Influence of preparation method on structural, optical and magnetic properties of synthesized Zn_0.5Mg_0.5−xNi_xFe₂O₄ nano-ferrite

X-ray diffraction (XRD) was employed to obtain the phase and structural parameters of ferrite samples. The type of method used in the synthesis has significant influence in crystal structures of nanoparticles as well as the annealing temperature [16]. The information of Zn_0.5Mg_0.5−XNi_XFe₂O₄ was confirmed by investigating the XRD pattern using MDI Jade 5.0 and Origin pro 8.6. Figures 1 and 2 show the x-ray diffraction patterns of Zn_0.5Mg_0.5−XNi_XFe₂O₄ nanoparticles which was carried out at room temperature for samples with five nickel concentration (x = 0, 0.1, 0.2, 0.3 and 0.4) prepared by co-precipitation and so-gel methods, respectively. XRD peaks were reported at 2θ of 30.72°, 35.91°, 43.55°, 53.82°, 57.32° and 62.88° for the samples prepared by co-precipitation, while were found to be 30.72°, 36.01°, 43.55°, 53.83°, 57.31° and 62.88° for sol–gel samples which are attributed to (220), (311), (400), (422), (511), and (440) hkl planes as shown in figures 1 and 2. The crystal structure is ascribed as cubic phase of space group Fd-3m:1 for the samples of both methods and each peak is in accordance with standard reference pattern (JCPDS card number 73–1720). These hkl planes may indicated the growth of the pure fcc spinel structures of Zn_0.5Mg_0.5−XNi_XFe₂O₄ nano-ferrite and the obtained peaks were found to be in agreement with the previous work [17]. The small shifting of the diffraction peaks in the direction of larger values of (2θ) could be attributed to replacement of Ni⁺² ions. The sharpest peak connected to (311) plane and the full width at half maximum (FWHM) was related to the growth of the crystallites [18]. The average crystallite size, D was calculated using Debye-Scherer's equation [19]:

$$\begin{eqnarray}&&D=\frac{0.9{\rm{\lambda }}}{\beta \cos \theta }\end{eqnarray} \tag{ 1 }$$

where β is FWHM of the XRD peak in radians, while θ and λ is the diffraction angle and the incident x-ray wavelength, respectively. Equation (1) was used to calculate the crystallite size and was found to be (19.15, 13.15, 12.75, 11.65 and 12.8 nm) for co-precipitation samples and (13.9, 15.05, 14.81, 21.9 and 21.9 nm) for sol–gel samples respectively. Table 1 reports the values of the crystallite size, lattice constants, density and volume of samples of both methods. The variation of crystallite size and lattice parameters with Ni concentration for the ferrite's nanoparticles were shown in figure 3. The crystallite size was found to decrease with Ni concentration in case of co-precipitation method as shown in figure 3(a). It might be due to the fact that the substitution of Mg refine the crystallite size. However, with the exception of sample with Ni concentration of 0.1, the opposite behavior is observed for samples of sol–gel method with different Ni concentration as shown in figure 3(b). Similar result with increase in crystallite size for samples prepared via sol–gel method was reported by Makiyyu Musa et al [20]. The increase in crystallite size figure 3(b) is related to the decrease in lattice parameter figure 3(d) through the repulsive force of unpaired surface electrons. According to Rajendran et al, this force becomes stronger with decreased lattice parameter which leads to expands in the particle size [21].

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The lattice parameter (a) was calculated using MDI Jade 5.0 program and the following equation [22]:

$$\begin{eqnarray}&&a=d/{[({{\mathtt{h}}}^{{\mathsf{2}}}+{k}^{{\mathsf{2}}}+{l}^{{\mathsf{2}}})]}^{1/2}\end{eqnarray} \tag{ 2 }$$

where d and h, k, l is the lattice spacing and the Miller indices, respectively.

The unit cell volume was measured using the relation (V = a³) [23]. The lattice constants obtained ranged (8.327–8.292 Å) for co-precipitation and (8.323–8.311 Å) for sol–gel samples. It can be observed from figures 3(c) and (d) that the lattice parameter is decreased for both methods with increasing of Ni concentration in Zn_0.5Ni_xMg_0.5−xFe₂O₄. The variation of lattice parameter with Ni concentration can be understood based on the ionic radii of both Ni⁺² and Mg⁺² ions where Ni⁺² ions have ionic radius of (0.69 Å) while Mg⁺² ions is (0.60 Å) in the octahedral sites [24, 25]. Thus, the replacement of the Ni ions by the Mg ions may cause little reduction of unit cell dimensions, thereby decreasing the lattice parameter. Furthermore, many metal oxides exhibit a decrease in lattice parameters with a decrease in particle size. This reduction in lattice parameter may be due to cation/anion vacancies, finite size effect or lattice stress. Previous works with a similar variation of lattice parameter were reported for Ni ferrite nanoparticles [26, 27]. Regarding the structural outcomes, it may suggest that co-precipitation method is preferred over sol–gel method for more refined structure.

