Journal of Materials Research and Technology Journal of Materials Research and Technology
Original Article
Effect of Mg addition on LaMnO3 ceramic system
García Iván Supelanoa,, , Aura J. Barón-Gonzáleza, Armando Sarmiento Santosa, César Ortíza, Julieth A. Mejía Gómezb, Carlos A. Parra Vargasa
a Universidad Pedagógica y Tecnológica de Colombia, Tunja 150003, Colombia
b Universidad Antonio Nariño, Boyacá 150003, Colombia
Received 07 October 2016, Accepted 28 May 2017
Abstract

In the present work we report the synthesis of La1−xMgxMnO3 (with x=0.10, 0.25, and 0.50) polycrystalline samples based on LaMnO3 (LMO) antiferromagnetic with low Neel temperature and insulating behavior. Structure was analyzed by Rietveld fitting of XRD patterns at room temperature by FullProf software, these show that La1−xMgxMnO3 (x=0.10, 0.25, 0.50) samples crystallize in the space group R-3c. Magnetic and electrical measurements exhibits ferromagnetic and semiconductor like behavior. A decreases of TC is observed when x doping value increases.

Keywords
Magnetic materials, Doped LaMnO3
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1Introduction

LaMnO3 (LMO) is an inorganic compound with perovskite structure. It is an A-type antiferromagnetic insulator with a low Neel temperature [1–4]. Depending on the synthesis process, LMO samples can be obtained as thin films [5], monocrystals [3,6] and polycrystalline powders [7]. To prepare stoichiometric LMO ceramics, the synthesis has to be carried out in low partial pressures of oxygen. In contrast, non-stoichiometric LaMnO3+d is obtained when the production is made in air [8].

The presence of Mn4+ influences the structural and magnetic behavior of LMO samples. For instance, it has been reported that for 12% of Mn4+ content, the structure is orthorhombic with antiferromagnetic ordering. Whereas for higher Mn4+ content, the structure is rhombohedral or cubic exhibiting ferromagnetism [9].

Partial substitution of lanthanum ions [3,9,10] or Mn ions [11,12] has an effect on the physical properties of LaMnO3 such as structural changes and Mn4+/Mn3+ ratio. The last one promotes phenomena such as charge and orbital ordering, which are controlled by interaction between electrons from eg and t2g levels [13].

The aim of this paper is to present the results from the production and characterization of polycrystalline La1−xMgxMnO3 (x=0.10, 0.25, 0.50).

2Experimental

Polycrystalline La1−xMgxMnO3 (x=0.10, 0.25, 0.50) samples were prepared by the usual solid state reaction method from a stoichiometric mixture of high purity La2O3 (99.99%), MgO (99.995%) and MnO2 (99.99%) in air at sintering temperature of 1150°C by 24h at 2.5°C/min of heating and cooling rate. X-ray powder diffraction (XRD) data were collected on PANalytical X’Pert's X ray diffractometer with Kα-Cu radiation at room temperature. XRD patterns were studied by Rietveld method with FullProf software. Surface was analyzed by scanning electron microscopy (SEM) with JEOL JSM-6490LV microscope and elemental analysis (EDS) with EDS Inca Energy 250 analyzer. Magnetization measurements were performed in vibrating magnetometer Versalab and Physical Property Measurements System (PPMS) of Quantum-Design. Electrical resistance measurements were performed with PPMS by using the standard 4-probe method.

3Results and discussion

XRD patterns for La1−xMgxMnO3 samples are displayed in Fig. 1(a). Based on these diffraction patterns, it can be observed that lanthanum manganites Mg-doped range from 0.25 to 0.50 exhibit the same phase. However, for the samples with x=0.25 and 0.5, additional reflections at 2θ=35.57° and 2θ=36.37° are detected. This corresponds to a secondary phase Mn3O4 (peaks marked as Δ).

Fig. 1.
(0.46MB).

(a) XRD patterns of La1−xMgxMnO3 system. Rietveld analysis for (b) La0.9Mg0.1MnO3, (c) La0.75Mg0.25MnO3 and (d) La0.5Mg0.5MnO3 sample at 300K.

Rietveld analysis (Fig. 1(b–d)) suggests that all the samples crystallize in the rhombohedral R-3c space group. This result differs from that reported by Zhao et al. [14]. They obtained the Pbnm structure for samples with x=0.05, 0.10, 0.20, 0.33 and 0.40. This difference can be attributed to different conditions of synthesis. The structural parameters of La1−xMgxMnO3 (x=0.10, 0.25, 0.50) were refined by fitting the XRD patterns using the program FullProf (Table 1). Substitution of La with Mg causes changes in the lattice parameters due to smaller Mg ionic radius than La ionic radius.

Table 1.

Structural details of La1−xMgxMnO3 obtained by Rietveld refinement of DRX. La/Mg occupy 6a Wyckoff position (0 0 ¼), Mn occupy 6b (0 0 0) and O occupy 18e (x 0 ¼).

