Influence of Nd Substitution by La in Nd_0.7Sr_0.3MnO₃ on Structural and Transport Properties for Sensing Applications

Abstract

The structural and transport properties of Nd_0.7−𝑥La_𝑥Sr_0.3MnO₃ manganites with 𝑥=0,0.1, and 0.2 prepared by solid state reaction route are studied. These compounds are found to be crystallized in orthorhombic structural form. A shift in the metal-semiconductor/insulator transition temperature (𝑇_MI) towards room temperature (289 K) with the substitution of Nd by La, as the value of 𝑥 is varied in the sequence (0, 0.1, and 0.2), has been provided. The shift in the 𝑇_MI, from 239 K (for 𝑥=0) to near the room temperature 289 K (for 𝑥=0.2), is attributed to the fact that the average radius ⟨𝑟_𝐴⟩ of site-A increases with the percentage of La. The maximum temperature coefficients of resistance (TCR) of Nd_0.7−𝑥La_xSr_0.3MnO₃ (𝑥=0.1 and 0.2) are found to be higher compared to its parent compound Nd_0.7Sr_0.3MnO₃ which is almost independent of ⟨𝑟_𝐴⟩. The electrical resistivity of the experimental results is explored by various theoretical models below and above 𝑇_MI. An appropriate enlightenment for the observed behavior is discussed in detail.

1. Introduction

In the past few decades, AMnO₃-type manganites have been extensively studied because of their richness in physical properties which is due to the simultaneous presence of spin, lattice, and orbital degrees of freedom [1–3]. Significant attention has been paid by many researchers in order to explore their potential for spacious technological applications such as read heads, magnetic information storage, low- and high-field magnetic sensors, IR detectors, and numerous other spintronic applications [4–11]. The substituted manganites provide high temperature coefficient of resistance (TCR) in bulk as well as in thin films at room temperature. This will motivate us to explore them for infrared radiation detectors (i.e., IR detector) for night vision applications [12]. Among all perovskite manganites, NdMnO₃ is an antiferromagnetic insulator, characterized by a superexchange coupling between Mn³⁺ sites. This coupling is facilitated by a single 𝑒_𝑔electron predominated by strong correlation effects. On the other hand, partial substitution of Nd³⁺ ions with divalent cations (Sr, Ca, and Ba) results in mixed valance states of Mn, that is, Mn³⁺/Mn⁴⁺ which is responsible for the ferromagnetic Zener double exchange mechanism [13].

The most prevalent experimental way of affecting the physical properties of the manganites is either substituting cations at the A- or B-sites or varying the oxygen content in the regular perovskite structure [20–22]. The size mismatch at A-site generates internal chemical pressure within the lattice. Due to this structural disorder effect, the local oxygen displacement occurs, ensuing into bond angle fluctuations and bond length variations, further leading to carrier localization in perovskite lattice. This distortion can be controlled by the average size of the A-site cation which in turn modifies the Mn–O–Mn bond angle and Mn–O distances. The Mn–O–Mn bond angle is directly related to the hopping integral between Mn³⁺ and Mn⁴⁺ degenerate states. Goldschmidt’s tolerance factor 𝜏is defined as𝜏=⟨𝑟_𝐴⟩+𝑟₀/√2(𝑟_𝐵+𝑟₀)where ⟨𝑟_𝐴⟩ and 𝑟_𝐵 are the radii of the average A-site and B-site ions and 𝑟₀is the radius of oxygen ion. For𝜏<1, Mn–O–Mn bond angle is decreasing due to rotation of MnO₆ octahedra which in turn leads to lower symmetric structure. The transport properties of the substituted manganites are influenced by the local distortion of the lattice. This local distortion occurs by the size mismatch of different radii of A-site cation and also by cationic vacancies of rare earth (3+) and divalent alkaline earth (2+)elements. This disorder is quantified by means of the variance of the A-site cation radius distribution(𝜎²)defined as ∑𝑦_𝑖𝑟²_𝑖−⟨𝑟_𝐴⟩² where𝑦_𝑖are the fractional occupancies of the species. Thus, the variations in ionic radii at A-sites lead to competing phases at a particular temperature, hence influencing electrical and magnetic transport properties of the perovskite manganites.

