The Effect of Reducing Time on the Magnetoresistance of Manganite La_0.67Sr_0.20Cu_0.10□_0.03MnO₃ at a Temperature of 30°C

Abstract

La_0.67Sr_0.20Cu_0.10□_0.03MnO₃ (“□” representing cation vacancy) polycrystalline manganite powder was synthesized by sol-gel method, which we used as parent materials. After reduced in hydrogen atmosphere for 30 and 60 minutes at a temperature of 400°C, the series bulk samples were obtained by sintering in argon atmosphere for 12 hours at 1100°C. The structure, electrical and magnetic properties, and colossal magnetoresistance of samples were researched in detail. Experiment results indicate that under an applied magnetic field of 1.8 T, the two bulk samples sintered in Ar atmosphere for 12 hours at 1100°C, with the powder reduced for 30 and 60 minutes in 400°C hydrogen atmosphere for La_0.67Sr_0.20Cu_0.10□_0.03MnO₃ parent powders, respectively, have the stable MR (11.0 ± 0.3)% and (10.0 ± 0.5)% in temperature region from 270 K to 330 K. this is important for the potential application of this kind of magnetoresistance materials.

1. Introduction

Manganite R_1-xT_xMnO₃ with ABO₃ perovskite structure, where R and T are rare earth and alkaline earth ions, respectively, has been extensively studied due to its abundant physics related to colossal magnetoresistance (CMR) and its potential application in magnetic devices [1–4]. In general, the MR magnitude of a perovskite manganite reaches the maximum only at the Curie temperature (𝑇_C), which is beyond the room temperature region generally. And the MR changes abruptly with the temperature approaching the Curie temperature. These properties severely limit the practical applications of these materials [5–8]. Many researchers have thus been concentrated on making new material system that provides low-field magnetoresistance (LFMR) around room temperature. On the other hand, recent efforts to broaden the CMR temperature range have been made by means of the Mn-site substitution or oxygen deficiency [9–12]. Brando et al. [11] studied the dependence of oxygen deficiency on electrical and magnetic properties of La_0.85Na_0.15MnO_3−δ (𝛿=0; 0.04; 0.10), they found that when the 𝛿 increased, the Curie temperature of the samples decreased. Liu et al. [12] studied the electrical properties and magnetoresistance effect and found that for La_0.67Sr_0.33MnO₃, Sr-site substituted by Cu and leading to vacancy could both change the peak temperature (TMR) of MR to room temperature and improve the peak value of MR.

In this paper, we chose La_0.67Sr_0.20Cu_0.10□_0.03MnO₃ as parent materials because its TMR is higher than room temperature, and its MR value is a little bigger. By means of reducing the powders in different temperature hydrogen atmosphere and changing the oxygen content of the parent materials, both the stabilization of MR and the peak value of MR are improved.

2. Experiment

La_0.67Sr_0.20Cu_0.10□_0.03MnO₃ polycrystalline manganite powder was prepared by sol-gel method [12], which we used as parent powder materials. After reduced in hydrogen atmosphere for 30 and 60 minutes at a temperature of 400°C, the series bulk samples were obtained by sintering in argon atmosphere for 12 hours at 1100°C.

The phase identification of samples at room temperature has been carried out by the X-ray diffraction (XRD) with an 18 kW Rigaku Max-RB diffractometer with CuK_𝛼 radiation in the 2𝜃 range of 20°–80°. The magnetization measurements were carried out with a Lake Shore vibrating sample magnetometer (VSM) in the temperature region of 120–370 K. All the Curie’s temperature measurements were performed at an applied field 0.05 T. The morphology was obtained by S-570 scanning electron microscopy (SEM). The temperature and magnetic field dependences of the resistivity were measured (using Oxford Maglab Exa Measurement System) with the standard DC four-probe method, and the applied field was parallel to the direction of current.

3. Results and Discussion

3.1. X-Ray Characterization

The structural characterization of the powder samples grinding from the bulk samples was determined. Figure 1 shows the X-ray diffraction patterns for bulk samples B1–B3. (B1: parent materials; B2: reduced for 30 min; B3: reduced for 60 min). The results indicate that the materials are a single phase with 𝑅3𝑐 perovskite phase without any other secondary or impurity phase.

Figure 1 

X-ray diffraction patterns for bulk samples B1 (a), B2 (b), and B3 (c).

We use a Rietveld refinement software Fullprof Suite to calculate the lattice parameter and crystal cell volume. The results are given in Table 1. It can be seen from Table 1 that the lattice parameter and crystal cell volume of reduced samples B2 and B3 are slightly lower comparing to parent sample B1, thus causing the Mn–O bond length of the unit cell to be slightly lower.

Table 1 

The lattice parameters and unit cell volume for bulk samples B1, B2, and B3.

3.2. Morphology of the Powder Sample

Figure 2 shows the scanning electron microscopy (SEM) morphology of powder samples B1, B2, and B3. From Figure 2, we can see that the La_0.67Sr_0.20Cu_0.10□_0.03MnO₃ parent sample B1 prepared by sol-gel method possesses homogeneously globular shape grains, and the grain size is about 100 nm. After being reduced by hydrogen, the phenomenon of conglobation in particles was improved.

