Journal of Nanomaterials

Volume 2014 (2014), Article ID 703072, 9 pages

http://dx.doi.org/10.1155/2014/703072

Research Article

Effect of Mn-Site for Al Substitution on Structural, Electrical and Magnetic Properties in Thin Films by Sol-Gel Method

H. Abdullah,¹ M. Syafiq Zulfakar,¹ and S. Kien Chen²

¹Department of Electrical, Electronic and Systems Engineering, Faculty Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia

²Physics Department, Faculty of Sciences, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia

Received 10 October 2013; Revised 14 January 2014; Accepted 28 January 2014; Published 26 March 2014

Academic Editor: Wanqin Jin

Copyright © 2014 H. Abdullah et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Nanocrystalline (, 0.05, 0.10, 0.15, 0.20, and 0.25) thin films have been prepared on quartz substrates by sol-gel method. The structural and morphology studies were investigated via X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM). XRD graph patterns show rhombohedral distorted perovskite structures. FESEM images show that the average grain size decreased as the concentration of increased. Electrical property was investigated using four-point probe technique. Resistivity results show that metal-insulator transition (MIT) temperatures () decreased when the concentration of increased. shifted to lower temperature when the concentration of increased. The data was analyzed based on theoretical models, where the ferromagnetic resistivity is followed with the equation , where is due to the significance of grain boundary effects and a second-term ~ appears that might be applied to the electrons scattering. In the high temperature regime , the resistivity data can be well described by small polaron hopping (SPH) and variable range hopping (VRH) mechanisms. Magnetic property was investigated using a vibration sample magnetometer. All samples that were obtained showed hysteresis curve with the highest value of magnetization for sample .

1. Introduction

The discovery of the colossal magnetoresistance (CMR) effect in epitaxial thin films of perovskite manganites, where R is alkaline earth elements and Ln is rare-earth element, gives an effect for usage in data storage and sensing application [1, 2]. Bulk compound is typically different with the properties of CMR thin films where in bulk compounds, doping concentration can change the CMR effects [3].

Perovskite-type lanthanum strontium manganate exhibits colossal magnetoresistance (CMR) with Curie temperature more than 370 K, where it can be operated at a room temperature [4]. The manganese oxide not only showed a metallic conduction below Curie temperature but also enhanced the ferromagnetic interaction when La³⁺ ions are replaced with alkaline earth elements or also known as divalent metal ions (Ca²⁺, Sr²⁺, Ba²⁺) in perovskite oxide structures [5]. By comparing to between Ln site with R site doping will not only modify the crucial Mn³⁺–O²⁻–Mn⁴⁺ interaction that will bring many complicated between Mn ions and dopands [6]. These compounds are Mn³⁺ and Mn⁴⁺ ions, which play an important part in double-exchange interaction when substituted with metallic resistivity. Distortion of John-Teller effect could give different result for transport properties, which removes the double degeneracy of Mn in orbital and provides a mechanism for coupling among the electronic, magnetic and lattice degrees of freedom [7].

The aim for this work is to investigate the effect of substituting Al at Mn sites with the structural and electrical properties of samples with , 0.05, 0.10, 0.15, 0.20, and 0.25.

2. Experimental Details

samples were prepared by using sol-gel method. The amounts of La(NO₃)₃ · H₂O, Sr(NO₃)₂, Mn(NO₃)₂ and Al₂O₃ were weighed accurately and dissolved in aqueous solutions that were added into deionized water, nitric acid, and triethanolamine (TEA). The produced solutions were dissolved completely and were stirred and heated at 90°C for 3 h to evaporate the excess solvent and water. Quartz substrate with dimensions (10 mm × 10 mm) was washed with acetone and methanol. The clean substrates were rinsed with distilled water and dried. All samples were deposited on the quartz substrate by using spin coating technique. The produced solutions were spin coated with 5 layers onto the substrate with 900 rpm for 25 s for each coating. All samples were annealed with temperature at 650°C for 1 h with heating and cooling rates of 1°C/min. The flowchart of the preparation of LSMOA thin film samples as shown in Figure 1. The morphological structures were investigated using X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM). Electrical property was investigated using 4-point probe technique within temperature range of 200–300 K. The magnetic property was investigated using a vibration sample magnetometer (VSM) analysis at room temperature with a magnetic field range of −10 kOe to +10 kOe. All experimental results were characterized by good repeatability.

