10 and 0.15). The samples are found to be in single phase as confirmed by Rietveld analysis. Both the samples exhibit the PM–FM phase transition accompanied by MIT temperature. However, with further decrease in temperature, the samples show the insulating phase characteristics accompanied with the charge-ordered region at low temperature. The Curie temperature Tc, the charge-ordering transition temperature TCO, and the MIT have been found to increase with Te doping. Asymmetric magnetization and the shifting in hysteresis loop have been noticed. The VRH model has been found useful in discussing the transport properties (resistivity). Dielectric constant exhibits a peak response with frequency and attains colossal values at higher temperatures for both the samples. Acknowledgements The authors are greatly thankful to the UGCDAE Consortium for Scientific Research, Indore and the Center of Excellence in Materials Science (Nanomaterials), Department of Applied Physics, Aligarh Muslim University, Aligarh for providing the experimental facilities. The authors are also thankful to Dr. R.K. Kotnala, the National Physical Laboratory, New Delhi for providing facilities in respect of magnetic measurements.
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