Scanning electron microscopy was used to accurately estimate the microstructure and morphology of Zn_0.5Ni_xMg_0.5−xFe₂O₄ nanoparticles. Figures 4 and 5(a)–(e) show the SEM micrographs of Zn_0.5Ni_xMg_0.5−xFe₂O₄ nano-ferrite samples prepared by both co-precipitation and sol–gel method, respectively. It can be observed from figures 4 and 5 that all samples revealed arrangement of homogeneous nanoparticles with granular [28]. For both methods, the micrographs showed nanostructure and agglomerated spherical particles for samples with x = 0 (figures 4(a) and 5(a)). In comparison with those of the ferrites with x = 0.1 to 0.4 (figures 4 and 5(b)–(e)) the grain appeared as agglomerated particles. The samples revealed ferrite nanoparticles of nearly a spherical shape and uniformly distributed while the size was ranged 7–29 nm for all samples. This difference in average particle size and particle diameter may arise from the effect of relatively ionic radius of Mg⁺² (60 Å) ions substituted by Ni⁺² (69 Å) ions in the octahedral sites [22]. An additional reason for the difference in average particle size could be the type of selected method of preparation. The existence of magnetic interactions among the particles may lead to the agglomeration of nanoparticles [29]. Besides, the formation of a microstructure with particles of random size is also noted in which small particles preferred the formation of soft agglomerates. The surface tension increased with smaller particles which generates driving force to increase of the state of agglomeration and aggregation [30].

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Fourier-transform infrared spectroscopy of Zn_0.5Ni_xMg_0.5−xFe₂O₄ (x = 0.0, 0.1, 0.2, 0.3 and 0.4) samples prepared using co-precipitation and sol–gel method was presented in figures 6 and 7. The FTIR frequency bands for different Ni concentration were reported in table 2 for samples of both methods of preparations. An insignificant shift was observed in the peaks of the two methods of preparation. In the case of co-precipitation method, the observed FTIR peak at 2427 cm⁻¹ was assigned to stretching vibration of O-H group [31]. The free absorbed water may indicate the presence of hydroxyl groups in the samples, which was reported in previously published works [32, 33]. For samples of both methods, the sharp peaks at the range of (1621 and 1350) cm⁻¹ are assigned to stretching vibration of (M-O), which may indicate the formation of the metal-oxygen in the ferrite samples [34]. The peaks observed around 1110–1157cm⁻¹ may indicate the coordination of oxygen atoms [35]. Two absorption bands were detected with low intensity in the range of (850–400 cm⁻¹), which could be allocated to the intrinsic stretching vibrations of the tetrahedral and octahedral sites. According to Rashmi Tiwari et al [36] this confirmed the existence of the cubic spinel phase in the nanoferrites. Accordingly, it can be inferred that the prepared samples have spinel structure which is in conformity with the result of the XRD analysis. Similar result was found by Yanchun Zhang et al for their Ni-Mg-Zn ferrite samples [37].

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Figures 8 and 9 show the absorbance as a function of wavelength for the samples of Zn_0.5Ni_xMg_0.5−xFe₂O₄ nano-ferrite prepared using co-precipitation and sol–gel methods (x = 0, 0.1, 0.2, 0.3 and 0.4). The absorption band was obtained at wavelength (197.2 nm) for co-precipitation samples, while for sol–gel samples the absorption was found to be at (255.19 nm). The Tauc formula derived the energy band gap Eg (αhυ)² as function of incident energy (hυ). This formula was used to determine the optical properties of nano ferrites [34]. The optical gap related to the Zn_0.5Ni_xMg_0.5−xFe₂O₄ nano-ferrites is determine from the intercept of the linear trend observed in the spectral dependence of (αhυ)² in a narrow range of photon energies with x-axis. The absorption coefficient α in many nano ferrites generally obeying the following relation [38]:

$$\begin{eqnarray}&&\left(\alpha h\upsilon \right)=A{(h\upsilon -{E}_{g})}^{n}\end{eqnarray} \tag{ 3 }$$

where, α is the absorption coefficient and A is the edge width parameter, Eg is the energy band gap, while (hυ) is the incident photon energy and n is the constant reliant on the degree of transition which has 1/2, 1 or 2 values and the value 1/2 was selected in this study. Based on the maximum absorption band, the band gap energy was calculated by [39]:

$$\begin{eqnarray}&&E={hv}=1240/\lambda \,\left({\rm{eV}}\right)\end{eqnarray} \tag{ 4 }$$

