The Comparative Study of the Structural and the Electrical Properties of the Nano Spinel Ferrites Prepared by the Soft Mehanochemical Synthesis

Nano spinel ferrites MFe2O4 (M= Ni, Mn, Zn) were obtained by soft mechanochemical synthesis in a planetary ball mill. The appropriate mixture of oxide and hydroxide powders was used as initial compounds. All of this mixture of powders was mechanically activated, uniaxial pressed and sintered at 1100°C/2h. The phase composition of the powders and sintered samples were analyzed by XRD and Raman spectroscopy. Morphologies were examined by SEM. In this study, the AC-conductivity and DC-resistivity of sintered samples of MFe2O4 (M= Ni, Mn, Zn) ferrites were measured at different frequencies and at room temperature. The values of the electrical conductivities show an increase with increasing temperature, which indicated the semiconducting behavior of the studied ferrites. The conduction phenomenon of the investigated samples could be explained on the basis of hopping model. The complex impedance spectroscopy analysis was used to study the effect of grain and grain boundary on the electrical properties of all three obtained ferrites.


Introduction
Ferrites are homogeneous materials composed of various oxides containing iron oxide (Fe 2 O 3 ) as their main constituent [1].Spinel ferrites have been investigated in recent years for their useful electrical and magnetic properties, and applications in several important technological fields such as ferrofluids, magnetic drug delivery and magnetic high-density information storage [1][2][3][4].The synthesis and magnetic structure characterization of spinel metastable nano-ferrites have been investigated with much interest.Among these spinel ferrites, the inverse type is particularly interesting due to its high magnetocrystalline anisotropy, high saturation magnetization from a typical crystal and magnetic structure.The properties of the synthesized materials are influenced by the composition and microstructure, which are sensitive to the preparation methodology used in their synthesis.
Mechanochemical treatment has been recognized as a powerful technique for synthesis of a wide range of materials.New approach to mechanochemical synthesis, based on reactions of solid acids and bases, crystal hydrates, basic and acidic salts, which react with each other releasing water, has been called soft mechanochemical synthesis [21].In many cases, when it comes to classical synthesis reaction sintering process, requires high temperatures, which can present an additional problem in industrial production.Mechanochemical derived precursors exhibit significantly higher reactivity and thus lower the calcination and sintering temperature.
In present work, nanosized nickel, manganese and zinc ferrites were synthesized using the soft mechanochemical treatment.The soft mechanochemical reaction leading to formation of the spinel phase was followed by X-ray diffraction and Raman spectroscopy.Scanning electron microscopy was used to analyze microstructure of the sintered sample.The electrical character of obtained ferrites was confirmed by measurements of electrical AC-conductivity and DC-resistivity at different frequencies and at room temperature.

Experimental
In present work MnFe 2 O 4 , NiFe 2 O 4 and ZnFe 2 O 4 ferrites were prepared from appropriate mixture of powders Mn(OH) 2 /α-Fe 2 O 3 , Ni(OH) 2 /α-Fe 2 O 3 and Zn(OH) 2 /α-Fe 2 O 3 by soft mechanochemical synthesis in a planetary ball mill for 25 h, 25 h and 18 h, respectively.The obtained ferrite powders were pressed into pallets and sintered at 1100 °C/2h.Heating rate was 10 °C min -1 , with nature cooling in air atmosphere.The formation of phase and crystal structure of ferrites was approved using the X-ray diffractometer (XRD, Model Philips PW 1050 diffractometer).Raman measurements of sintered samples were performed using Jobin-Ivon T64000 monochromator.Room temperature Raman spectra are in spectral range from 100 to 800 cm -1 .
TEM studies were performed using a 200 kV TEM (JEM-2100 UHR, Jeol Inc., Tokyo, Japan) equipped with an ultra-high resolution objective lens pole piece having a point-to-point resolution of 0.19 nm, being sufficient to resolve the lattice images of nanoparticles.
The morphology and microstructure of sintered samples were examined using scanning electron microscope (SEM, Model JEOL JSM-6460LV).The sintered samples in the disc shape were prepared for microstructure examination and electrical properties by polishing to thickness of 1 mm with silicon carbide and alumina powder and cleaning in an ultrasonic bath in ethanol.
In this study, the ferrite samples used for electrical measurements were coated with silver paste to ensure good ohm contacts.Thus prepared samples with silver electrodes deposited on both sides can be considered electrically equivalent to a capacitance C p in parallel with a resistance R p .These AC parameters were measured directly in the frequency range 100 Hz to 1 MHz at room temperature using an Impedance Analyzer HP-4194A.The AC conductivity was determined using the following relation: where ε'' = (ωR p C 0 ) -1 , ω = 2πf is the angular frequency of the applied field and ε 0 represents the permittivity of vacuum equal to 8.85×10 -12 F m -1 .The capacitance C 0 is determined by area of electrode A and distance between the electrodes d, as follows C 0 = Aε 0 /d.The DC resistivity of the synthetized ferrites was measured at room temperature by simple two-probe method.A Source Meter Keithley 2410 was used for the said purpose.The DC resistivity was calculated by using the following formula: where R is the measured resistance, A is area of electrode and d is the thickness of the sample.

