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Abstract. Modern technologies can not be realized without magnetic components. Crystallite size, crystallinity, composition and cations distribution influences the properties of nanoferrite particles. Co1-xZnxFe2O4 nanoparticles with (x) varying from 0.0 to 1.0 were synthesized by co-precipitation method. Prepared samples were characterized to investigate the microstructures developed due to Zn doping with different concentration in cobalt ferrites and the impact on electrical transport and magnetic properties. X-ray diffraction (XRD) patterns confirmed the FCC spinel structure of synthesized particles. The crystallite sizes were calculated from the most intense peak (311) using the Debye-Scherrer formula. At reaction temperature of 70°C, the obtained crystallite size was 8 - 21 nm. Then samples were sintered at 550°C for 2 hours and again characterized by XRD. The crystallite sizes and lattice constants for all compositions were calculated before and after sintering to investigate the effects of sintering on microstructure. It was observed that the grain size increased and strains were removed due to sintering. DC electrical resistivity measurements were done as a function of temperature in range 370 K to 690 K. The DC electrical resistivity decreases with increase in temperature ensuring the semiconductors like nature of the material. DC electrical resistivity increased with increase of Zn concentration and also for decreases in crystallite size. Activation energies calculated from DC electrical resistivity were in the range from 0.505 eV to 0.676 eV. Magnetic properties characterized by Vibrating Sample Magnetometery showed that the coericivity and remanence decreases with increase of Zn concentration.
Keywords: Co precipitation, Co-Zn nanoferrites, Structural properties, Electrical properties, Magnetic properties.
The study of nanocrystalline materials is an active area of research in now days. Nanocrystalline spinel ferrite particles continue to be intensively investigated because of their remarkable combination of electrical and magnetic properties and wide range of practical applications. Infect nanostructures bridge the gap between an isolated atom and its bulk counterpart in terms of the role of interatomic interactions. A nanosolid has high surface to volume ratio and hence high ratio of undercordinated atoms at the surface skin. New physical and chemical properties are expected to occur in such systems, because of the large fraction of the under coordinated atoms at the surface and from the confinement of electrons to a rather small volume . Soft ferrites with spinel structure finds a wide variety of technological applications especially in information storage system , Ferrofluid technology , medical applications , magnetocaloric refrigeration , and in high frequency devices due to high resistivity and low eddy current losses . Among spinel ferrites, cobalt ferrites CoFe2O4 are especially interesting because of their high cubic magnetocrystalline anisotropy, high coercivity and moderate saturation magnetization . Recently, cobalt ferrite nanoparticles are also known to be a photomagnetic material, which shows an interesting light-induced coercivity change . There are a number of reports with regards to the study of nanoferrite materials in litrature but only few of them are systematic which combines the effects of dopent elements on microstructures and consequent effects on the electrical and magnetic properties. The structural, electrical and magnetic properties of ferrites are very much sensitive to the method of synthesis and synthesis conditions. Further more composition, microstructures and cation distribution affects the properties of ferrites [9,10]. So the concept of microstructural engineering has been used very successfully to tailor and hence having the desired properties.
In the present study Co1-xZnxFe2O4 nanoparticles with (x) varying from 0.0 to 1.0 were prepared by the chemical co-precipitation method. Spinel ferrites have cubic close-packed arrangement of oxygen atoms, with divalent metal cations and trivalent Fe3+ at two different crystallographic sites. These A and B sites have tetrahedral and octahedral oxygen coordination respectively. In a cubic unit cell only 8 out of 64 available octahedral A sites and 16 out of 32 available octahedral B sites are occupied to maintain the charge neutrality. In respect of cation distribution among these tetrahedral A sites and octahedral B sites, spinal structure can be normal, inverse or partially inverse. The structural formula for a spinel structure can be written as (M2+δFe3+1-δ)A [M2+1-δFe3+1+δ]B O4. Where M stands for a divalent metal cation and δ inversion parameter. For complete normal spinel δ =1, for complete inverse δ = 0, for mixed ferrite, this δ ranges between these two extreme values . Depending upon the cation distribution the Co1-xZnxFe2O4 could be considered between inverse CoFe2O4 ferrites and a normal ferrite ZnFe2O4 . Zinc is known to play a vital role in determining the ferrite properties ; hence the composition was varied by changing the zinc concentration. Synthesized samples were characterized by XRD to investigate the effects of Zn concentration and sintering on the structural properties. Effects of temperature and Zn substitution on electrical transport properties were examined by temperature dependent resistivity measurements. Alteration in magnetic properties like coericivity and remanence againest Zn substitution was studied by vibrating sample magnetometery. It was found that the coericivity and remanence decreases with increase of Zn concentration.