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The absorption coefficient α is given by [40]:

$$\begin{eqnarray}&&\alpha =\frac{2.303A}{d}\end{eqnarray} \tag{ 5 }$$

Where A is absorption and d is particle size which is determined by Scherrer relation. The band gap was calculated by plotting (hυ) against (αhυ)² and extrapolating the tangent on the x-axis (Tauc plots). Figures 10 and 11 show the optical band gap energy of Zn_0.5Ni_xMg_0.5−xFe₂O₄ nano-ferrite synthesized by co-precipitation and sol–gel methods as a function of the incident energy and it is value was found to be 6.04, 6.05, 6.06, 6.07 and 6.08 eV for co-precipitation samples and 4.44, 4.46, 4.47, 4.49 and 4.50 eV for sol–gel samples. The absorbance, absorption coefficient, wavelength and energy band gap were demonstrated in table 3. The absorbance of the samples synthesized by co-precipitation was 1.00, 0.85 0.72, 0.62 and 0.53 a.u, while for sol–gel method samples was 1.00, 0.80, 0.64, 0.51 and 0.20 a.u. The absorption coefficient was found to decrease with the increase of nickel concentration and was found to be 329.7, 281.1, 238.2, 202.5 and 172.4 cm⁻¹ and 329.6, 263.8, 210.5, 169.02 and 135.03 cm⁻¹ for co-precipitation and sol–gel samples respectively.

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For both methods, the results show that the energy band gap increases slightly with Ni⁰. It is suggested that the band gap energy can be changed by different factors such as crystallite size, structural parameter, and presence of impurities [41]. The increase in case of co-precipitation samples may attributed to the smaller crystallite size with increase in nickel concentration. This is supported by the quantum confinement theory, which suggests that the holes in the valence band and the electrons in the conduction band are confined by the potential barriers of the surface or potential well of quantum box [42]. The band gap energy increases between the valence band and the conduction band with decreasing the particle size due to the confinement of the electrons and holes [43]. This effect is more obvious in smaller nanoparticles and for materials with a large band gap. As the size of the nanoparticles increases the band gap approaches the value of the bulk material. In addition, the increase of the energy band gap for the samples may be due to decrease in lattice constant which is consider one of the factors that affects the size of the band gap of a material. As interatomic distance decreases, extra energy is required to set free the valence electrons in the conduction band. Therefore, a direct result of decreasing the lattice constant is the increase in the energy gap.

The magnetic properties are strongly dependent on the doping, sintering, and synthesis methods. According to Almessiere et al [17] the magnetic characteristics of spinel ferrites samples are affected by several factors, such as synthesis route, cations distribution on tetrahedral and octahedral sites, doping of nickel with magnesium and crystallite size. Therefore, to verify the existence of a magnetic order in our ferrite nanoparticles, the Saturation magnetization (Ms), coercive field (coercivity) (Hc) and remanent magnetization (retentivity) (Mr) of Zn_0.5Mg_0.5−XNi_XFe₂O₄ nano-ferrite samples (x = 0.0, 0.1, 0.2, 0.3 and 0.4) were investigated using VSM to examine the magnetic properties of the samples. Figures 12 and 13 show the magnetic hysteresis curve of Zn_0.5Mg_0.5−XNi_XFe₂O₄ nano-ferrite synthesized using co-precipitation and sol–gel, respectively. The magnetic parameters as saturation magnetization M_s, coercivity H_c, remnant magnetization M_r and loop squareness ratio (M_r/M_s) were quantified at the room temperature and hysteresis loop for different concentrations of Ni⁺² ions are demonstrated in table 4.

