Abstract
Zinc substituted magnesium ferrite nanomaterials ( = 0, 0.1, 0.3, 0.5, 0.7) powders have been prepared by a sol-gel autocombustion method. The lattice parameter increases with increase in Zn concentration, but average crystallite size tends to decrease by increasing the zinc content. SEM results indicate the distribution of grains and morphology of the samples. Some particles are agglomerated due to the presence of magnetic interactions among particles. Room temperature Mössbauer spectra of shows that the A Mössbauer absorption area decreases and the B Mössbauer absorption area increases with zinc concentration increasing. The change of the saturation magnetization can be explained with Néel’s theory. It was confirmed that the transition from ferrimagnetic to superparamagnetic behaviour depends on increase in zinc concentration by Mössbauer spectra at room temperature. Saturation magnetization increases and coercivity decreases with Zn content increasing.
1. Introduction
Magnesium ferrite is a soft magnetic n-type semiconducting material, which is used in catalysis, gas sensors, transformers, ferrofluids, fuel cells, and magnet core of coils [1, 2]. It has been reported [3, 4] that the structure of magnesium ferrite is partially inverse spinel, with 0.1 of ions and 0.9 of ions distributed over the A and B sites in the following way . The magnetic properties of nonmagnetic Zn substituted ferrites have attracted considerable attention because of the importance of these materials for high-frequency applications [5, 6]. Zinc ferrite possesses a normal spinel structure, and all ions reside on tetrahedral A sites. Therefore, substitution of Mg by Zn in is expected to increase the magnetic moment up to a certain limit; thereafter, it decreases for the canting of spins in octahedral B sites. Choodamani et al. [7] investigated thermal effect on magnetic properties of Mg-Zn ferrite nanoparticles, and magnetic properties were found to be affected by particle size. In this paper, ferrite (, 0.1, 0.3, 0.5, 0.7) powders were prepared by a sol-gel autocombustion method. The aim of this study is to investigate variation structural and magnetic properties of magnesium ferrite powders by partial replacement of nonmagnetic zinc cations.
2. Experimental
2.1. Sample Preparation
Zinc substituted magnesium ferrite (, 0.1, 0.3, 0.5, 0.7) powders were prepared by a sol-gel autocombustion method. The analytical grades Mg(NO3)2·6H2O, Zn(NO3)2·6H2O, Fe(NO3)3·9H2O, citric acid (C6H8O7·H2O), and ammonia (NH3·H2O) were used as raw materials. The molar ratio of metal nitrates to citric acid was taken as 1 : 1. The metal nitrates and citric acid were, respectively, dissolved into deionized water to form solution. The solution of metal nitrates was added to ammonia to change the pH value from 7 to 9. The mixed solution was poured into a thermostat water bath and heated at 80°C under constant stirring to transform into a dried gel. Citric acid was dropped continually in the process of heating. The gel was dried at 120°C in a dry oven for 2 h and, being ignited in air at room temperature, the dried gel burnt in a self-propagating combustion way to form loose powder. The powder was ground and annealed at temperature of 800°C for 3 h.
2.2. Characterization
The crystalline structure was investigated by X-ray diffraction (D/max-2500V/PC, Rigaku) with Cu radiation ( nm). The micrographs were obtained by scanning electron microscopy (NoVa Nano SEM 430). The Mössbauer spectrum was performed at room temperature (25°C), using a conventional Mössbauer spectrometer (Fast Com Tec PC-moss II), in constant acceleration mode. The γ-rays were provided by a 57Co source in a rhodium matrix. Magnetization measurements were carried out with super conducting quantum interference device (MPMS-XL-7, Quantum Design) at room temperature.
3. Results and Discussion
3.1. XRD Patterns Analysis
Figure 1 shows the XRD patterns of (, 0.1, 0.3) ferrites calcined at 800°C for 3 h. The impurity peak of is detected in the samples with , 0.1 and 0.3, and increasing the content of Zn is favorable for the synthesis of pure Mg-Zn ferrites. Similar results also were reported in the other literature [6].
Table 1 indicates that the lattice constant increases with the increasing substitution of ions. The increase in lattice parameter is probably due to replacement of smaller ions (0.72 Å) by larger ions (0.74 Å) [8, 9].
The X-ray density was calculated using the relation [4, 10, 11]:where is relative molecular mass, is Avogadro’s number, and “” is the lattice parameter. Table 1 shows the X-ray density increase with concentration for all samples. The atomic weight of Zn is greater than that of Mg, so the relative molecular mass increases with Zn concentration increasing. The increase in X-ray density is attributed to the fact that relative molecular mass increases more than the negligible rise of the lattice parameter.
The average crystallite size of the investigated samples estimated by Scherrer’s formula [10–12] is found to be around 3241 nm. The slight decrease in the crystallite size by the addition of Zn indicates that the presence of zinc obstructs the crystal growth [13, 14].
3.2. Structures and Grain Sizes
The SEM micrographs of MgFe2O4 annealed 800°C for 3 h are shown in Figure 2. The distribution of grains with almost uniform size can be observed. Figure 3 shows the histogram of grain size distribution of MgFe2O4 ferrites. The average grain size of MgFe2O4 is approximately 96.26 nm by using a statistical method. The average grain size is slightly larger than the average crystallite size determined by XRD.
