Background

Mn-Zn ferrites are widely used in electronic applications, such as magnetic recording heads, transformers, choke coils, noise filters, electromagnetic gadgets, and memory or data storage devices (Dasgupta et al. 2006). Ferrimagnetic cubic spinels viz., ferrites possess properties of both magnetic materials and electric insulators making them an important material in many technological applications. Ferrites are preferred for their high permeability in the radio frequency region, high electrical resistivity, mechanical hardness, chemical stability, etc. (Zheng et al. 1998). Along with the development of science and technology, devices containing ferrites needed to get smaller in size for higher performance and the interest in soft magnetic materials therefore turned to the synthesis of nanocrystals and their substitutions, expecting to improve the properties (Yeong et al. 2003). Recently, cobalt ferrite nanoparticles were found to be photo-magnetic, exhibiting interesting light-induced coercivity changes (Arulmurugan et al. 2005) and Co-Zn ferrite was found to possess good elastic properties (Patil Sagar et al. 2009). Preparing stable colloidal suspensions of nanoparticles is challenging due to Vander Waal’s forces and magnetic dipolar interactions. Hence, coating magnetic nanoparticles with a surfactant during chemical synthesis is essential in order to prepare well-dispersed nanoparticle colloids. Coating with oleic acid limits the growth of the nanoparticles and prevents the Ostwald ripening process as its surface layer acts as a barrier to mass transfer (Ayyappan et al. 2009; Davies et al. 1993). Oleic acid has also been widely used as a surfactant in the synthesis of mono dispersed nanoparticles (Tadmor et al. 2000), and it contains a carboxyl functional group that is used in bio applications, for example, to immobilize oligo nucleotides, and anti-cancer drug on particle surfaces (Kim et al. 2005; Somiya et al. 2003).

However, very less attention has been paid to the study of the electrical and leakage properties of cobalt-doped ferrite nanopowders coated with oleic acid. The investigations of electrical and leakage current properties of cobalt-doped ferrite nanopowders are thus important from the point of view of its use in electrical and electronic industry applications and hence the synthesis, leakage current properties, and the effects of strain on electrical properties are discussed in this report.

Methods

Apparatus and chemicals

The X-ray diffraction (XRD) patterns of the samples were recorded on a BRUKER-binary V2 (RAW) powder diffractometer using CuKα (λ = 1.5406 Å) radiation. The V-I characteristics were studied using KEITHLEY 6517A Electrometer/High Resistance Meter and the dielectric properties were studied using HIOKI 3532-50 LCR HITESTER.

The materials that were procured for synthesizing the samples were of high grade purity. The synthesis of the nanoferrite material was done by the chemical co-precipitation method. For synthesizing the nano ferrite particles from the pure chemicals the salts of constituent metals were taken as starting materials. Sodium hydroxide (NaOH), a reducing agent was used as a base. The chemicals, Cobalt II chloride (CoCl2.6H2O), Manganese chloride (MnCl2.4H2O), Zinc chloride (ZnCl2), Iron (III) Chloride (FeCl3.6H2O) were purchased from Merck, India. All the chemicals used were of analytical grade.

Synthesis of cobalt-doped Mn-Zn ferrites coated with oleic acid

The cobalt-doped ferrite nanoparticles synthesized by co-precipitation depends mostly on parameters viz., reaction temperature, pH of the suspension, initial molar concentration etc., (Jeyadevan et al. 2003). Ultra fine particles of CoxMny ZnyFe2O4 (x = 0.1, 0.5, and 0.9 and y = 0.45, 0.25, and 0.05) were prepared by co-precipitating aqueous solutions of CoCl2, MnCl2, ZnCl2, and FeCl3 mixtures in alkaline medium in their respective stoichiometric ratios at 333 K (60 °C).

This mixture was added to a boiling solution of NaOH (0.1 M dissolved in 1000 ml of distilled water) under constant stirring. Nanoferrites are formed due to the conversion of metal salts into hydroxides that take place immediately, followed by the transformation of hydroxides into ferrites. The solutions were maintained at 358 K (85 °C) for 1 h, and sufficient amount of fine particles were collected at this stage using magnetic separation. The particles were washed several times with distilled water followed by washing with acetone and are then dried at room temperature. For the preparation of the coated particles, a suitable surfactant (oleic acid) is added to the boiling solution of NaOH along with the solution mixture. The powdered samples were then pelletized at a pressure of 300 Kg/cm2 to yield pellets of 10 mm diameter and approximately 2 mm thickness for all the three compositions.