In the present research, the structural and transport properties of Nd-based manganites with composition Nd_0.7−𝑥La_𝑥Sr_0.3MnO₃ (where 𝑥 = 0, 0.1, and 0.2) are studied in order to tune the TCR and 𝑇_MI for application aspect. The present work is targeted to achieve favorable properties of manganites for IR detector applications.

2. Experimental Details

The polycrystalline samples of Nd_0.7−𝑥La_𝑥Sr_0.3MnO₃, where 𝑥 = 0, 0.1, and 0.2 are synthesized by the solid state reaction route using ingredients Nd₂O₃, La₂O₃, SrCO₃, and Mn₂O₃. The mixed powders are calcined at 1100°C in air for 24 hours. Thereafter, powder is pressed into pellets by applying a uniaxial pressure of 4-5 tons followed by sintering at 1300°C for 5 hours. The sintered pellets are annealed in an oxygen environment at 1000°C for 5 hours to retain the oxygen stoichiometry. The structure and phase purity of the samples are analyzed by powder X-ray diffraction (XRD) performed on a diffractometer (PANanalytical X’pert Pro) using CuK_α radiation at 40 kV and 30 mA. The resistivity measurement without and with magnetic field (5T) are carried out using a four-probe method in the temperature range from 5 to 300 K on a quantum design Physical Property Measurement System (PPMS Model no. 6000).

3. Results and Discussion

3.1. XRD Results

The XRD patterns of polycrystalline manganites Nd_0.7−𝑥La_𝑥Sr_0.3MnO₃ where 𝑥=0 (NLSMO 0), 0.1 (NLSMO 1), and 0.2 (NLSMO 2) prepared by solid state route are shown in Figure 1. The XRD patterns of all compounds exhibit single-phase orthorhombic unit cell with Pnma (no. 62, PCPDF ref no. 861534) space group. It can be observed from patterns that the peaks are slightly shifted toward lower angle side with the substitution of Nd by La. This small shift arises due to the mismatch of radius at A-site, that is, larger radius of La-ion (1.36 Å) in comparison to the radius of Nd-ion (1.27 Å) which causes increase in the volume of the lattice. Subsequently, an internal chemical pressure generated within the lattice due to the size mismatch at A-site, which results in slight shift in the peaks of XRD pattern. The increase in tolerance factor from 0.9408 (for NLSMO 0) to 0.9470 (for NLSMO 2) indicates that the Mn–O–Mn bond angle approaches towards angle 180° which reduces the distortion in MnO₆. Less distortive structure promotes the hopping integral (𝑡=cos(𝜃/2))and reduces the charge localization (especially below 40 K in our case) which is further supported by the resistivity data.

Figure 1 

XRD patterns of perovskite manganites Nd0.7−𝑥La𝑥Sr0.3MnO3 where 𝑥 = 0, 0.1, and 0.2.

3.2. Electrical Transport

The design and development of uncooled IR detector (Bolometer) require 𝑇_MI around the room temperature with high TCR for improved sensitivity [12, 23]. The temperature-dependent resistivities𝜌(𝑇)studied in the temperature range 5–300 K are shown in Figure 2. All samples are exhibiting metal-semiconductor/insulator transition (𝑇_MI). The shift in the 𝑇_MI, from 239 K (for 𝑥=0) to near the room temperature 289 K (for 𝑥=0.2), is attributed to the fact that the average radius ⟨𝑟_𝐴⟩ of site-A increases with the percentage of La. In perovskite manganites, the oxygen ions tend to move towards the center of MnO₆ octahedra as ⟨𝑟_𝐴⟩ decreases, which in turn leads to distortion. Hence, average A-site ionic radii induced distortion is influencing the reduction in Mn–O bond distances and Mn–O–Mn bond angle. This lattice distortion offers a localized state for the 𝑒_𝑔 electron and causes possible electronic phase separation within the lattice. Therefore, hopping amplitude of the charge carrier from Mn³⁺ to Mn⁴⁺ is decreasing due to suppression of delocalized hopping sites [24]. As the average radius ⟨𝑟_𝐴⟩ increases, the local lattice distortion reduces due to the shifting of Mn–O–Mn bond angle towards symmetrical side (180°). As a consequence, hopping amplitude increases which leads to shift in 𝑇_MI towards higher temperature.

