Microstructural and Mössbauer Spectroscopy Studies of Mg_1−𝑥Zn_𝑥Fe₂O₄(𝑥=0.5,0.7) Nanoparticles

Abstract

Zinc substituted magnesium ferrite Mg_1−𝑥Zn_𝑥Fe₂O₄ (𝑥=0.5,0.7) powders have been prepared by a sol-gel autocombustion method. XRD patterns show that the specimens with 𝑥=0.5 and 0.7 exhibit single-phase spinel structure, and more content of Zn in specimens is favorable for the synthesis of pure Mg-Zn ferrites. Room temperature Mössbauer spectra of Mg_1−𝑥Zn_𝑥Fe₂O₄ annealed at 800°C display transition from ferrimagnetic behavior to super paramagnetic behavior with increase in zinc concentration. The Mössbauer spectras of Mg_0.5Zn_0.5Fe₂O₄ annealed at different temperatures display the magnetic phase change of the ferrite particles.

1. Introduction

MgFe₂O₄ is regarded as an important candidate of the spinel ferrite family. Nanocrystalline Mg ferrite shows more enhanced magnetization than its crystalline counterpart and has been found to exhibit some unusual magnetic properties, such as super paramagnetism and a noncollinear ordering of the magnetic moments of Fe³⁺ ions, known as spin canting [1]. If the MgFe₂O₄ structure was completely inversed, its magnet moment would be zero because the magnetic moment of Mg²⁺ ion is zero. Manjurul Haque et al. [2] studied saturation of Zn²⁺ substitution on the magnetic properties of Mg_1−𝑥Zn_𝑥Fe₂O₄ ferrites. Saturation magnetization and magnetic moment are observed to increase up to 𝑥=0.4 and thereafter decrease due to the spin canting in B-sites. Similar results of saturation magnetization’s variation should be reported in the other literature [3, 4]. In this paper, ferrite Mg_1−𝑥Zn_𝑥Fe₂O₄(𝑥=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 Mg_1−𝑥Zn_𝑥Fe₂O₄(𝑥=0.5,0.7) powders were prepared by a sol-gel autocombustion method. The analytical grade 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 dissolved into deionized water to form solution, respectively. The pH value of metal nitrates solution was changed from 7 to 9 by adding ammonia. And then, the mixed solution was maintained at 80°C in a thermostat water bath under constant stirring to transform into a dried gel. Citric acid was dropped continually in the process of heating. The gel were dried at 120°C in a dry-oven for 2 h, being ignited in air at room temperature, the dried gel burnt in a self-propagating combustion way to form loose powder. The powder was grounded and annealed.

2.2. Characterization

The crystalline structure was investigated by X-ray diffraction (D/max-2500 V/PC, Rigaku) with Cu K_α radiation (𝜆=0.15405nm). The micrographs were obtained by scanning electron microscopy (NoVa Nano SEM 430). The Mössbauer spectrum was performed at room temperature, using a conventional Mössbauer spectrometer (American Fast Com Tec PC-mossII), in constant acceleration mode. The γ-rays were provided by a ⁵⁷Co source in a rhodium matrix.

3. Results and Discussion

3.1. XRD Patterns Analysis

Figure 1 shows the XRD patterns of Mg_1−𝑥Zn_𝑥Fe₂O₄(𝑥=0.5,0.7) ferrites calcined at 800°C for 3 h. It is clear that the specimens with 𝑥=0.5 and 0.7 exhibit single-phase spinel structure. Obviously, increasing the content of Zn is favorable for the synthesis of pure Mg-Zn ferrites. Similar results also are reported in the other literature [4]. Table 1 indicates that the X-ray density increases with Zn²⁺ concentration for all samples. The increase in lattice parameter is probably due to replacement of smaller Mg²⁺ ions by larger Zn²⁺ ions [5, 6].

Table 1 

Lattice parameters, average crystallite size, and X-ray densities date of Mg_1−𝑥Zn_𝑥Fe₂O₄ annealed at 800°C.

Figure 1 

Room temperature X-ray diffraction patterns of Mg1−𝑥Zn𝑥Fe2O4 annealed at 800°C.

The X-ray patterns of Mg₀₅Zn_0.5Fe₂O₄ annealed at different temperatures are shown in Figure 2. All the samples are the single-phase cubic spinel structure. No additional phase was detected. The lattice parameter showed changes for all the samples from Table 2. Average crystallite size of Mg₀₅Zn_0.5Fe₂O₄ tends to increase with the increase of the calcining temperature, due to the coalescence of small grains through grain boundary diffusion [7]. In other people’s work [8], the diffraction peaks of Cu_0.7Mg_0.3Fe₂O₄ annealed at low temperature are not very sharp, but in our result the diffraction peaks of Mg₀₅Zn_0.5Fe₂O₄ without burning are very sharp. The result suggests that magnesium substituted cobalt ferrite powders prepared by a sol-gel autocombustion method still have a good crystallinity without calcining.

