Metal-Insulator Transition in (La_0.7Sr_0.3Mn_0.98Co_0.02O₃)_1−x(BaTiO₃)_x Multiferroic

Abstract

(La_0.7Sr_0.3Mn_0.98Co_0.02O₃)_1−x(BaTiO₃)_x (x = 0.1; 0.2; 0.3) mixed perovskite were prepared by the solid state reaction method. It is shown that this compound exposes the multiferroic (magnetic and ferroelectric) behavior comparing with magnetic pristine La_0.7Sr_0.3Mn_0.98Co_0.02O₃. The increase of BaTiO₃ fraction from x = 0.1 to 0.3 leads to weakening (enhancing) of the ferromagnetic (ferroelectric) order. The metal-insulator transition (MIT) in the ferromagnetic La_0.7Sr_0.3Mn_0.98Co_0.02O₃ (x = 0) and the multiferroic x = 0.1, 0.2 samples were registered with reduction of the MIT temperature T_MI from 380 K of the x = 0 to 117 K of the x = 0.2 samples. Temperature dependence of resistivity and MIT of these samples are well described by the mixed conducting carrier model, which has crossover between the low temperature spin-scattering electron and the hopping small polarons conductions at T_MI.

Fig. 8 Temperature dependence of the volume fraction functions for metallic like (f) and small polaron (1 − f) electrical carriers in x = 0 (a), x = 0.1 and 0.2 samples (b).

1. Introduction

Recently, multiferroic materials attract much attention of researchers due to their coupling magnetic and ferroelectric properties, which open a wide field for technological application and fundamental investigation.¹⁾ Composites compose of magnetic colossal magnetoresistance (CMR) perovskite R_0.7A_0.3MnO₃ (R = rare earth metals, A = Ca, Sr, Ba…) and ferroelectric (FE) perovskite are potentially multiferroic with applicable features.

Except for CMR effect, there are other interesting phenomena occurred in mixed phase doped manganite compounds like: an enhancement of the low field magnetoresistance,²⁾ first order magnetisation process and step-like change of resistivity³⁾… It was reported²⁾ that the doping of the FE insulating BaTiO₃ (BTO) component in (La_0.7Sr_0.3MnO₃)_1−x(BaTiO₃)_x (x = 0, 0.06, 0.12, 0.18) composite enhances the low field magneto-resistance and shifts the MIT temperature to the lower temperature region, but mechanism of the MIT in this compound has not been analyzed yet. Influence of the BTO doping on magnetic and transport properties of La_0.7Sr_0.3MnO₃/BaTiO₃ nanocomposite was investigated in Ref. 4). Besides, the core-shell La_0.7Sr_0.3MnO₃/BaTiO₃ nanocomposite prepared by sol-gel method shows an enhancement of magnetization and good magneto-electric voltage coefficient due to BTO doping.⁵⁾ Nature of the magnetic order in cobalt doped La_0.7Sr_0.3Mn_1−xCo_xO₃ was examined in Ref. 6). The multiferroic behavior of La_0.7Sr_0.3Mn_0.96Co_0.04O₃/BaTiO₃ composite was shown early,⁷⁾ but the MIT in such system was not studied.

Aim of our research is to study magnetic & electrical behavior and the metal-insulator transitions in (La_0.7Sr_0.3Mn_0.98Co_0.02O₃)_1−x(BaTiO₃)_x(LSMCO_1−xBTO_x) multiferroic composites. The two-type of electrical conducting carrier model is applied to explain the MIT in these compounds.

2. Experiment

(La_0.7Sr_0.3Mn_0.98Co_0.02O₃)_1−x(BaTiO₃)_x, x = 0.1, 0.2, 0.3 compounds were synthesized by the solid state reaction method. The CMR perovskite component LSMCO was prepared from high purity La₂O₃, Sr(CO₃), Co₂O₃, MnO oxides. These metallic salts were weighed in appropriate proportion and mixed, crushed in 1 hour and dried. After grounding of the obtained mixture in absolute alcohol for 1 hour, powders were pressed and pre-heated at 500°C in air for 2 hours. Afterwards, the pre-heated product is subjected to dry and wet grinding processes. Fine powder product was pressed into the pellets and sintered at 1300°C for 24 hours in air to get final LSMCO component. The second nanosized BTO component was prepared by the sol-gel method. High purity chloride salts of Ba²⁺, Ti³⁺ were mixed at 120°C for 5 hours and washed several times to obtain fine BTO powders. The LSMCO and BTO powders were weighed in suitable molar ratio and crushed in a high-energy ball grinder for 1 hour. After that, the homogeneous mixtures were pressed in pellets and sintered at 1200°C in air for 2 hours. The crystal structure of materials was analysed using the X-ray diffractometer Bruker D5005 and the magnetic characteristics were measured by the Vibrating Sample Magnetometer (VSM). Dielectric property of materials was registered by impedance analyzers 41920A and HP4193A. Resistivity of the materials was determined by the four-probe method using rectangular samples with the size of 2 × 2 × 8 mm³.

