Electrical Properties of La0.67Sr0.33Mn0.8Ni0.2O3 Synthesized by Sol Gel Method

The previous study of lanthanum manganites showed that physical properties of LaMnO3 compound has changed when Sr doped the La site. La0.67Sr0.33MnO3 compound give the highest TC = 370 K and showed metal insulator transition. It happens as a result of changed in Mn3+/Mn4+ ratio. Its indicated that Mn has the most important rule to the physical properties changed. In order to shift Tc of the compound to the near room temperature, Ni doped was given to Mn site of LSMO. In this paper, 20% Mn site was doped by Ni. La0.67Sr0.33Mn0.8Ni0.2O3 compound synthesized by sol gel method and characterized using XRD. Then, single phase compound resisitivity was measured to see the electrical properties this compound. The XRD result showed that sample have single phase with rhombohedral structure and R-3c space group. Resistivity as a temperature function graph does not show resistivity peak and metal insulator transition. The resistivity measurement showed that below 300 K, resistivity inscreased with decreased of temperature that indicates insulator behavior. It caused by decreased of double exchange interaction and increase of superexchange interaction. From this result, we can conclude that Ni doped decreased electrical properties of the sample.


Introduction
Lanthanum manganites La1-xAxMnO3 (A = Sr, Ba, Ca) have been studied and become subject of research interest because of their magnetic and electrical properties which can be varied and applied in magnetic random access memory, and magnetic sensor [1,2]. Doping in La site would change structure and electrical properties of material. The previous study of lanthanum manganites showed that physical properties of LaMnO3 compound has changed when Sr doped the La site. La0.67Sr0.33MnO3 compound give the highest TC = 370 K and showed metal insulator transition [3]. It happens as a result of changed in Mn 3+ /Mn 4+ ratio. Its indicated that Mn has the most important rule to the physical properties changed.
In order to shift Tc of the compound to the near room temperature, Ni doped was given to Mn site of LSMO. In this paper, 20% Mn site was doped by Ni. La0.67Sr0.33Mn0.8Ni0.2O3 compound was synthesized by sol gel method. Sample was characterized using XRD and the resistivity was measured using cryogenic magnetometer. starting material mixed and stirred at 80C. Ammonia solution was added to the mixed solution to adjust pH until reached 5,6. Then, the solution was stirred and dried it at 120C for 2 h until sol gel formed. The gel then calcined at 500C, grinded, and re-calcined at 850C. The re-calcined sample was pressed into pellet and sintered at 1200C for 2 h. The sample then characterized using XRD, while the resistivity was measured using cryogenic magnetometer.

Structure Characterization
Structure of sample is characterized using X-Ray Diffraction (XRD), and the result shows in figure 1. The figure 1 shows that sample has single phase with no impurity. The result of XRD characterization is analyzed using Rietveld refinement analysis method. The Rietveld refinement results show that sample has Rhombohedral structure with R-3c space group. Refinement result of La0.67Sr0.33Mn0.8Ni0.2O3 sample compared with La0.67Sr0.33MnO3 sample [4] and summarized in table 1.

Resistivity
The result of resistivity measurement show in figure 2. It show that sample has insulator behavior in temperature range 100-300 K with resistivity value range 0.01-0.34 .m. The insulator behavior is indicated by increasing of resistivity with temperature decreased. The result does not show resistivity peak as shown in La0.67Sr0.33MnO3 sample and denote higher resistivity compared with La0.67Sr0.33MnO3 sample resistivity value range [4]. When Ni doped Mn site, there are more localization of electron, because the ratio of Mn 3+ /Mn 4+ will be decreased and suppress the Double Exchange (DE) interaction [5]. Resisitivity of this sample could be analyzed using Small Polaron Hoping (SPH) model with equation (1) [5,6,7,8].
Where A E is the sum of the activation energy required to create free carriers and the energy that activates carriers for hoping [5], , and   is residual resistivity [6,7,8]. When Ni doped Mn site, DE interaction will be reduced due to decrease of Mn 3+ -O 2--Mn 4+ bond. In the insulator state as shown by La0.67Sr0.33Mn0.8Ni0.2O3 sample, many electrons are localized, due to the decrease of Mn 3+ /Mn 4+ ratio and the coupling between electrons and phonons that form polarons. When given temperature increases, the energy supplied is also getting bigger, and more polaron activated, so that the resistivity decreases. It explain the insulator behavior that showed by decreases of resistivity when temperature increased.
Value of obtained fitting parameters of La0.67Sr0.33Mn0.8Ni0.2O3 sample are showed in table 2. Quality of fitting is evaluated by the value of squared linear correlation coefficients (R 2 ) [6]. We obtained R 2 value above 99.9%, and it means that the fitting that has been done has a good quality.

Conclusion
La0.67Sr0.33Mn0.8Ni0.2O3 sample was synthesized by sol gel method. XRD result showed that sample has single phase with no peak of impurity with rombhohedral structure and R-3c space group. The resistivity measurement showed that sample has insulator behavior with resistivity value range 0.01-0.34 .m in temperature range 100-300 K. Analysis of resistivity found that small polaron hoping (SPH) model fit the experimental data with R 2 value 99.93%.