Results and discussions
Formations mechanisms of materials from nanopowders by soft mechanically assisted synthesis (mechanochemical synthesis) are complex have not been fully understood yet.During the ferroelectric materials formation, the process passes through few steps.Generally, it starts with the decrease in particle and grain size of starting materials following by the nucleation and crystallization of target compound.As the result of mechanically assisted synthesis, nanocrystalline powders can be obtained directly from their oxide/hydroxide mixtures after milling.The Fig. 1 shows the X-ray diffraction spectra of NiFe   1) is due to translational movement of the tetrahedron (metal ion at tetrahedral site together with four oxygen atoms).There is a negligible displacement of metal atoms in modes A 1g , E g and F 2g (3) [22].
All five Raman peaks are asymmetric, with shoulder on the low energy side.Each peak can be presented like a doublet.At a microscopic level the structure of MFe 2 O 4 (M= Ni, Mn, Zn) can be considered as a mixture of two sublattices with Fe 3+ and M 2+ (M= Ni, Mn, Zn).It is supposed that Fe 3+ and M 2+ are ordered over the B-sites.In nanocrystalline samples asymmetry is partly due to confinement and size-distribution of nanoparticles.Relatively uniform distribution of grain size polygonal shape was formed.In the case of sintered nickel-ferrite grain size in the range of 0.3-1.5 μm, while in the sintered samples of manganese-and zink-ferrites grain size are in the range of 0.3-1.2μm and 0.2-1.0μm, respectively (Fig. 3).It is obvious that this difference in the grain size is due to different starting precursors and not to the conditions of the synthesis process.Based on the micrographs it can be concluded that the sintered samples, in the case of the ZnFe Density can be attributed to the difference in specific cations of the ferrite components, as NiO (6.72 g cm -3 ) is heavier than MnO (5.28 g cm -3 ) and ZnO (5.60 g cm -3 ).In order to understand the conduction mechanism and the hopping of charge carriers responsible for the conduction mechanism, the variation of electrical AC conductivity of ferrites under investigation is determined from measurement data using the relations (1).Generally, conductivity is an increasing function of frequency if it takes place by hopping of charges and it is a decreasing function of frequency if the band conduction is used [23].
The variation of AC conductivity is represented as a function of frequency in the range 100 Hz to 1 MHz at room temperatures.It is observed that electrical conductivity of all sintered samples increases with increasing frequency of the applied field.This behavior could be explained on the basis of Maxwell-Wagner model and Koops phenomenological theory [24], which assumes that the ferrites consist of conductivity grains separated by highly resistive thin layers (grain boundaries).As the frequency of the applied filed increases, the conductive ferrite grains became more active by promoting the hopping of the electrons between Fe 2+ and Fe 3+ ions (n-type) on the octahedral (B) sites [25,26].As a result, the AC conductivity of all ferrites under study increases.But, various reports show that the hole hopping between Ni 2+ and Ni 3+ .(p-type) on B site also contribute to the electric conduction in the case of NiFe 2 O 4 ferrites [25,27].
In our present case, frequency dependent AC conductivity of samples under study varies from 10 -7 to 10 -3 (Ωcm) -1 and the conductivity behavior for all ferities is analogous with each other.From the Fig. 5   In the present study, the complex impedance spectroscopy [28] as well-developed tool has been used to separate out the grain boundary and grain contribution to the total electrical conductivity of sintered ferrites.In this regard, impedance spectra (Cole-Cole plots) have been drawn in the frequency range from 100 Hz to 1 MHz at room temperature.It is evident from Fig. 6(a) that one semicircle are obtained in the impedance spectra of NiFe 2 O 4 and MnFe 2 O 4 ferrites indicating one dominant relaxation phenomenon and suggesting a dominant role of the grain boundary contribution.But, if we analyze the impedance response measured for ZnFe 2 O 4 ferrite, it is noticeable that the impedance spectrum includes both grain and grain boundary effects (see Fig. 6b)).The diameters of these semicircles correspond to the resistance: a larger one at low frequency represents the resistance of the grain boundary and a smaller one obtained at the higher frequency side corresponds to the resistance of grain properties [29].In order to correlate the electrical properties of MFe 2 O 4 (M= Mn, Ni, Zn) samples with the microstructure of these ferrites, the equivalent circuit models shown in the insets of Fig. 6 have been used to interpret their impedance response.In the proposed models, R gb and R g represent the grain boundary and grain resistance, while CPE gb and CPE g are the constant phase elements for grain boundaries and grain interiors, respectively [30].The CPE is used to accommodate the non-ideal Debye-like behavior of the capacitance which is given by relation C = Q 1/n R (1-n)/n , where the value of parameter n is 1 for a pure capacitor [25,27,30,31].The electrical parameters of equivalent circuits were obtained by the impedance data using EIS Spectrum Analyzer software [32].The calculated values of these impedance parameters are given in Table 2.In the case of ZnFe 2 O 4 ferrite, it is observed that resistance and capacitance have higher values for the grain boundary than for the grain.Higher value of capacitance can be explained by the fact that capacitance is inversely proportional to the thickness of the media [30].Among all prepared samples, the ZnFe 2 O 4 exhibits the lowest value of resistances which means greater polarizability for this ferrite.The lower total resistance at ZnFe 2 O 4 promotes rate of electron hopping, which is the sole process for both conduction and polarization in ferrites [33].Thus, the trend observed in both the electrical conductivity and complex impedance of present MFe 2 O 4 (M= Ni, Mn, Zn) ferrites are in good agreement with each other.