2.1. Synthesis of Co1-xZnxFe2O4
Co1-xZnxFe2O4 ferrite nanoparticles with x varying from 0.0 to 0.1 were prepared by co-precipitation method. The chemical reagents used in this work were ferric nitrate (hydrated) Fe(NO3)3.9H2O, cobalt nitrate (hydrated) Co(NO3)2.6H2O and zinc nitrate (hydrated) Zn(NO3)2.6H2O. All reagents were of analytical grade and used without further purification. To have equal conditions for all three metals, they all were taken in the form of their nitrates. The aqueous solutions of Co(NO3)2.6H2O, Zn(NO3)2.6H2O and Fe(NO3)3.9H2O used were 0.4 M in their stoichiometry and mixed by a constant stirring using magnetic stirrer until a clear solution was obtained. The soloution of the co-precipitation agent (NaOH) used was 1.5 M. The aqueous solution of precipitating reagent (NaOH) was added quickly into metal solution, with constant stirring at room temperature. The required Co1-xZnxFe2O4 nanoferrite particles obtained at reaction temperature of 70°C. The pH of the reaction was kept between 12.5 and 13. The precipitates were thoroughly washed in deionized water and then dried in an electric oven at 100°C to remove water contents. The dried powder was grinded and mixed homogeneously in agate and mortar. The grinded powder was then palletized using hydraulic press. The pellets were sintered at 550 °C for 2h and then slowly cooled to room temperature.
The structure and crystallite sizes were determined from X-ray diffraction (XRD) data. The XRD patterns were obtained at room temperature using CuK¡ (λ=1.5406 Å) radiation. The X-ray patterns were recorded by varying 2θ from 20ο to 70ο. The crystallite sizes were calculated by Debye-Scherrer formula  using the full width at half maximum (FWHM) value of the most intense (311) peak.
t = 0.89λ/βcosθ (1)
The lattice constant 'a' was calculated using the relation given below.
a = d (hkl) (2)
The theoretical density of the prepared samples was calculated by using the relation.
ρx = nM/Na3 (3)
Where n is the number of molecules per unit cell, M is the molecular weight of the samples, N is the Avogadro's number and 'a' is the lattice parameter. The measured density, ρm was determined by using the relation
ρm = m /π r2h (4)
Where m is the mass, r the radius and h is the thickness of the sample pellet. The porosity ( P ) of the prepared samples for all compositions was determined by using the relation.
P = 1- ρm / ρx (5)
Where ρm is measured density and ρx is theoretical density. Considering all the particles to be spherical, the specific surface area was calculated from the relation
Where D is the diameter of the particle and ρm is the measured density in g/cm3. The DC electrical resistivity as a function of temperature was measured by two probe method. The two probe method is the standard and most commonly used method for the measurement of very high resistivity samples. The relationship between resistivity and temperature may be expressed as
where ρ is the DC electrical resistivity at temperature T, kB the Boltzmann constant and ΔE the activation energy corresponding to electrical process. Drift mobility (µd) of all the samples has been calculated using the relation
µd =1/neρ (8)
Where e is the charge of electron, ρ the electrical resistivity at a particular temperature and n is the concentration of charge carrier, which can be calculated from the relation,
n = Na ρm PFe/M (10)
where M is the molecular weight, Na the Avogadro's number, ρm the measured density of sample and PFe is the number of iron atom in the chemical formula of the ferrites.
3. Results and discussion
3.1. Structural properties
The X-ray diffraction (XRD) patterns confirmed FCC spinel structure for all prepared samples of Co1-xZnxFe2O4 nanoparticles with 'x' changing from 0.0 to 1.0. The crystallite sizes of as prepared samples calculated by Debye-Scherrer formula using the full width at half maximum (FWHM) value of the most intense (311) peaks, were in the range from 8nm to 21nm. The obtained values of lattice constant 'a' in our study were in good agreement with the reported data. The lattice constant 'a' is found to increase with increase of Zn concentration. It is due to the fact that the radius of the Zn ion (0.82 Å) is larger than that of the Co ions (0.78Å). Addition of Zn at the expense of Co in the composition is expected to increase the lattice constant.