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(b)

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Figure 2 

SEM morphology of powder samples B1, B2, and B3.

3.3. Magnetization versus Temperature of Power Samples

Figure 3(a) shows curves of the special magnetization 𝜎 versus temperature 𝑇 for the power samples, under an applied magnetic field of 0.05 T. Figure 3(a) shows that in the temperature region measured, the samples experienced a shift from ferromagnetic to paramagnetic; the changes have gone through a transition region, because samples still in the transition zone show varying degrees of ferromagnetism, so we define 𝑑𝜎/𝑑𝑇 which tends to zero corresponding to the temperature as the Curie temperature 𝑇_𝐶.

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(b)

Figure 3 

(a) Curves of special magnetization 𝜎 versus temperature T for samples B1 (a), B2 (b), and B3 (c). (b) Curves of 𝑑𝜎/𝑑𝑇 calculated from Figure 3(a) versus temperature 𝑇 for samples B1 (a), B2 (b), and B3 (c).

Figure 3(b) shows curves of the 𝑑𝜎/𝑑𝑇 calculated from Figure 3(a) versus temperature, in which the Curie temperature (𝑇_𝐶) of the samples is determined by 𝑑𝜎/𝑑𝑇 tending to zero. Figure 3(b) indicates that reducing treatment can decrease Curie temperature 𝑇_𝐶 of the sample obviously, and the more reducing time, the more Curie temperature 𝑇_𝐶 decreaseng. Abdelmoula et al. [13] studied the dependence of oxygen deficiency on electrical and magnetic properties of La_0.7Sr_0.3MnO_3-δ(0≤𝛿≤0.15), they found that when the 𝛿 increased, the Curie temperature of the samples decreased, which was the same as our conclusion. This phenomenon can be explained by the weakness of double exchange theory. Reducing treatment could result in the deficiency of oxygen. With oxygen deficiency increased, Mn⁴⁺ ion is reduced, Mn³⁺ ion increased, and Rn (the ratio of Mn⁴⁺ and Mn³⁺ ion) decreased quickly, causing 𝑇_𝐶 to lower temperature.

Figure 4 shows the magnetic hysteresis loop of the powder samples at room temperature (300 K).

Figure 4 

The magnetic hysteresis loop for samples B1 (a), B2 (b), and B3 (c) at room temperature (300 K).

As shown in Figure 4, at room temperature (300 K), our samples are in the transition region from ferromagnetic to paramagnetic; the Curie temperature is higher than room temperature; magnetization has not yet reached saturation under an applied magnetic field of 500 mT.

Table 2 shows the special saturation magnetization 𝜎 (Am²/kg) under an applied magnetic field 500 mT. It can be seen from Table 2 that: Reducing treatment could reduce the special magnetization 𝜎 significantly, as the reducing time longer, 𝜎 decreases more.

Table 2 

The Curie temperature 𝑇_𝐶 (K) and special saturation magnetization 𝜎 (Am²/kg) for samples B1 (a), B2 (b), and B3  (c).

3.4. Colossal Magnetoresistance of Samples

The temperature dependence of the resistivity measured in zero field and in an applied field (1.8 T) for the samples B1 (a), B2 (b), and B3 (c) is plotted in Figure 5. From Figure 5, we know that with the increase of temperature, the conductivity of the samples experienced a metal-semiconductor transition. The definition of metal-semiconductor transition temperature TMI for resistivity and temperature curve of resistivity maximum point corresponds to the temperature. For sample B1 (a), as in the temperature deviation TMI, its resistivity decreases rapidly, similarely to most of what is reported in the reference. But for samples B2 (b) and B3 (c), their resistivity changes a little near TMI.

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Figure 5 

Temperature dependence of the resistivity measured in zero field and in an applied field (1.8 T) for samples B1 (a), B2 (b), and B3 (c). The corresponding magnetoresistances are also indicated.

Define the magnetoresistance by MR(%)=[(𝜌₀−𝜌_𝐻)/𝜌₀]×100%, where 𝜌₀ and 𝜌_𝐻 stand for the resistivities at 0 and 1.8 T, different from most of the reports in the past; as the temperature decreases, the changes in samples B2 and B3 can be divided into three stages: in the high temperature area, MR increases as the temperature decreases; in near room temperature, MR is basically unchanged,in the low-temperature area, with decreasing temperature MR continues to increase.

For B2 and B3 samples at room temperature (300 K), their MR was 11.0% and 10.0%, not only higher than the sample B1 (9.5%), but also the temperature stability of MR is better; in the temperature range from 270 K to 330 K, their MR was maintained at 11.0% (±0.3%) and 10.0% (±0.5%) almost flat with temperature, which is important for the potential application of this kind of magnetoresistance materials.

4. Conclusions

By using reducing treatment, we can improve the MR of La_0.67Sr_0.20Cu_0.10□_0.03MnO₃ polycrystalline manganite. The reduced samples B2 and B3 have the stable MR (11.0±0.3)% and (10.0±0.5)% in a temperature range between 270 K and 330 K. this is important for the potential application of this kind of magnetoresistance materials.

Acknowledgment

This work was supported by Science and Technology Research Program for Colleges and Universities in Hebei Province, China (Grant no. Z2012002) and supported by foundation science of Hebei united university (Grant no. Z201216).