Mössbauer Spectroscopy , Structural and Magnetic Studies of Zn 2 + Substituted Magnesium Ferrite Nanomaterials Prepared by Sol-Gel Method

Zinc substituted magnesium ferrite nanomaterials Mg1−xZnxFe2O4 (x = 0, 0.1, 0.3, 0.5, 0.7) powders have been prepared by a solgel 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 Mg1−xZnxFe2O4 shows that the A Mössbauer absorption area decreases and the B Mössbauer absorption area increases with zinc concentration increasing.The change of the saturationmagnetization 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.


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 Mg 2+ ions and 0.9 of Mg 2+ ions distributed over the A and B sites in the following way (Mg 0.1 Fe 0.9 )[Mg 0.9 Fe 1.1 ]O 4 .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 Zn 2+ ions reside on tetrahedral A sites.Therefore, substitution of Mg by Zn in Mg 1− Zn  Fe 2 O 4 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 Mg 1− Zn  Fe 2 O 4 ( = 0, 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.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.

Characterization.
The crystalline structure was investigated by X-ray diffraction (D/max-2500V/PC, Rigaku) with Cu K  radiation ( = 0.15405 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 -rays were provided by a 57 Co source in a rhodium matrix.Magnetization measurements were carried out with super conducting quantum interference device (MPMS-XL-7, Quantum Design) at room temperature.

Results and Discussion
3.1.XRD Patterns Analysis.Figure 1 shows the XRD patterns of Mg 1− Zn  Fe 2 O 4 ( = 0, 0.1, 0.3) ferrites calcined at 800 ∘ C for 3 h.The impurity peak of Fe 2 O 3 is detected in the samples with  = 0, 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 Zn 2+ ions.The increase in lattice parameter is probably due to replacement of smaller Mg 2+ ions (0.72 Å) by larger Zn 2+ 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 Zn 2+ 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][11][12] is found to be around 32∼41 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].

Structures and Grain Sizes.
The SEM micrographs of MgFe 2 O 4 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 MgFe 2 O 4 ferrites.The average grain size of MgFe 2 O 4 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.
The SEM micrographs of Mg 0.5 Zn 0.5 Fe 2 O 4 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 Mg 1− Zn  Fe 2 O 4 ( = 0.5).Some particles are agglomerated due to the presence of magnetic interactions among particles [14].
Figure 5 shows the histogram of grain size distribution of Mg 0.5 Zn 0.5 Fe 2 O 4 ferrites.The average grain size of Mg 0.5 Zn 0.5 Fe 2 O 4 ( = 0.5) 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].The sextet with the larger isomer shift is assigned to the Fe 3+ ions at the B site and the one with the smaller isomer shift is assumed to arise from the Fe 3+ ions occupying the A site.May be it is due to difference in Fe 3+ -O 2− internuclear separation.Compared with A site ions, the bond separation is larger for B site Fe 3+ ions.In addition, overlap-ping of orbit is smaller for Fe 3+ and O 2+ ions at B site, which results in smaller covalency and larger isomer shift for Fe 3+ ions at B site [17,18].It is reported that the values of IS for Fe 2+ ions lie in the range 0.6∼1.7 mm/s, while for Fe 3+ 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 Fe 3+ 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  When  = 0.3, 0.5, the spectra of Mg 1− Zn  Fe 2 O 4 are only the B magnetic sextet, and the magnetic sextet of A site vanishes which indicates the presence of Fe 3+ ions only in the octahedral B site [21].The spectrum obtained for the composition with  = 0.5 shows features of relaxation effects and was analyzed to a single sextet.Mössbauer spectra for the samples with  = 0.7 consist only of a central doublet, and it exhibits superparamagnetic character.The central doublet can be attributed to the magnetically isolated Fe 3+ ions which do not participate in the long-range magnetic ordering due to a large number of nonmagnetic nearest neighbors [20,21].3 that saturation magnetization increases as Zn content  increases.

Magnetic Property of Particles.
The saturation magnetization could be expressed by means of the following relation [22,23]: where  B is magnetic moment with Bohr magneton as the unit and  is relative molecular mass.The relative molecular mass of Mg 1− Zn  Fe 2 O 4 decreases as Zn content  increases.The change of magnetic moment  B can be explained with Néel's theory.The magnetic moment for Zn 2+ , Mg 2+ , and Fe 3+ ions is 0 B , 0 B , and 5 B , respectively [3,4].According to Néel's two sublattice model of ferrimagnetism, using the cation distribution of (Zn  Mg  Fe 1−− ) [Mg 1−− Fe 1++ ]O 4 , since Zn 2+ ions have a stronger preference for the tetrahedral sites [11,12], and Mg 2+ ions exist in both sites but have a preference for the octahedral  site [3,4,[10][11][12].The magnetic moment  B is expressed as [4,5,11].
where  B and  A 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 Mg 1− Zn  Fe 2 O 4 with  = 0.1 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 Mg 1− Zn  Fe 2 O 4 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 Zn 2+ and Mg 2+ ions have no unpaired electrons and lead to zero total electron spin.So replacing Mg 2+ ions with the Zn 2+ ions will not have much effect on the magnetic anisotropy constant.The reduction in magnetic coercivity is related to the grain size [28][29][30].

Conclusion
The analysis of XRD patterns for Mg 1− Zn  Fe 2 O 4 ( = 0, 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 Mg 2+ ions

3. 3 .
Mössbauer Spectroscopy.The Mössbauer spectra recorded at room temperature are shown in Figure6for Mg 1− Zn  Fe 2 O 4 .All samples have been analyzed using Mösswinn 3.0 program.For the Mg 1− Zn  Fe 2 O 4 with  = 0, 0.1, the spectra exhibit two normal Zeeman-split sextets due to Fe 3+ at tetrahedral and octahedral sites, indicating the ferromagnetic behavior of the samples.
Figure 7  shows hysteresis loops of Mg 1− Zn  Fe 2 O 4 at room temperature.The magnetization of all samples nearly reaches saturation at the external field of 5000 Oe.It is observed from Table

Figure 8 :
Figure 8: Variation in experimental and theoretical magnetic moment with zinc content .

Table 1 :
Lattice parameters, average crystallite size, and X-ray densities date of Mg 1− Zn  Fe 2 O 4 annealed at 800 ∘ C.

Table 2 :
2+substitutes Mg ferrite and occupies the A site, leading to transfer of Fe 3+ from A site to B site.

Table 3 :
Magnetic data for Mg 1− Zn  Fe 2 O 4 annealed at 800 ∘ C. 2+ 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 Mg 1− Zn  Fe 2 O 4 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.