3. Result and Discussion

Figure 1 shows the powder X-ray diffraction patterns of the LSMCO_1−xBTO_x. One sees that the increase of the BTO doping fraction leads generally to the reduction of the intensity of the Bragg diffraction peaks of LSMCO. This reduction is robust in the x = 0.1, 0.3 compounds. The sufficient high sintering temperature (1200°C) leads to solid-state reactions between two precursors of La_0.7Sr_0.3Mn_0.98Co_0.02O₃ and BaTiO₃. Then, Bragg’s peaks corresponding to BTO are extremely weak and compounds are almost single phase. That is different from the La_0.7Sr_0.3MnO₃–BaTiO₃ nanocomposite case,^3,4) where the presence of the minor second BaTiO₃ crystal phase is evidenced. The lattice constants of the polycrystal LSMCO_1−xBTO_x system are calculated by CelRef software and the crystallite size is derived from the Scherrer equation   

\begin{equation} D = \frac{0.9\lambda}{\beta \cos \theta}, \end{equation} (1)

where λ is the wavelength of the Cu-K_α radiation λ = 1.54056 Å, θ is the Bragg angle, β is the line broadening at half the maximum intensity (FWHM), respectively. The LSMCO_1−xBTO_x crystalline phase exhibits Pbam symmetry and orthorhombic structure. Data show the prepared materials consist of nano-crystallites with sizes of 17–29 nm and the crystal structure parameters are given in Table 1.

Fig. 1

X-ray diffraction patterns of the LSMCO_1−xBTO_x with x = 0.0, 0.1, 0.2, 0.3.

Table 1 Structural parameters of the LSMCO_1−xBTO_x crystal phase.

Figure 2 shows the thermo-magnetic plots of the samples recorded at the magnetic field strength H = 300 Oe. The ferromagnetic (FM) to paramagnetic (PM) phase transition temperature $T_{C}^{m}$ for x = 0.0 and 0.1 samples is found by the fastest slope tangent of the M(T) curve (dM/dT, see insets in the picture for the x = 0.2, 0.3 compounds or intersection between tangents of the M(T) curves and temperature axes for the x = 0.1, 0.3 samples) and listed in Table 2. $T_{C}^{m} = 359$ K of the La_0.7Sr_0.3Mn_0.98Co_0.02O₃ pristine sample is higher than the value 320 K of La_0.7Sr_0.3Mn_0.95Co_0.05O₃⁶⁾ indicating reduction tendency of the FM-PM phase transition temperature with increasing cobalt content in this compound.

Fig. 2

Temperature dependence of the LSMCO_1−xBTO_x magnetization measured at H = 300 Oe.

Table 2 Experimental phase transition temperatures and parameters derived from the fits.

Figure 3 exhibits the LSMCO_1−xBTO_x magnetization curves measured at 300 K. One sees that doping BTO leads to the room temperature magnetization reduction tendency. The room temperature saturation magnetizations at 4000 Oe reduce with increasing BTO fraction and have the lowest value for the x = 0.2 sample (see insets in Fig. 3).

Fig. 3

Field dependence of LSMCO_1−xBTO_x magnetization registered at T = 300 K. Insets: magnetization of the x = 0 sample (upper corner), magnetization decreases with increasing BTO fraction (lower corner).