To study the effects of sintering pellets of prepared powder of all compositions of Co1-xZnxFe2O4 were sintered at 550°C for two hours and then characterized by XRD. The XRD results showed that the grain size increases by sintering where lattice constants get uniformity. The values of lattice constants and crystallite sizes are given in table 1. It is due to the reason that by heating the strains in the crystal structures were removed and texture improved. The change in lattice constants with Zn concentration is shown in figure 2. Increase in crystallite size occurred due to coalescence, during sintering two or more particles seem to fuse together by melting of their surfaces. In nano size particles surfaces melts below the melting point of their bulk counter part because of large fraction of low coordinated atoms that lies at surface skin. The values of theoretical density (ρx), measured density (ρm), and porosity (P), are given in table 2. It was observed that due to increase of Zn concentration, there is no dominant change occurred in theoretical density. Because with increase of Zn concentration lattice constant increased which decreased the density but at the same time the Zn atoms which replaces Co atoms, has greater atomic mass (The atomic mass of Co is 58.933 g/mol and that of Zn is 65.37 g/mol) as compared with cobalt atoms and compensate this decrease in density. The decrease in measured density for x = 0.2 and for x = 0.8 is due to greater values of porosity of these two samples as compared to the other compositions as shown in table2. The specific surface area decreases with increase in particle size.
3.2. Electrical properties.
Electrical properties of ferrite nanoparticles depend upon the method of preparation, chemical composition, cation distribution, grain size and sintering temperature generally, because these parameters affects the structure of the particles. Conduction in ferrites is reported to occur as a result of electron hopping between ions of the same element existing in different valence state on equivalent lattice sites . The DC electrical resistivity for all these samples was measured by two probe method from 370 K to 690 K temperature range. The DC electrical resistivity of the ferrite particles was found to vary from 8.7-103 Ω cm to 1.33-105 Ω cm at 625K (figure 3) with change of Zn concentration from x = 0.0 to 1.0 in Co1-xZnxFe2O4. It is observed that DC electrical resistivity increases significantly with increas of Zn concentration. The variation of resistivity can be explained on the basis of actual location of cations in the spinel structure and also by microstructures of the material.
The spinel structure of ferrites consists of two interpenetrating sub-lattice forming two types of sites where metal ions are located, the tetrahedral (A) and tetrahedral (B) sites. Conduction mechanism in ferrites is considered as the electron hopping between Fe2+and Fe3+in (B) sites and mobility of holes between Co2+ and Co3+ ions. The addition of Zn2+ ions leads to increase of Fe3+ ions in octahedral site and a simultaneous decrease of Co2+ ions present at the same time. Also with increase of Zn concentration the lattice expands that increase the activation energy, therefore DC resistivity increases with increase of Zn concentration. At higher temperature at about 400 K conductivity is attributed to the hoping of holes between Co2+ and Co3+ ions. So the placement of zinc at the expense of cobalt will consequently decrease the process and as a result conductivity decreases . The change in DC electrical resistivity as a function of Zn concentration at 625 K temperature is shown in figure 4. In our prepared material the crystallite size decreases gradually with increase of Zn concentration as given in table 2. The structure of the prepared material consists of conducting grains separated by highly resistive thin layers (grain boundaries). As the crystallite size decreases the total grain area increases and due to increase in grain area resistivity increases. In temperature range from 370 K to 690 K, decrease in resistivity with increase of temperature showes the semi conducting nature of the material in this temperature range. The values of activation energies evaluated from the slopes of the linear plots of DC electrical resistivity (Figure 5) were ranges from 0.505 eV to 0.676 eV as shown in figure 4, which is greater than 0.4 eV and clearly suggests that the conduction is due to polaron hopping . The hopping depends upon the activation energy, which is associated with the electrical energy barrier experienced by the electrons during hopping.
3.3. Magnetic Measurements.
The magnetic properties such as coercivity (Hc) and remanence (Mr) of Co1-xZnxFe2O4 for x = 0.0, 0.2, 0.4 were measured at room temperature by vibrating sample magnetometer (VSM). Figuers 6(a), 6(b) and 6(c) shows the effects of Zn concentration on coercivity and remanence. It was found that the coercivity and remanence were maximum with minimum value of Zn in Co1-xZnxFe2O4. The magnetic properties of the nano ferrite particles depend upon the cations types and their specific site position. Super exchange interaction takes place between the metal ions in the tetrahedral A sites and octrahedral B sites which give rises the magnetic order in the cubic system of spinel ferrites . Exchange interaction between magnetic ions of sublattices A and B is the strongest, A-A interaction is about ten times weaker and B-B interaction is the weakest. Cobalt ferrite have partially inverse spinel structure. In the inverse spinel ferrites one half of Fe3+ is placed in A-sites and half in B-sites. Their magnetic moments are mutually compensated and the resulting moment of the ferrite is due to the magnetic moments of bivalent cations Me2+ in the B-positions . For higher Zn concentrations ferrite becomes normal spinel. It means that there are no more Zn2+ in B-sites and no more Fe3+ in A-sites. So A-B exchange interaction becomes weak and the role of B-B interaction increases. Zn is a non magnetic ion and prefers tetrahedral A site. The placement of Zn at tetrahedral A site reduces the exchange interaction between A and B sites. Therfore the coercivity and remanence decreased with increase of Zn concentration and are given in table 2.