Figure 2 

Temperature dependent resistivity behavior of Nd0.7−𝑥La𝑥Sr0.3MnO3 where 𝑥 = 0, 0.1 and 0.2.

In order to understand the electrical transport mechanism of manganites, the temperature-dependent resistivity is categorized into two parts: low temperature𝑇<𝑇_MI and high temperature (𝑇>𝑇_MI)  behaviors. In case of𝑇<𝑇_MI, TCR is positive (i.e., 𝑑𝜌/𝑑𝑇>0), while it is negative (𝑑𝜌/𝑑𝑇<0) for𝑇>𝑇_MI.

3.2.1. Low Temperature (𝑇<𝑇_MI) Behavior

Based on the temperature-dependent polynomial equations, the variation of resistivity at low temperatures (𝑇<𝑇_MI)and comparative strengths of the different scattering mechanisms are explained [14–19]. These equations are used to explain the low temperature resistivity of manganites which is given in Table 1. From these equations, the conduction phenomenon of different scattering mechanisms is well explained.

Table 1 

Various models found in the literature to explain the conduction mechanism in low-temperature regime (below 𝑇_MI) of manganites.

In the equations, 𝜌₀is the temperature-independent residual resistivity which arises due to the grain/domain boundary effects, scattering by impurities, defects, and domain walls [18, 25]. Since polycrystalline materials have many grain boundaries, their substantial contribution to the resistivity is proved in microwave measurement [26]. Hence, 𝜌₀plays a main role in the conduction phenomena. The term 𝜌₂𝑇²  describes the resistivity due to electron-electron scattering phenomenon [27, 28], while the term 𝜌_2.5𝑇^2.5  gives resistivity due to the phenomenon of single magnon scattering process in ferromagnetic phase [18, 26, 29]. The last terms 𝜌_4.5𝑇^4.5  ascribe due to the process of electron-magnon scattering process in the ferromagnetic region [28] and 𝜌₅𝑇⁵  contributes to the resistivity due to the phenomenon of electron-phonon interaction [19].

The experimental data of our samples are fitted with general polynomial 𝜌=𝜌₀+𝜌₂𝑇²+𝜌_4.5𝑇^4.5 equation and their related fitting graphs and parameters are shown in Figure 3 and Table 2, respectively. From these data, the corresponding fitted parameters are decreasing with increase of A-site average radius ⟨𝑟_𝐴⟩. It can be observed from the graphs of Figure 3; the residual resistivity of the original data is slightly higher than the fitting parameter values. This slight variation in residual resistivity arises may be due to the presence of grain boundary effects in the polycrystalline materials as well as localization effects particularly at low temperature. From Figure 4, it can be concluded that the residual resistivity is decreasing with increase of the average radius ⟨𝑟_𝐴⟩ of A-site. These results are consistent with the reported literature [30]. As shown in Table 2, the values of 𝜌₂and 𝜌_4.5 are obtained from fitting polynomial (Figure 3) and are also decreasing with increase of A-site average radius ⟨𝑟_𝐴⟩.

Table 2 

Theoretical fitted parameters below 𝑇_MI, and maximum %TCR (see Figure 4) with respect to average radius ⟨𝑟_𝐴⟩ of NLSMO series.

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Figure 3 

Fitted curves of the resistivity data using polynomial equation 𝜌=𝜌0+𝜌2𝑇2+𝜌4.5𝑇4.5 below the metal-insulator transition (𝑇<𝑇MI) temperature.

Figure 4 

Residual resistivity𝜌0versus A-site average radius ⟨𝑟𝐴⟩. Inset shows maximum TCR% versus A-site average radius ⟨𝑟𝐴⟩.