Conclusions
MFe 2 O 4 (M=Mn, Ni, Zn) ferrite powders and sintered samples were prepared by soft mechanochemical synthesis.Single phase nanosized NiFe 2 O 4 and MnFe 2 O 4 ferrite were synthesized by 25 h ball milling, while the ZnFe 2 O 4 ferrite powder was obtained after 18 h of milling.All three samples obtained ferrites were sintered at the same temperature (1100 °C) and for the same time (2 h).X-ray diffraction of the prepared samples shows single phase cubic spinel structure.All of five first-order Raman active modes characteristic for spinel structure were observed in obtained Raman spectra.The obtained sintered ferrite samples have polygonal grains.The value of AC conductivity of the ZnFe 2 O 4 is higher than the values of the MnFe 2 O 4 and NiFe 2 O 4 at different frequencies and at room temperature.Also, the values of DC resistivity are 2.72×10 5 , 2.01×10 4

FFig. 2 .
Fig. 2. Raman spectra for the sample of the MFe 2 O 4 (M= Ni, Mn, Zn) sintered at 1100 °C/2h.The Fig. 2 shows Raman spectra for the NiFe 2 O 4 , MnFe 2 O 4 and ZnFe 2 O 4 prepared by the soft mechanochemical synthesis.To simplify, peaks are assigned as for normal spinel

Fig. 3
Fig.3shows SEM images of NiFe 2 O 4 , MnFe 2 O 4 and ZnFe O 4 ferrites sintered at 1100 °C/2h.Relatively uniform distribution of grain size polygonal shape was formed.In the case of sintered nickel-ferrite grain size in the range of 0.3-1.5 μm, while in the sintered samples of manganese-and zink-ferrites grain size are in the range of 0.3-1.2μm and 0.2-1.0μm, respectively (Fig.3).It is obvious that this difference in the grain size is due to different starting precursors and not to the conditions of the synthesis process.Based on the micrographs it can be concluded that the sintered samples, in the case of the ZnFe 2 O 4 ferrite has lower porosity than the NiFe 2 O 4 and MnFe 2 O 4 ferrite, which is in agreement with the determined density based on Archimedes principle.The density are 3.93 g cm-3  , 4.2 g cm -

3 and 5 .
Fig.3shows SEM images of NiFe 2 O 4 , MnFe 2 O 4 and ZnFe O 4 ferrites sintered at 1100 °C/2h.Relatively uniform distribution of grain size polygonal shape was formed.In the case of sintered nickel-ferrite grain size in the range of 0.3-1.5 μm, while in the sintered samples of manganese-and zink-ferrites grain size are in the range of 0.3-1.2μm and 0.2-1.0μm, respectively (Fig.3).It is obvious that this difference in the grain size is due to different starting precursors and not to the conditions of the synthesis process.Based on the micrographs it can be concluded that the sintered samples, in the case of the ZnFe 2 O 4 ferrite has lower porosity than the NiFe 2 O 4 and MnFe 2 O 4 ferrite, which is in agreement with the determined density based on Archimedes principle.The density are 3.93 g cm -3 , 4.2 g cm - 3 and 5.58 g cm -3 for NiFe 2 O 4 , MnFe 2 O 4 and ZnFe 2 O 4 ferrite sintered samples, respectively.Density can be attributed to the difference in specific cations of the ferrite components, as NiO (6.72 g cm-3 ) is heavier than MnO (5.28 g cm -3 ) and ZnO (5.60 g cm-3 ).
, one can see that the ZnFe 2 O 4 has the highest values of AC conductivity compared to the NiFe 2 O 4 and MnFe 2 O 4 at different frequencies and at room temperature.The measured values of DC resistivity of present MFe 2 O 4 (M= Ni, Mn, Zn) ferrites are given in the Table 1.It is observed that NiFe 2 O 4 has the highest value of the DC resistivity compared to the other two ferrite samples, which is in good agreement with results of AC conductivity.The high value of electrical resistivity makes NiFe 2 O 4 ferrite good candidate for microwave devices applications that require negligible eddy currents.

Fig. 6 .
Fig. 6.Impedance spectra for (a) NiFe 2 O 4 and MnFe 2 O 4 ferrites and (b) ZnFe 2 O 4 ferrite at room temperatures.Insets: proposed equivalent circuits model for analysis of the impedance data.
Tab. I.The values of DC resistivity at room temperature for the MFe 2 O 4 (M=Mn, Zn) samples.
Tab. II.Impedance parameters for MFe 2 O 4 (M=Mn, Ni, Zn) ferrites calculated from their impedance response at room temperature.