Table 2 

Lattice parameters, average crystallite size, and X-ray densities date of Mg_0.5Zn_0.5Fe₂O₄ annealed at different temperatures.

Figure 2 

X-ray diffraction patterns of Mg0.5Zn0.5Fe2O4 annealed at different temperatures.

3.2. Structures and Grain Sizes

The SEM micrographs of Mg_1−𝑥Zn_𝑥Fe₂O₄(𝑥=0.5) annealed 800°C for 3 h are shown in Figure 3. It can be observed the distribution of grains with almost uniform size, well crystallized for Mg_0.5Zn_0.5Fe₂O₄. Some particles are agglomerated due to the presence of magnetic interactions among particles [9].

(a)

(b)

(c)

(d)

(e)

(f)

Figure 3 

SEM micrographs depict Mg0.5Zn0.5Fe2O4(𝑥=0.5) ferrites with diameters of 100 μm (a), 50 μm (b), 20 μm (c), 10 μm (d), 5 μm (e), and 3 μm (f).

Figure 4 shows the histogram of grain size distribution of Mg_1−𝑥Zn_𝑥Fe₂O₄(𝑥=0.5) ferrites. The average grain size of Mg_0.5Zn_0.5Fe₂O₄ estimated by a statistical method is approximately 90.74 nm, respectively. It shows that the ferrite powers are nanoparticles, and the average grain size decreases with the increase of Zn content. The average grain size is slightly larger than the average crystallite size determined by XRD. This shows that every particle is formed by a number of crystallites [10, 11].

(a)

(b)

Figure 4 

Histogram of grain size distribution of Mg0.5Zn0.5Fe2O4 annealed at 800°C.

3.3. Mössbauer Spectroscopy

Mössbauer absorption spectra measured at room temperature for Mg_0.5Zn_0.5Fe₂O₄ powders annealed at different temperatures are shown in Figure 5. All samples have been analyzed using Mösswin 3.0 program.

Figure 5 

Room temperature Mössbauer spectra of Mg0.5Zn0.5Fe2O4 annealed at different temperatures.

Spectra of the samples without calcining and annealed at 400°C are fitted into a single sextet and a central paramagnetic doublet, and the sample annealed at 800°C was analyzed to only a single sextet. The Mössbauer spectra of Mg_0.5Zn_0.5Fe₂O₄ sample show a paramagnetic doublet, which is due to the super paramagnetic relaxation, in other words, some particles are a single domain. Table 3 shows that the Mössbauer absorption area of paramagnetic doublet decreases with the increase of the annealed temperature; it is attributed to the change in particle size as a function of heat treatment [12]. Therefore the Mössbauer spectra of Mg_0.5Zn_0.5Fe₂O₄ annealed at different temperatures display the magnetic phase change of the ferrite particles.

Table 3 

Mössbauer parameters of isomer shift (IS), quadrupole splitting (QS), magnetic hyperfine field (𝐻), line width (Γ), and absorption area (𝐴₀) for Mg_0.5Zn_0.5Fe₂O₄ annealed at different temperatures.

4. Conclusion

The analysis of XRD patterns for Mg_1−𝑥Zn_𝑥Fe₂O₄ annealed at 800°C shows that the specimens with 𝑥=0.5 and 0.7 exhibit single-phase spinel structure, and more content of Zn in specimens is favorable for the synthesis of pure Mg-Zn ferrites. The XRD patterns of Mg_0.5Zn_0.5Fe₂O₄ annealed at different temperatures indicate that all Mg_1−𝑥Zn_𝑥Fe₂O₄ ferrite powders prepared by a sol-gel autocombustion method have good crystallinity. SEM results indicate the distribution of grains and morphology of the samples. Room temperature Mössbauer spectra of Mg_1−𝑥Zn_𝑥Fe₂O₄ annealed at 800°C display transition from ferrimagnetic behavior to super paramagnetic behavior with increase in zinc concentration. Furthermore, the Mössbauer spectra of Mg_0.5Zn_0.5Fe₂O₄ annealed at different temperatures display the magnetic phase change of the ferrite particles.

Conflict of Interests

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

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (nos. 11364004, 11164002); Innovation Project of Guangxi Graduate Education under Grant (no. 2010106020702 M47).