The observed maximum %TCR {=100[(1/𝜌)∗(𝑑𝜌/𝑑𝑇)]}versus average radius of A-site, ⟨𝑟_𝐴⟩ is given in the inset of Figure 4. The maximum %TCR values of the compounds are increasing with average radius ⟨𝑟_𝐴⟩ of A-site but %TCR values are slightly equal in 𝑥 = 0.1 and 0.2 as compared to the parent compound. It is worth to mention here that %TCR have values 2.66 (for 𝑥=0.1) and 2.65 (for 𝑥=0.2) which are independent with⟨𝑟_𝐴⟩. To design a room temperature uncooled resistive bolometer (IR detector), the potential materials are α-Si, polycrystalline SiGe, semiconducting YBCO, VO₂, and VO_x having maximum TCR values of 2% to 6% [31, 32], 7% [33], 2.9% [34], 1.7% [35, 36], and 3.3% K⁻¹)  [37], respectively. The present results of our %TCR values are comparative with the existing materials. Hence, the present study enriches the possibility of further improvement in operating around room temperature for bolometer applications without sacrificing an optimum %TCR value.

3.2.2. High Temperature (𝑇>𝑇_MI)  Behavior

In paramagnetic or semiconducting/insulating phase, the electrical resistivity generally exhibits strong temperature dependence. Several conduction mechanisms are used to explain the electrical transport properties at high temperature above 𝑇_MI. The most prevalent models are used to describe the conduction mechanisms by Mott’s variable range hopping model (𝑇_MI<𝑇<𝜃_𝐷/2)  [38] and another one small polaron hopping model (𝑇>𝜃_𝐷/2) where 𝜃_𝐷 is the Debye temperature [39]. Two models are linearly best fitted with experimental results. The experimental data of NLSMO 2 are fitted within a narrow temperature regime due to higher 𝑇_MI; however, the obtained theoretical parameters are reasonably accountable as summarized in Table 3.

Table 3 

Fitting parameters by using different models for NLSMO series above 𝑇_MI.

According to Mott’s variable range hopping model (VRH), the characteristic hopping length increases with lowering temperature and density of states are obtained from the well-established Mott law. Hopping conduction results from the states whose energies are focused in a narrow band near the vicinity of Fermi level is given by the equation 𝜌=𝜌₀exp(𝑇₀/𝑇)^1/4 where𝑇₀=16𝛼³/𝑘_𝛽𝑁(𝐸_𝐹), 𝑘_𝛽 Boltzmann’s constant, and 𝑁(𝐸_𝐹) is the density of states at the Fermi level. Here we have taken 𝛼 value 2.22 nm⁻¹ which is estimated and reported for manganites in [40]. (𝑇₀)^1/4values are measured by slope of ln(𝜌) versus 1/(𝑇)^1/4 which is useful for measurement of density of the states at the Fermi level, and the values obtained from Figure 5 are given in Table 3. The density of states 𝑁(𝐸_𝐹) are increasing with increase of A-site average radius that affect the conducting nature of the samples.

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Figure 5 

Fitted curves of ln(𝜌) versus (1/𝑇)1/4 for   𝜌=𝜌0exp(𝑇0/𝑇)1/4 equation above 𝑇MI.

Small polaron hopping models are used to explain the conductivity mechanism by either adiabatic polaron hopping 𝜌=𝜌₀𝑇exp(𝐸_𝑃/𝑘_𝛽𝑇) or nonadiabatic polaron hopping𝜌=𝜌₀𝑇^3/2exp(𝐸_𝑃/𝑘_𝛽𝑇). Here 𝜌₀ is the residual resistivity and 𝐸_𝑃 is the polaron activation energy. Jung [41] have pointed out that the higher value of 𝑁(𝐸_𝐹) is due to the effect of adiabatic small polaron hopping process. The higher order density of states 𝑁(𝐸_𝐹)(10²²)is indicating the applicability of the adiabatic hopping mechanism. Based on this, the adiabatic small polaron hopping model is used in the present investigation rather than nonadiabatic small polaron hopping model. Polaron activation energy (𝐸_𝑃)of the samples has been obtained from the slopes of ln(𝜌/𝑇)  versus  1/𝑇curves of Figure 6. These values are given in Table 3. Henceforth, it can be concluded that the decrease in polaron activation energies with the increase in average radius ⟨𝑟_𝐴⟩occur due to the shift in Mn–O–Mn bond angle towards 180° which is shown in Figure 7. The hopping of charge carrier from one site to another site in the lattice will be enhanced with the decrease of polaron activation energy.