Characterization

The X-ray diffraction (XRD) patterns of the samples were recorded on a BRUKER-binary V2 (RAW) powder diffractometer using CuKα (λ = 1.5406 Å) radiation. Slow scans of the selected diffraction peaks were carried out in step mode (step size 0.02°, measurement time 5s, measurement temperature 323 K (25 °C), standard; Si powder). The crystalline size of the nanocrystalline samples was measured using Debye-Scherrer formula. The V-I characteristics were studied using KEITHLEY 6517A Electrometer/High Resistance Meter and the dielectric properties were studied using HIOKI 3532-50 LCR HITESTER.

Results and discussion

Structural characterization

The precipitated fine particles were characterized by XRD for structural determination and estimation of crystallite size. The XRD patterns were analyzed using JCPDS with all the peaks matching with the theoretically generated file (PDF#871171) and indexed. Analysis of the diffraction pattern using powder-X software (Creanga and Calugaru 2005) confirms the formation of tetragonal spinel structure in all the samples. The X-ray diffraction patterns (Fig. 1) of Co-doped Mn-Zn ferrites [Co x Mn y Zn y Fe2O4 (x = 0.1, 0.5, and 0.9 and y = 0.45, 0.25, and 0.05) (C1, C5, C9)] coated with oleic acid indicate that the functionalization does not degrade the core cobalt ferrite (Andrade et al. 2014).

Fig. 1
figure 1

XRD analysis of CoxMnyZnyFe2O4 (x = 0.1, 0.5, and 0.9 and y = 0.45, 0.25, and 0.05)

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The broadening of XRD peaks shows refinement of grains and the strain associated with the nanocrystalline material. Williamson and Hall (W-H) proposed a method of deconvoluting size and strain from the mathematical expression,

$$ {\beta}_{hkl}\ \cos \theta = \left[\frac{k\lambda}{D}\right]+4{\varepsilon}_{\mathrm{s}}\ \sin \theta $$
(1)

where k is the shape factor, λ is the X-ray wavelength, θ is the Bragg angle, D is the effective crystallite size (calculated by Debye-Scherrer equation), ε s is the strain, and β hkl is the full width at half maximum of the corresponding hkl plane.

A plot drawn between 4 sinθ along the X-axis and β hkl cosθ along the Y-axis is shown in Fig. 2a–c. From the data of linear fit, the value of strain was calculated from the slope of the line. Negative slope in plots a and b (Fig. 2a, b) indicates that the system is under compressive strain (Maurya et al. 2011). Positive slope in plot c (Fig. 2c) indicates that the system is under tensile strain. The average crystallite sizes of the prepared ferrite particles were calculated by means of the Debye-Scherrer equation (Sharma et al. 2005). The crystallite size was found to decrease with increase in cobalt content as was reported by many researchers (Zhang et al. 2009).

Fig. 2
figure 2

a W-H plot of Co x Mn y Zn y Fe2O4 with (C1) x = 0.1 and y = 0.45. b W-H plot of Co x Mn y ZnyFe2O4 with (C5) x = 0.5 and y = 0.25. c. W-H plot of Co x Mn y Zn y Fe2O4 with (C9) x = 0.9 and y = 0.05

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The lattice parameter, average crystallite size, and lattice strain are given in Table 1. From the table, strain (ε s) values were found to be −0.01254 for x = 0.1, −0.00549 for x = 0.5 and 0.00287 for x = 0.9.

Table 1 Lattice constants, strain, and crystallite size of nanopowders
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Dielectric studies

Figures 3 and 4 illustrate the frequency dependence of the real and imaginary part of the dielectric constants for all composition at room temperature. The dielectric constant decreases with increasing frequency exhibiting normal ferrimagnetic behavior for all the compositions (Smit and Wijn 1959). As the frequency increases, the dielectric constant remains almost constant. This is a normal behavior of ferrites and can be explained with the help of Maxwell-Wagener model (1951) (Wagner 1913).