Figure 4 illustrates the temperature dependence of the relative dielectric constant (ε) of the LSMCO_1−xBTO_x compounds measured at 10 kHz. The paraelectric-ferroelectric (PE-FE) transition temperature $T_{C}^{e}$ is defined as the temperature at which the dielectric constant reaches a maximum. From Fig. 4, we can see the maxima of the dielectric permeability at $T_{C}^{e} = 330$ K, 317 K, 304 K for x = 0.1, 0.2 and 0.3 composite samples, respectively. The dielectric constant increases from almost temperature independent ε ∼ 10⁻⁵ to ε ∼ 3 of the x = 0 and x = 0.1 samples, respectively. It shows that presence of the BTO leads to the appearance of the FE behavior of the LSMCO_1−xBTO_x compounds.

Fig. 4

Variation of the LSMCO_1−xBTO_x (x = 0, 0.1, 0.2 and 0.3) dielectric constant with temperature measured at 10 kHz.

The inverse of the dielectric constant ε⁻¹ as the function of temperature is plotted in Fig. 5 and the intersection points $T_{0}^{e}$ of the tangent lines with temperature axes are very closed to the $T_{C}^{e}$. It means ε⁻¹ is agreeably follows the Curie-Weiss law for the PE-FE second order phase transition with reducing temperature.   

\begin{equation} \varepsilon^{-1} = \frac{T-T_{C}^{e}}{C}. \end{equation} (2)

Fig. 5

Temperature dependence of the LSMCO_1−xBTO_x inverse dielectric constant recorded at 10 kHz.

Figure 6 shows that the x = 0.0, 0.1, 0.2 samples exhibit the metal (dρ/dT > 0) to insulator (dρ/dT < 0) transition at the phase transition temperature T_MI (MIT temperature). The MIT temperature shifts to the lower temperature which is from 380 K at x = 0.0 to 170 K at x = 0.1 and to 117 K at x = 0.2. The x = 0.3 sample shows semiconducting behavior upto 300 K, having a step change of resistivity near 80 K. This can be explained by good conducting LSMCO becomes semiconductor under strong doping of insulating BTO. Comparing experimental data for $T_{C}^{m}$, $T_{C}^{e}$, T_MI of x = 0.1, 0.2 samples (see Table 2) we note that $T_{C}^{e}$ is higher than $T_{C}^{m}$ and T_MI is the lowest temperatures among them. Therefore, the MIT completely occurs in the multiferroic state where the FM and FE orders coexist, except for the x = 0 sample (in the FM state).

Fig. 6

Temperature dependence of LSMCO_1−xBTO_x sample resistivity: x = 0.0 (a) (b) x = 0.1, 0.2, 0.3.

Several models are used to elucidate the below room temperature transport in CMR perovskites basing on the coexistence of localized and itinerant carriers^8–10) or Kondo like transport model.¹¹⁾ Because the LSMCO_1−xBTO_x composite is multiferroic ionic then polaron conducting picture is more reasonable. Polarons in such multiferroic materials are seemed “composite polarons” originating from interaction of electron with local magnetic and ferroelectric environment.

In this work, we apply the two-component polaronic transport model developed by Rubinstein⁹⁾ to explain the MIT in LSMCO_1−xBTO_x (x = 0, 0.1, 0.2) samples. In the high temperature region above T_MI, the resistivity is described by a small polaron (SP) hopping law, which is   

\begin{equation} \rho_{I} = \rho_{a}T\exp \left(\frac{E_{a}}{k_{B}T} \right), \end{equation} (3)

where E_a is the SP thermal activation energy, ρ_a is the temperature independent parameter, and k_B the Boltzmann constant. In the temperature region below T_MI, where the FM and FE orders are strong, the electrical carriers are scattered by magnetic and ferroelectric elementary excitations. According to experiment⁴⁾ the spin-disordered scattering increases with the increasing amount of BTO in LSMO-BTO composite. In our polycrystalline compound with the weak FE behavior, the spin dependent and grain boundary scattering mechanism seem more important. Then the metallic-like resistivity in the temperature region below T_MI is taken as   

\begin{equation} \rho_{\textit{II}} = \rho_{0} + \rho_{2}T^{2}. \end{equation} (4)

In eq. (4), ρ₀ is the temperature independent residual resistivity due to the grain boundary and other temperature independent scattering mechanism. The second term represents the scattering of itinerant electron by lattice spin waves.¹²⁾ Since the SP conductivity $\sigma _{I} = \rho _{I}^{ - 1}$ and the metallic conductivity $\sigma _{II} = \rho _{II}^{ - 1}$ are given with probabilities 1 − f and f, respectively. The total conductivity σ = ρ⁻¹ is defined by   

\begin{equation} \sigma = (1 - f)\sigma_{I} + f\sigma_{\textit{II}}, \end{equation} (5a)

or in term of resistivity   

\begin{equation} \rho = \left\{\frac{1 - f}{\rho_{I}} + \frac{f}{\rho_{\textit{II}}} \right\}^{-1}. \end{equation} (5b)