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Figure 6 

Fitted curves of the ln(𝜌/𝑇)versus 1/𝑇 for adiabatic equation 𝜌=𝜌0𝑇exp(𝐸𝑝/𝐾𝛽𝑇) above 𝑇MI.

Figure 7 

Variation of polaron hopping energy and (𝑇0)1/4 versus A-site average radius ⟨𝑟𝐴⟩.

From Table 3, density of states𝑁(𝐸_𝐹)values are increasing with average radius⟨𝑟_𝐴⟩. This𝑁(𝐸_𝐹)specifies the carrier effective mass in other sense narrowing of the band width, consequently results in a large change in the resistivity and sharpening of the resistivity peak in the vicinity of 𝑇_MI [42]. The higher values of density of states at the Fermi level lead to a higher value of conductivity [41].

3.3. Magnetoresistance

Figure 8 shows the electrical resistivity behavior of the Nd_0.7−𝑥La_𝑥Sr_0.3MnO₃ where 𝑥 =0, 0.1, and 0.2 samples in a constant magnetic field of 5T. In the presence of an external magnetic field, the resistivity decreases significantly. This suggests that the external magnetic field (5T) facilitates the hopping of 𝑒_𝑔electron between neighbouring Mn ions, which agrees with the double exchange mechanism [13].

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Figure 8 

Variation of resistivity with temperature of NLSMO series with (5T) and without magnetic field. Inset shows the variation of %MR with temperature.

The magnetoresistance MR is defined as  %MR =100∗[𝜌(0,𝑇)−𝜌(𝐻,𝑇)]/𝜌(0,𝑇)where𝜌(0,𝑇)and 𝜌(𝐻,𝑇)are the resistivities at temperature without magnetic field and in the applied magnetic field 𝐻, respectively. The highest percentage MR (at 𝑇_MI) values of the Nd_0.7−𝑥La_𝑥Sr_0.3MnO₃ are 46%, 52%, and 50% for 𝑥 = 0, 0.1, and 0.2 as shown in inset of Figure 8. The %MR values are increasing slightly higher than the parent compound with varying of ⟨𝑟_𝐴⟩; this will be useful for potential applications [8, 43]. The misalignment of adjacent magnetic domains/grains or phase separation scenario are responsible for low-temperature MR. Hopping of charge carriers become easier across the domain wall boundaries and the resistivity decreases, which in turn leads to significant MR at low temperature. Colossal magnetoresistance at 𝑇_MI is due to the alignment of adjacent Mn ions in the presence of field which directly influence the double exchange mechanism.

4. Conclusion

In this paper, the influence of La substitution at Nd site on the electrical and magnetotransport properties in Nd_0.7−𝑥La_𝑥Sr_0.3MnO₃ (𝑥 = 0, 0.1 and 0.2) has been extensively studied. 𝑇_MI is shifting towards room temperature with La content. Conduction mechanism behavior is explained by the polynomial equation in low-temperature region and the high-temperature behavior is described using different prevalent existing models. From the present study, it can be concluded that the residual resistivity𝜌₀and the polaron activation energies are decreasing with increase of average radius of site-A. An interesting observation is that NLSMO 2 provides 𝑇_MI around the room temperature and maximum percentage of TCR values are independent with average radius ⟨𝑟_𝐴⟩ in 𝑥=0.1 and 𝑥=0.2. However, the TCR value is not satisfactorily high for the development of sensitive microbolometer; this study will be useful in future to optimize the working temperature without affecting the TCR. Moreover, the present research can further be extended for the optimization of the composition to achieve high TCR with 𝑇_MI around the room temperature for the development of MEMS-based uncooled microbolometer for night vision cameras.