Figure 1: Flowchart for the preparation of LSMAO sol-gel and thin films.

3. Results and Discussion

Field emission scanning electron microscope (FESEM) analysis was used to investigate the morphological structure of thin film samples with different concentrations of . Based on Figure 2, the clear image of grain was observed at compared to the other samples. Based on the average grain size of the sample that was measured during the SEM analysis, it shows that when the concentration of is increased, the size of grain decreased. Structural morphology of samples becomes more compact and homogenous when the concentration of increased. The grain size of all samples was observed with round shape, but for sample () it is shown that the particles are submerged between each other. Similar finding has also reported that, by increasing the concentration, the nanoparticles were tightly tied together and the size of the particles reduced for sample [8]. This was also reported by Abdullah and Halim [9], where the reduction of size and linkage between the particles can be clearly seen, as the samples were doped for several concentrations. Based on Figure 3, it shows the graph of particles size against concentration. The grain sizes decreased as the concentration of increased. The reduction size of Al plays the main role where the Al³⁺ ion is smaller than Mn³⁺/Mn⁴⁺ ion. This indicates that more Al³⁺ ions will be substituted or take place in Mn³⁺/Mn⁴⁺ ion.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 2: FESEM image of with different concentrations of (a) , (b) , (c) , (d) , (e) , and (f) .

Figure 3: Particle size of as a function of concentration .

Morphological structure of thin film samples, with concentration of , 0.05, 0.10, 0.15, 0.20, and 0.25, was characterized using XRD analysis. XRD graph pattern indicates no clear formation of single phase with rhombohedral distorted perovskite structures as shown in Figure 4. Sahu et al. [10] have reported that the formation of LSMO phase calcined at 600°C. There are several peaks observed with Miller indices (104) and (111) plane at , 38.24°, and 39.96°. These indicate that LSMAO thin film samples for and have a fine crystal structure compared with other samples. The intensity of the width diffraction decrease when the concentration of increased. This showed that when the level of doping increased, it may have effect on the crystal structures of the LSMAO sample.

Figure 4: XRD patterns of with different concentrations of (a) , (b) , (c) , (d) , (e) , and (f) .

Figure 5 shows a resistivity, of the systems. All samples show a peak of a resistivity curve where it has the highest resistivity at the particular temperature which was known as peak temperature (). According to the plot, it shows that when the temperature increases, the resistivity value decreases. This indicates a semiconducting behavior. All samples follow the metal-insulator transition (MIT) at . In a metallic region, in a semiconducting region the plot slightly decreases. Based on the plot, should shifted at lower temperature for samples and 0.05, but for sample , 0.20 and 0.25 showing otherwise. Figure 6 shows the schematic dependents of the peak resistivity temperature () on the Al concentration. This phenomenon indicates the electron hopping between Mn³⁺ and Mn⁴⁺ ions. As a result, double exchange has been suppressed as the concentration increased. This result indicates the weakened double exchange (DE) ferromagnetic interactions. It shows that when the concentration of increased, peaks of resistivity decreased from to 0.05 and for samples , 0.20, and 0.25 increased, respectively, and do not follow the theory of double exchange (DE) ferromagnetic interactions. Peak temperature, , shifted to lower temperature when the concentration of increased because of the charge transfer mechanisms of Mn³⁺–O²⁻–Mn⁴⁺ network which has been replaced with Mn³⁺–O²⁻–Al⁴⁺. This finding has also been reported by Abdullah et al. [11] as when the concentration of Al increased, this may affect the charge transfer mechanism. This is obviously because of replacing conducting regions of a conducting matrix by insulating regions.

Figure 5: The temperature dependence of resistivity for with concentration; (a) , (b) , (c) , (d) , (e) , and (f) .

Figure 6: Peak resistivity temperature, (), as a function of concentration .