Fig. 3
figure 3

Variation of ε′ with frequency for C1(x = 0.1, y = 0.45), C5(x = 0.5, y = 0.25), and C9(x = 0.9, y = 0.05)

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Fig. 4
figure 4

Variation of ε″ with frequency for C1(x = 0.1, y = 0.45), C5(x = 0.5, y = 0.25), and C9(x = 0.9, y = 0.05)

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Since the grain boundaries of lower conductivity are found to be effective at lower frequencies while the ferrite grains of high conductivity are effective at high frequencies (Koops 1951; Kumar and Srivastava 1994), higher values of ε′ at lower frequencies and vice versa are observed. For example, the ε′ values in the order of 104 in the frequency range 102 to 103 Hz decreases to the order of 103 with increasing frequency. The low dielectric values make these ferrites useful in high-frequency applications (Pathan and Shaikh 2012). The variation of dielectric constant with various concentrations of cobalt at 5, 10 and 100 KHz, for particles coated with oleic acid is shown in Fig. 5. At a frequency of 10 KHz, the dielectric constant was found to decrease slightly and then increase with increasing cobalt content (Bhuvaneswari et al. 2016). However the dielectric constant increases and then decreases with increase in cobalt content at the frequency of 100 KHz which may be attributed to the corresponding changes in microstructures due to the coating of oleic acid.

Fig. 5
figure 5

Variation of dielectric constant with cobalt content at 5, 10 and 100 kHz

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Loss tangent

The loss tangent represents the energy dissipation of dielectric material. The variation of tan δ with frequency for various compositions at room temperature is shown in Fig. 6. The curves suggest that tan δ strongly depend on the applied frequency (Rani et al. 2013). The dielectric loss decreases rapidly with frequency up to 10 kHz for all the compositions. For the composition Co0.5Mn0.25Zn0.25Fe2O4 (C5), the loss was found to be very low (<0.4) at a frequency of 100 kHz.

Fig. 6
figure 6

Variation of tan ∂ with frequency for C1(x = 0.1, y = 0.45), C5(x = 0.5, y = 0.25), and C9(x = 0.9, y = 0.05)

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The dependence of the imaginary part (ε″) with frequency is similar to that of the dielectric loss on frequency. This shows that ε″ is directly proportional to tan δ according to the following equation:

$$ \upvarepsilon^{\prime \prime } = \upvarepsilon^{\prime }\ \tan\ \updelta $$
(2)

where ε″ is the imaginary part of dielectric, ε′ is the real part and tan δ is the dielectric loss. ε′ was calculated using the following equation.

$$ {\upvarepsilon}^{\prime } = \frac{\mathrm{C}\ \mathrm{d}}{\upvarepsilon_0\mathrm{A}} $$
(3)

where ε 0 = 8.854x10−12 F/m is the absolute permittivity of free space, C is the capacitance, A is the area, and d is the thickness.

Impedance analysis

Impedance spectroscopy is a relevant technique to obtain information about the electrical behavior of nano- and polycrystalline materials as the impedance measurement data gives both resistive (real) and reactive (imaginary) components. To separate the grain and grain boundary contributions of the cobalt-doped Mn-Zn ferrite samples, complex impedance plane plots (Cole-Cole plots) have been drawn in the frequency range from 100Hz to 100 KHz at room temperature. One or two successive semicircles may occur due to grain or grain boundaries. These contributions can conventionally be displayed in a complex plane plots (Nyquist diagram) in terms of the formalism.

Complex impedance is given by as follows:

$$ {\mathrm{Z}}^{*}\left(\upomega \right)=\mathrm{Z}^{\prime }-\mathrm{j}\mathrm{Z}^{\prime \prime } $$
(4)

where Z′ = |Z |cosθ and Z″ = |Z| sinθ. In Fig. 7a–c, two semicircles are seen for all compositions [Co x Mn y Zn y Fe2O4 (x = 0.1, 0.5, and 0.9 and y = 0.45, 0.25, and 0.05)] even at room temperature. The semicircle in the higher frequency region is attributed to the grain conduction of the materials which occurs due to parallel combination of the grain resistance R g and grain capacitance C g of the material. The semicircle in the lower frequency region is attributed to the grain boundary conduction which occurs due to parallel combination of the grain boundary resistance R gb and grain boundary capacitance C gb of the material (Barik et al. 2011; Dasari et al. 2011). These two semicircles can be modeled by two parallel RC equivalent circuits in series configuration as shown in Fig. 8. It is noticed that the circular arcs in the lower and higher frequency regions are not completed which may be due to the fact that grain boundary resistance and grain resistance are out of measurement scale (Ali et al. 2015).

