A study on leakage current and electrical properties of oleic acid-coated cobalt-doped Mn-Zn ferrite nanocrystalline powders
© The Author(s). 2017
Received: 8 December 2016
Accepted: 12 April 2017
Published: 25 April 2017
Mn-Zn ferrites have drawn a continuously an increasing interest because of their potential applications as multifunctional devices. These materials simultaneously exhibit ferroelectricity and ferromagnetism. The dielectric and leakage current properties of Cobalt substituted Mn-Zn ferrites coated with oleic acid were not reported.
This paper presents the synthesis, electrical, and leakage properties of nanoparticles of cobalt-doped Mn-Zn ferrite [CoxMnyZnyFe2O4 (x = 0.1, 0.5, and 0.9 and y = 0.45, 0.25, and 0.05)] coated with oleic acid and prepared by chemical co-precipitation method. The crystal structure was determined by X-ray diffraction (XRD), the effect of strain on the electronic structure was analyzed using Williamson-Hall plot. Complex impedance spectroscopic analysis was carried out, and the impedance plots show the resistive and reactive parts of the impedance. Frequency dependence on AC conductivity was investigated for all the compositions, and leakage current properties were also studied.
The nanoparticles were found to have an average size of 13.62 nm. The average crystallite size (DaveXR) of the precipitated particles found to decrease from 15.22 to 12.65 nm with increasing cobalt substitution. The presence of two semicircular arcs at the lower and higher frequency regions indicates the grain boundary conduction and grain conduction at room temperature. Leakage current density of the order of 10-4 A/cm2 (at field strength of 0.02 kV/cm) was observed for all compositions.
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 104to 103 with increase in frequency. 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. AC conductivity is found to be compositional dependent.
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.
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.
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
Lattice constants, strain, and crystallite size of nanopowders
Lattice parameter (Å)
Volume of the unit cell (Å3)
Strain (ε s)
Crystallite size (nm)
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.
Leakage current density
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.
All authors acknowledged that the Anna University–UCE Pattukkottai for providing platform and technical help for this work.
MB carried out the all the experimental work, RT participated in the interpretation of experimental and analysis, and VS participated in the sequence alignment and drafted the manuscript. SS participated in its design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.
The all authors declare that they have no competing interests.
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