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The variation observed in the saturation magnetization values at room temperature for different Ni content is presented in table 4 and figure 14(a). The value of M_s for sample with x = 0 is 56.675 emu g⁻¹ for the sample prepared by co-precipitation method, while the value is reduced to 29.952 emu g⁻¹ in case of sol–gel method. For co-precipitation, the introduction of nickel resulted in decrease of saturation magnetization of the samples with Ni concentration and then start to increase up for sample of x = 0.3, and it was found to be maximum for sample of x = 0.4 with value of M_s equal to 127.570 emu g⁻¹. On the other hand, the introduction of nickel in the case of sol–gel method samples resulted in an increase of M_s from 29.952 emu g⁻¹ to 31.563 emu g⁻¹ for the sample with x = 0.1 and then decreased up to x = 0.3. The saturation magnetization then increases for sample of x = 0.4. For both methods, the M_s was maximum for sample with x = 0.4. The increase in M_s values of prepared samples with x = 0.4 are most likely attributed to the increasing of crystallinity and particle size of the sample [44].

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The following equation is used to calculate the magnitudes of squareness ratio (M_rs) of the sample of nano-ferrite [45].

$$\begin{eqnarray}&&{M}_{{rs}}={M}_{r}/{M}_{s}\end{eqnarray} \tag{ 6 }$$

It can be noticed from table 4 that the numerical values of M_rs were increased with Ni concentration for co-precipitation samples, while opposite behaviors were observed for samples prepared with sol–gel method. However, for both methods, the prepared samples own single domain. According to references [46, 47], the particles possess single domain if the squareness ratio M_r/M_s value 0.5 or less. Table 4 shows that, for all the samples, the ratio M_r/M_s are significantly less than 0.5 suggesting that the samples under investigation are soft magnetic materials.

The saturation magnetization has related to coercivity through Brown's relation [44],

$$\begin{eqnarray}&&{H}_{C}=2{K}_{1}/{\mu }_{0}{M}_{s}\end{eqnarray} \tag{ 7 }$$

where K₁ is the magneto-crytalline anisotropy constant and μ₀ is the magnetic permeability of free space. The magnitude of K₁ and the volume of nanocrystals control the strength of grain size and spin–orbit (L-S) coupling and define the degree of the super-paramagnetism. The magneto-crytalline anisotropy constant K₁ for the samples of both methods was determined and was listed in table 4. These values are comparable with previous studies of NiZnAl ferrite [43], NiFe₂O₄ [48]. The sample with x = 0.4 of sol–gel method shows the highest value of K₁ and Hc as well. Materials with high magnetic anisotropy usually have high coercivity. Comparison of magneto-crytalline anisotropy constant (K₁), crystallite size (D), saturation magnetization (M_s), and coercivity (H_c) for samples of both methods with previous studies were summarized in table 5. The coercivity for sample with x = 0.0 was found to be 16.5960 (Oe) and the introduction of Ni resulted in decreasing of H_c values with Ni concentration up to x = 0.3 for the sample prepared by co-precipitation method. In the case of sol–gel method, as indicated by figure 14(b), the Hc was found to increase from 8.5577 (Oe) up to x = 0.3 (164.6200 Oe) when Ni concentration increased. The coercivity of sol–gel samples, generally, increased with Ni concentration with the exception of sample x = 0.4. Similar findings were reported by Xingrong Wu et al [49] for their samples. The increase in the value of Hc could be attributed to the strong magnetic interaction. The magnetism of cubic spinel ferrite mainly comes from metal cation super exchange between A and B sites. There are three types of magnetic interactions that occur between A and B sites. The strongest and most dominant is A-B sub-lattice exchange, while A-A type and B-B type are relatively weak [50]. The ferromagnetic behavior of the soft magnetic material was detected in the samples as mentioned earlier. Specifically, non-permanent magnetic materials are due to their small M_r and H_c values, which could be magnetized and demagnetized easily at low field values. This may indicate the formation of the narrow magnetic hysteresis loop [51]. Cationic magnetic moments were produced at tetrahedral and octahedral sites by the super exchange interaction between metal cations that are responsible for the magnetic properties [52]. For both methods, the remanent magnetization of coercive field deduced from the hysteresis loops, are very low, suggesting the super-paramagnetic behavior of the samples. This magnetic behavior of the nickel nanoferrites makes them a good candidate for use in hyperthermia treatment [53]. According to Wahajuddin and Arora [54], these nanoparticles are very important for their use as drug delivery vehicles. They can drag the drug molecules to certain part in the body by the effect of an applied magnet field. Furthermore, ferrites can be classified as soft ferrites that are possible to magnetize and demagnetize easily, while it is difficult in the case of hard ferrites because of greater H_c and M_r [51, 55]. The synthesized samples of co-precipitation method possessed the lowest values of remanent magnetization, M_r and coercive field, H_c compared to samples of sol–gel method.