  La0.9Mg0.1MnO3  La0.75Mg0.25MnO3  La0.5Mg0.5MnO3 
Lattice parameters
a=b (Å)  5.534(5)  5.529(4)  5.526(1) 
c (Å)  13.354(5)  13.345(3)  13.339(9) 
Statistical parameters of fitting
RBragg (%)  6.28  7.27  6.12 
Rf (%)  4.20  5.34  6.04 
Atomic positions
O x  0.44699  0.43746  0.44501 
Cell volume V (Å3)
  354.267  353.351  352.784 
  Mn–O–Mn bond angle (°)
  162.8(6)  164.2(5)  163.9(9) 
  Mn–O bond length (Å)
  1.969(6)  1.965(3)  1.964(6) 

Fig. 2 shows the scanning electron micrograph (SEM) of La1−xMgxMnO3 (x=0.10, 0.25, 0.50) samples. These images show that, there is no significant change in the morphology with increasing Mg concentration in LMO. The grain size distributions for each sample are shown in the insets of Fig. 2. For x=0.1 the grain size ranges between 3.23 and 1.83μm, for x=0.25 between 2.45 and 1.65μm, and for x=0.5 between 1.93 and 1.13μm. Hence, it is concluded that in La1−xMgxMnO3 (x=0.10, 0.25, 0.50), the Mg substitution at the lanthanum positions induces decreasing of grain size. This result is in agreement with the decreasing of cell volume that is clearly observed from the Rietveld analysis (Table 1). EDS analysis ruled out the presence of other traces elements.

Fig. 2.
(0.4MB).

SEM micrographs of (a) La0.9Mg0.1MnO3, (b) La0.75Mg0.25MnO3 and (c) La0.5Mg0.5MnO3.

The ZFC-FC M(T) curves of La1−xMgxMnO3 (x=0.10, 0.25, 0.50) are illustrated in Fig. 3. The plots evidence the ferromagnetic behavior for all the samples. In order to understand the influence of Mg doping at La site, the Curie temperature (TC) change of the samples (TC determined by the minimum value of the derivative of magnetization as a function of the temperature in the ZFC curve) is shown in the inset of Fig. 3. The values of TC were found to decrease with Mg content.

Fig. 3.
(0.12MB).

Curves ZFC-FC for the La1−xMgxMnO3 (x=0.1, 0.25 and 0.5) samples. Inset displays the TC obtained for the three samples produced in this work plus data for x=0.05 [2], 0.33 [14], 0.40 [14].

The M vs H hysteresis plot of La1−xMgxMnO3 (x=0.10, 0.25, 0.50) at 5 and 50K (Fig. 4(a and b)) indicate ferromagnetic behavior at low temperatures. Whereas at 300K (Fig. 4(c) the behavior is paramagnetic.

Fig. 4.
(0.15MB).

Curves M vs H for La1−xMgxMnO3 (x=0.1, 0.25, 0.5) samples measured with: (a) PPMS system to 5K, (b) Versalab system to 50K and (c) VersaLab system to 300K.

La1−x(Ca, Sr)xMnO3 samples exhibit similar behavior [9]. In this work, the content of Mn4+ increases as x value increases. Mn–O–Mn bond angle values were found to be 164° approximately. Mn–O bond length values range between 1.92Å and 1.96Å. Furthermore, there is colossal magnetoresistance (CMR) which is observed only for values of Mn–O bond length below 1.97Å. In our study, Mn–O–Mn bond angle values lie between 162.8(6)° and 164.2(5)°. The values of Mn–O bond length are around 1.969(6)Å. However, CMR was not observed.

Log R is plotted in Fig. 5 as a function of the inverse of the absolute temperature. Samples with x=0.10, 0.25 and 0.50 present a linear behavior, first and second exhibit an inflection at 139K and 197K, above TC. It suggests that samples have a normal semiconductor behavior. Semiconducting behavior also occurs in the sample with x=0.05 [2]. Inset in Fig. 5 shows electrical resistance as a function of the temperature, at applied field of H=0, and H=1kOe. For sample with x=0.10, resistance increases while the temperature decreases, there is no evidence of magnetoresistance. A similar behavior was found in the other samples.

Fig. 5.
(0.1MB).

Electrical resistance logarithm as a function of the inverse of temperature for La1−xMgxMnO3 (x=0.1, 0.25, 0.5) samples. Inset: electrical resistance as a function of temperature for the La0.9Mg0.1MnO3 sample.

For this system, based on LMO, the results suggest that previously values of Mn–O–Mn bond angles and Mn–O bond lengths are the reason why the La1−xMgxMnO3 (x=0.10, 0.25, and 0.50) system present ferromagnetism and does not present CMR.

Comparing the present results with those reported by Zhao et al. [14], the TC obtained is in the same order of magnitude, which permitted to conclude that the better doping value of x is around 0.1. Table 2 listed the values of TC.

Table 2.

Data of magnetization: Curie temperature (TC), remnant magnetization (Mr), coercive field (Hc) and saturation magnetization (Ms).

Doping x  TC (K)  Mr (emu/g) at 5Hc (Oe) at 5Ms (emu/g) at 5
0.1  157  1.53  15  79.35 
0.25  106  5.86  43  49.59 
0.5  73  3.05  137  20.88 
4Conclusions

Polycrystalline La1−xMgxMnO3 (x=0.10, 0.25, 0.50) samples were synthetized by solid state reaction method. The system crystallize in rhombohedral structure (R-3c) at room temperature. The inclusion of Mg in the system, induce interactions which arise the ferromagnetic phase at low temperature. The system present a decrease of TC when the doping increases. The electrical Response have semiconductor behavior, in which the resistance is proportional the inverse of the temperature. Results suggest that this behavior is due to the strong influence of Mn–O–Mn bond angles and Mn–O bond lengths values.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

To Institut de Ciéncia de Materials de Barcelona (CSIC), Dr. Carlos Frontera and Dr. Bernat Bozzo. This work was supported by the Gobernación de Boyacá (N° 733 Colciencias).

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Corresponding author. (García Iván Supelano ivan.supelano@uptc.edu.co)
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