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The SEM micrographs of annealed 800°C for 3 h are shown in Figure 4. The distribution of grains with almost uniform size can be observed, well crystallized for (). Some particles are agglomerated due to the presence of magnetic interactions among particles [14].
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Figure 5 shows the histogram of grain size distribution of ferrites. The average grain size of () is approximately 90.74 nm by using a statistical method. It shows that the ferrite powers are nanoparticles, and the average grain size decreases with Zn content increasing. This shows that every particle is formed by a number of crystallites [15, 16].
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3.3. Mössbauer Spectroscopy
The Mössbauer spectra recorded at room temperature are shown in Figure 6 for . All samples have been analyzed using Mösswinn 3.0 program. For the with , the spectra exhibit two normal Zeeman-split sextets due to at tetrahedral and octahedral sites, indicating the ferromagnetic behavior of the samples.
The sextet with the larger isomer shift is assigned to the ions at the B site and the one with the smaller isomer shift is assumed to arise from the ions occupying the A site. May be it is due to difference in Fe3+–O2− internuclear separation. Compared with A site ions, the bond separation is larger for B site ions. In addition, overlap-ping of orbit is smaller for and ions at B site, which results in smaller covalency and larger isomer shift for ions at B site [17, 18]. It is reported that the values of IS for ions lie in the range 0.6~1.7 mm/s, while for they lie in the range 0.1~0.5 mm/s [19]. From Table 2, values for IS in our study indicate that iron is in state.
Table 2 shows the values of magnetic hyperfine field at A and B sites decrease by increasing nonmagnetic zinc substitution. The value of quadrupole shift of the A and B magnetic sextets is very small in the samples indicating that the local symmetry of the ferrites obtained is close to cubic [20]. The A Mössbauer absorption area decreases and the B Mössbauer absorption area increases with increasing zinc concentration, since substitutes Mg ferrite and occupies the A site, leading to transfer of from A site to B site.
When , the spectra of are only the B magnetic sextet, and the magnetic sextet of A site vanishes which indicates the presence of ions only in the octahedral B site [21]. The spectrum obtained for the composition with shows features of relaxation effects and was analyzed to a single sextet. Mössbauer spectra for the samples with consist only of a central doublet, and it exhibits superparamagnetic character. The central doublet can be attributed to the magnetically isolated ions which do not participate in the long-range magnetic ordering due to a large number of nonmagnetic nearest neighbors [20, 21].
3.4. Magnetic Property of Particles
Figure 7 shows hysteresis loops of at room temperature. The magnetization of all samples nearly reaches saturation at the external field of 5000 Oe. It is observed from Table 3 that saturation magnetization increases as Zn content increases. The saturation magnetization could be expressed by means of the following relation [22, 23]:where is magnetic moment with Bohr magneton as the unit and is relative molecular mass. The relative molecular mass of decreases as Zn content increases.
The change of magnetic moment can be explained with Néel’s theory. The magnetic moment for , , and ions is , , and , respectively [3, 4]. According to Néel’s two sublattice model of ferrimagnetism, using the cation distribution of () [], since ions have a stronger preference for the tetrahedral sites [11, 12], and ions exist in both sites but have a preference for the octahedral site [3, 4, 10–12]. The magnetic moment is expressed as [4, 5, 11].where and are the B and A sublattice magnetic moments. According to the literature [3], we assumed that the value of is equal to 0.1. Figure 8 shows the change in experimental and theoretical magnetic moments with Zn content .
From Figure 8, the experimental and theoretical magnetic moments increase as Zn content increases. Furthermore, according to (3), the theoretical saturation magnetization increases with Zn content increasing. The result of the experimental is in a good agreement with theoretical saturation magnetization for all samples. However, the saturation magnetization of with and 0.5 has no significant changes, maybe because the average grain size decreases with increasing Zn content from the SEM. It is known that porosity is inversely proportional, while the particle size is directly proportional to the magnetization for nanoferrites [24, 25].
It is observed from Table 3 that the coercivity of is less than 100 Oe, which indicating the all sample is soft magnetic materials. And the coercivity tends to decrease with Zn content increasing. The magnetic coercivity of the particles depends significantly on their magnetocrystalline anisotropy, microstrain, interparticle interaction, temperature, size, and shape [9, 26, 27]. However the and ions have no unpaired electrons and lead to zero total electron spin. So replacing ions with the ions will not have much effect on the magnetic anisotropy constant. The reduction in magnetic coercivity is related to the grain size [28–30].
4. Conclusion
The analysis of XRD patterns for (, 0.1, 0.3, 0.5, 0.7) annealed at 800°C show that the increase in lattice constant is due to replacement of smaller ions by larger ions. SEM results indicate the distribution of grains and morphology of the samples. Some particles are agglomerated due to the presence of magnetic interactions among particles. And the ferrite powers are nanoparticles. Room temperature Mössbauer spectra of display that the A Mössbauer absorption area decreases and the B Mössbauer absorption area increases with increasing zinc concentration. The change of the saturation magnetization can be explained with Néel’s theory. The coercivity decreases with increasing Zn content is attributed to the grain size.
Conflict of Interests
The authors declare that they have no conflict of interests regarding this work.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (nos. 11364004 and 11164002) and Innovation Project of Guangxi Graduate Education under Grant (no. 2010106020702M47).