Figure 7 shows plotted graph for resistivity data against temperature. Below , according to double exchange theory, the mechanism of electronic conduction can be applied. The Mn³⁺–O–Mn⁴⁺ coupling allows conduction through charge transfer from half-filled to empty orbital. In this regime, the metallic behavior of the samples can be explained in terms of electron-magnon scattering of the carriers. The resistivity data fit quite well with the following expression:

where the first term corresponds to the resistivity arising due to domain, grain boundary, and other temperature independent scattering mechanisms. The second term appears as a result of electron-magnon scattering in ferromagnetic phase. Based on Table 1, the values of parameters increased with the disorder increment as values of for and 0.15 decreased and the values for increased. As the concentration of doping increased, the value of should increase due to the suppression of spin fluctuation. Thus, the spin scattering cannot be neglected in this regime as the measured data can be best explained by electron-magnon scattering. The best fitted parameters are given in Table 1. It is noted that the values of both and increase with the increase of . As the doping increases, the size of the domain boundary decreases and becomes larger. It means that both these parameters are increasing with decreasing grain size, which may be an evidence for increasing the scattering processes due to the reduction of grains of the material. Thus decreasing the grain size may increase the grain boundary region and hence the net grain boundary scattering term as well as electron-magnon scattering term. Therefore, grain boundary plays a dominant role in the conduction process, and it acts as the region of reduced scattering centre for conduction electron. The increase of with is due to spin fluctuation [9].

Table 1: Best fitted parameters obtained from the fitting of the low temperature resistivity data in the metallic regime of manganites with .

Concentration
(cm )
(cm )

0.00
−8459.91
0.11378

0.05
−9483.61
0.11758

0.10
−9284.09
0.00671

0.15
−6841.67
0.00511

0.20
−517.36
0.16998

0.25
−130.44
0.12156

(a)

(b)

(c)

(d)

(e)

(f)

Figure 7: Resistivity data showing dependence for system. Solid lines are the best fit with equation .

3.1. The High-Temperature () Regime

The high-temperature electronic transport properties among these materials may be divided into 2 distinct phenomena based on 2 different models, each one predicting different temperature dependence for the resistivity. For example, to explain the conduction just above , the variable range hopping (VRH) model has been suggested, while the small polaron hopping model is considered at temperatures beyond (where is the Debye temperature). In the latter case, a polaron can be thought to be trapped inside a local energy well of height and, when the field is applied, one side of the well is lowered slightly with respect to the other. This makes the polaron likely to hop more in that direction [11–13].

3.1.1. Variable Range Hopping (VRH) Model ()

The samples show semiconducting-like behaviors for . The transport data in the semiconducting regime of compounds have been analysed by the Mott variable range hopping (Mott-VRH) model [14], according to equation

where is a preexponential factor, is a constant , and is the density of state (DOS) at the Fermi level, which is calculated from the slope of the log versus curves shown in Figure 8.

(a)

(b)

Figure 8: The slope of the log versus curves.

Equation (2) is used to explain the conductivity data at temperatures for which . From the resistivity data, it can be seen that temperatures above are fitted by plotting log() versus . values or it can be estimated based on the graph for , where deviation from linearity occurs in the temperature region above . values for each of the samples were calculated from the slopes of log() versus . Finally, using the values, , the Fermi level for each material was also obtained. All of the values that were obtained are presented in Table 2. For the samples with higher resistivity values, the VRH region becomes smaller. At the high temperature , conductivity data are better fitted with the small polaron hopping (SPH) model.

Table 2: Values of , phonon frequency, (Hz), the density of states, , at Fermi level, and the activation energies from the resistivity plot.

(K)

() (eV⁻¹ cm⁻³) 10¹⁶
/2
(Hz) 10¹²
(meV)

0.00
282
37428689.3
5.4267
284.0283
5.9183
977.521

0.05
256
46396954.9
4.3777
258.003
5.3759
711.172

0.10
270
2798410000
0.07258
271.001
5.6469
867.893

0.15
233
280200294
0.72489
252.000
5.2509
1448.356

0.20
244
1600000000
0.01269
247.003
5.1468
1760.435

0.25
245
9927117782
0.0204
249.003
5.1885
3019.785

3.1.2. Small Polaron Hopping Model ()

For the conduction mechanism of these materials at high temperatures (), the resistivity data of the Al-doped and undoped LSMO films can be well fitted with the thermally activated small polaron hopping (SPH) model of Mott, given by equation

where is the residual resistivity and is the activation energy. is the optical phonon frequency and can be estimated from the relation . The resistivity has been replotted as versus , and, from the slope of the curve, the activation energy, , can be estimated (using Table 3). The plots are shown in Figure 9. Table 2 also shows the value of the phonon frequency against the concentration, . It indicates that the frequency of the lattice wave decreases with increasing Nd content. As the concentration of Nd increased, the vibrational energy in periodic solids decreased.