3.4. Dielectric Properties.
The dielectric constant (ε) of zinc substituted cobalt ferrites have been measured at room temperature in the high frequency range from100 Hz to1MHz. The plots of dielectric constant versus frequency show a normal dielectric behavior of the spinel ferrites. The variations of dielectric constant as a function of frequency for mixed Co-Zn ferrites with different compositions are shown in Figure 7. The characteristic features of the dielectric properties have been known to arise from distribution and type of the cations among thetetrahedral A-site and octahedral B-sites in the spinel structure . Graphs shows that the dielectric constant is maximum for CoFe2O4 and decreases as the zinc content is increased. Also the dielectric constant decreases with increase of frequency. All the samples show the frequency dependent behavior. As the frequency of the externally applied electric field increases gradually, the dielectric constant decreases. This reduction occurs because beyond a certain frequency of the externally applied electric field the electronic exchange can not follow the frequency of the external applied electric field and as a result dielectric constant (ε) decreases. The decrease of dielectric constant with increase of frequency as observed in the case of mixed Co-Zn ferrites is a normal dielectric behavior observed in most of the ferromagnetic materials which is due to interfacial polarization as predicted by Maxwell Wanger . To explain the compositional dependence of the dielectric constants of mixed Co-Zn ferrites, it can be seen that the number of ferrous ions on the octahedral sites which take part in the electron exchange interaction between Fe2+ and Fe3+ and hence are responsible for the polarization is maximum in the case of cobalt ferrite (CoFe2O4), therefore a high value of the dielectric constant is expected and is observed. It has been already observed for spinal structure that the electron hopping between Fe3+ and Fe2+ leads to the large dielectric constant (ε) and dielectric relaxation . As the zinc content in the mixed Co-Zn ferrites is continuously increased at the expense of cobalt the number of ferrous ions on the octahedral sites which are available for polarization decreases, resulting in a continuous decrease in the dielectric constant (ε).
Co1-xZnxFe2O4 nanoparticles (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared by the chemical co-precipitation method. X-ray diffraction analyses confirmed the FCC spinel crystal structure. Grain growth was found to occur with sintering, also the strains in the lattice were removed. Lattice constant increased with increase in Zn concentration. DC electrical resistivity decreased with increase in temperature. With increase in Zn concentration DC electrical resistivity increased. Activation energy for all the samples is greater than 0.4 eV ensuring that the conduction in Zn doped Co nanoferrites is due to polaron hoping. The coericivity and remanence decreases with increase of Zn concentration. The dielectric constant decreases with increase of frequency and also with increase of Zn concentration. All of above discussed properties has strong dependence on microstructures and composition of the prepared materials.
Higher Education Comission (HEC) of Pakistan is highly acknowledged for providing financial support for this research work.
Captions of Figures and Tables
Figure 1: X-ray diffraction pattern of Co1-xZnxFe2O4
Figure 2: Change in lattice constants with Zn concentration
Figure 3: Variation of resistivity ρ(Ω-cm) with change in temperature
Figure 4: Variation in resistivity at temperature of 625 K for different concentrations of Zn
Figure 5: Variation of DC electrical resistivity with temperature with linear fit diagram to
measure activation energy
Figure 6(a), 6(b) and 6(c):Vibrating Sample Magnetometery graphs of CoFe2O4 , Co0.8Zn0.2Fe2O4
and Co0.6Zn0.4Fe2O4 respectively
Figure 7: Effect of Zn concentration and frequency of the applied electric field on dielectric
constants for Co1-xZnxFe2O4
Table 1: Comparison between lattice parameter, particle size with Zn concentration, before and
Table 2: Grain size d(311), grain volume (V), lattice constant(a), X-ray density(ρx), measured
density(ρm), activation energy ΔΕ(eV), drift mobility (µd), porosity (P), specific surface
area (S) , resistivity(ρ), coercivity (Hc) and remanence (Mr) for different values of 'x' in
Fig. 1 Fig. 2
Fig. 6(a) Fig 6(b)
x = 0
x = 0.2
x = 0.4
x = 0.6
x = 0.8
x = 1.0
x = 0
x = 0.2
x = 0.4
x = 0.6
x = 0.8
x = 1.0
ρ (Ω-cm) at 625 K
8.7 x 103
µd(cm2/Vs) at 625K
3.2 x 10-7