The temperature dependent probability functions f and 1 − f are relative volume fractions of the itinerant electron and SP gas, correspondingly   

\begin{equation} f = \cfrac{1}{1 + \exp \biggl(\cfrac{T-T_{0}}{\Delta} \biggr)}, \end{equation} (6)

where T₀ is fitting metal-insulator transition temperature and the constant Δ characterizes the width of MIT. Figure 7 shows the fitting result using eqs. (3)–(6) for the curves given in Fig. 6. The parameters obtained by this theory are listed in Table 2.

Fig. 7

Fitting and experimental curves for the temperature dependent resistivity of the LSMCO_1−xBTO_x (x = 0, 0.1, 0.2) samples.

Table 2 exhibits the calculated MIT phase transition temperature T₀, which is almost coincided with the experimental T_MI. The SP thermal activation energy E_a has reduction tendency with the increase of BTO fraction. It is noted that the high resistivity x = 0.1 sample has the highest residual resistivity ρ₀ = 1.229 Ωcm and the sample has higher T_MI owning higher SP thermal activation energy E_a. It is clearly seen that BTO doping leads to reduction of the MIT width from 103.1 K of LSMCO to about 80 K of the x = 0.1, 0.2 composite samples. The reduction of both $T_{C}^{m}$, $T_{C}^{e}$ in multiferroic x = 0.1, 0.2, 0.3 composites comparing with that of the pristine feromagnetic LSMCO ($T_{C}^{m} = 359$ K) and ferroelectric BTO ($T_{C}^{e} \sim 393$ K) components is due to the dilution of one by the other order.

Table 3 shows a comparison between the SP resistivity ρ_I and the metallic-like resistivity ρ_II values taken at the metal-insulator transition temperature. It is observed clearly that both ρ_I, ρ_II and resistivity by spin wave scatterings have the highest value for the x = 0.1 sample, which also has the largest resistivity below the room temperature. The data exhibit the main contribution to total resistivity at the MIT temperature is come from the SP resistivity ρ_I comparing with the spin dependent part ρ_II. It is in agreement with the assumption that the SPs are main carriers at high temperature.

Table 3 Small polaron (ρ_I) and metallic (ρ_II) resistivity values at T_MI.

Figure 8 illustrates temperature dependence of the volume fraction function f, and (1 − f) for metallic-like (small polaron) conductivity of the x = 0, 0.1, 0.2 samples. The T_MI can be defined as the temperature, at which the two type of conducting carriers have an equal probability 1/2 = f = 1 − f. As we expect, the metallic–like (SP) conductivity has larger probability in the temperature T < T_MI (T > T_MI). One sees that MIT in multiferroic LSMCO_1−xBTO_x is interpreted well by crossover between spin-scattered itinerant electron and SP hopping conducting mechanisms with increasing temperature.

Fig. 8

Temperature dependence of the volume fraction functions for metallic like (f) and small polaron (1 − f) electrical carriers in x = 0 (a), x = 0.1 and 0.2 samples (b).

4. Conclusion

Almost single phase LSMCO_1−xBTO_x (x = 0.0, 0.1, 0.2, 0.3) compounds are experimentally fabricated and their electrical and magnetic properties have been examined. Among the prepared specimens, the compounds with x = 0.1, 0.2, 0.3 apparently expose multiferroic behavior with different Curie magnetic $T_{C}^{m}$ and ferroelectric $T_{C}^{e}$ temperatures. MIT is observed in the x = 0, 0.1, 0.2 samples and the phase transition temperatures $T_{C}^{m}$, $T_{C}^{e}$, T_MI reduce with the increasing BTO content. MIT and temperature dependent resistivity of these compounds are properly fitted by Rubinstein mixed conducting model.

Acknowledgments

This work is funded by National Foundation for Science and Technology of Viet nam (NAFOSTED) under grant number 103.01-2019.324. N. N. Dinh is thankful to Viet nam National University for support (grant number QG.16.04).

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