Table 3: Value of magnetization with concentration of Al at 10 kOe, measured at room temperature.

Concentration
Magnetization (emu/g)

0.00
0.0457

0.05
0.1855

0.10
0.0287

0.15
0.1293

0.20
0.0537

0.25
0.0717

(a)

(b)

Figure 9: Plot of ln versus for .

Jung [15] suggested that higher values (2 to 3 orders than those of the usual oxide semiconductors) for the of the present manganite system could be due to the high values of conductivity. Furthermore, these higher values of are clear signatures of the applicability of adiabatic small polaron hopping mechanisms. Based on this fact, it has been concluded that the adiabatic small polaron hopping model explains the conduction mechanism of the samples used in the present investigation. Furthermore, it is also clear from Table 2 that the values of are less for the first 2 samples as compared to the remaining 2 samples. This may be attributed to oxygen deficiency. In the case of the last 2 samples, the oxygen deficiency induces an increase in the bending of the Mn–O–Mn bond angle, thereby narrowing the bandwidth and enhancing the effective mass of the charge carrier. Due to this fact, the effective band gap increases with increasing oxygen deficiency. Therefore, higher values for the activation energies are needed for the charge carriers to overcome this band gap [16]. The activation energy () values increase [13] and decrease depending on the grain size, and the observed behaviour may be explained as follows. It is known that, with increasing grain size, the interconnectivity between grains, which enhances the possibility of conduction electrons to hop to neighbouring sites, increases [17], and the value of decreases.

Magnetic property was investigated using a vibration sample magnetometer (VSM) at room temperature. All samples were measured by applying magnetic field in a range −10 kOe to +10 kOe. Figure 10 shows the hysteresis curve graph pattern for sample with concentration of , 0.05, 0.10, 0.15, 0.20, and 0.25. Graph pattern that was obtained was known as a magnetization curve or hysteresis curve. Based on Table 3, it indicates that the highest value of magnetization is for samples and 0.15 where the values are 0.1855 emu/g and 0.12933 emu/g. Samples and 0.15 show the highest value of magnetization compared to other samples, which indicates weak ferromagnetic properties. Different finding has reported LSMO particles that were prepared by sol-gel method. The magnetic properties show a good ferromagnetic behavior at room temperature which gives 29.60 emu/g. It indicates that the magnetic ordering of Mn³⁺ and Mn⁴⁺ ions in LSMO might be improved under the influence of a magnetic field [18]. The other samples may indicate the properties of weak paramagnetic due to the fact that the value of magnetization is low where the effect can only be measured by VSM analysis. The slope of loop for samples is not narrow where it was reported by Krishna and Venugopal Reddy [19] in which the loop width of PSMO, PPMO, and PBMO is narrow, which gives an impression that these three samples might have soft magnetic nature.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 10: Vibration sample magnetometer (VSM) pattern of thin film at room temperature with concentration of (a) , (b) , (c) , (d) , (e) , and (f) .

4. Conclusions

Nanocrystalline (, 0.05, 0.10, 0.15, 0.20, and 0.25) thin film samples were successfully prepared on quartz substrates by sol-gel method prepared at room temperature. Based on SEM images, samples with Al³⁺ doped show a round shape and become more compact, when the composition of concentrations increased. XRD graph pattern of samples showed a single phase with rhombohedral distorted perovskite structures, where the highest peaks are (104) and (111). Temperature dependence shows that is the influence by the concentration of . Undoped sample gave the higher compared to the others. shifted to lower temperature when the concentration of increased. Metallic conduction in these systems follows dependences, indicating the importance of electron-magnon contribution. The electrical conduction mechanism of these materials at low temperatures may be due to the electron-magnon scattering processes. While, in the high temperature regime , the conduction can be explained by the variable range hopping (VRH) model and the small polaron hopping (SPH) mechanisms. SPH conduction is observed above the MIT temperature for all of the samples. Magnetic property shows a weak ferromagnetic property in magnetic field −10 kOe to +10 kOe.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors gratefully acknowledge IMEN, Universiti Kebangsaan Malaysia, for the permission to use all the facilities and the staff for the support to finish this paper. The Ministry of Higher Education (MOHE) is gratefully acknowledged for the Grant under FRGS vote: UKM-KK-07-FRGS0026-2009 (Growth of Nanostructured Colossal Magnetoresistive Material for Low-Field Magnetic Sensing Device).

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