Fig. 7
figure 7

a–c Nyquist’s plot (Z′ vs Z″) for C1(x = 0.1, y = 0.45), C5(x = 0.5, y = 0.25), and C9(x = 0.9, y = 0.05)

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Fig. 8
figure 8

Equivalent circuit for Nyquist’s plot (Z′ vs Z″) for C1(x = 0.1, y = 0.45), C5(x = 0.5, y = 0.25), and C9(x = 0.9, y = 0.05)

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AC conductivity

Figure 9 shows the frequency dependence of AC conductivity at room temperature. The curves suggest that the values of AC conductivity increases gradually with increase in frequency, which is the normal behavior of ferrites (Sathishkumar et al. 2010). The increase in AC conductivity with applied field can be explained on the basis of pumping force of the applied field which promotes the transfer of charge carriers between the two Fe ion states and also the liberation of charges from different trapping centers. These charge carriers along with electrons produced from the valence exchange between different metal ions participate in conduction process. From Fig. 9, it is evident that AC conductivity is higher for the composition Co0.9Mn0.05Zn0.05Fe2O4 indicating that AC conductivity is compositional dependent (Batoo 2011).

Fig. 9
figure 9

Frequency vs AC conductivity for C1(x = 0.1, y = 0.45), C5(x = 0.5, y = 0.25), C9(x = 0.9, y = 0.05) at room temperature

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Leakage current density

The leakage current density (J) is measured as a function of applied field for all the compositions as shown in Fig. 10a–c. For a given applied voltage, the current was found to decay with time and the measurements were taken after the stabilization of current. The V-I characteristic curves show the voltage polarity dependence (Stolichnov et al. 2000). For coated particles, a low leakage current density of the order of 10−4 A/cm2 is measured at a field strength of 0.02 kV/cm.

Fig. 10
figure 10

Leakage current density of coated particle as a function of electric field of a Co0.1 Mn0.45Zn0.45Fe2O4, b Co0.5 Mn0.25Zn0.25Fe2O4, and c Co0.9 Mn0.05Zn0.05Fe2O4

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Conclusions

From the above study it could be seen that the oleic acid prevents agglomeration of the cobalt ferrite nanoparticles, and their crystallinity is not affected by the oleic acid coating. The crystallite size was found to decrease with increase in cobalt content, and the micro strain values of materials identified from the Williamson-Hall plot model shows all the samples having a very low value of micro strain due to calcinations. The variation of the strain values from negative to positive indicates that the strain changes from compression to tensile. The dielectric permittivity was found to decrease from 104 to 103 with increase in frequency. The variation in the behavior of dielectric constant with cobalt concentration at 10 and 100 KHz may be attributed to the changes in microstructures due to the coating of oleic acid. The dielectric loss was found to decrease with frequency up to 10 KHz for all the compositions, and the loss was found to be very low (<0.4) at a frequency of 100 KHz for the composition Co0.5 Mn0.25Zn0.25Fe2O4 (C5). The impedance spectroscopy analysis shows two semicircular regions for all the compositions even at room temperature. The semicircle in the higher frequency region is attributed to the grain conduction of the materials, and the semicircle in the lower frequency region is due to the grain boundary conduction. Both the grain and grain boundary are found to be active at room temperature. The AC conductivity is observed to increase gradually with the increase in frequency, which is the normal behavior expected of ferrites. AC conductivity was found to be higher for the composition Co0.9Mn0.05Zn0.05Fe2O4 indicating compositional dependence of AC conductivity. A low leakage current density of the order of 10−4 A/cm2 was also observed at a field strength of 0.02 kV/cm for all compositions.