Synthesis, investigation on structural and electrical properties of cobalt doped Mn–Zn ferrite nanocrystalline powders

The crystalline structure of the CMZF nanopowders was examined using BRUKER-binary V2 (RAW) powder diffractometer using CuKα (λ = 1.54060 Å) radiation. The surface morphologies of the nanopowders were measured with a scanning electron microscope. For the electrical measurements, the powder sample was pelletized at a pressure of 300 kg/cm2 to yield a pellet of 10 mm diameter and 2 mm thickness. The dielectric constant and loss tangent of these powders were measured by using LCR HiTESTER (HiOKI 3532-50). The crystalline structure of CMZF nanopowders was determined by using FT-IR spectroscopy.

The XRD patterns of Co_xMn_yZn_yFe₂O₄ (x = 0.1, 0.5, 0.9 and y = 0.45, 0.25, 0.05) nanopowders are shown in Fig. 2. The diffraction peaks compared to the standard XRD pattern (PDF# 871171) correspond to (1 1 2), (1 0 3), (2 0 2), (2 1 3), (3 0 3), (2 2 4) and (3 1 4) planes. The peaks detected in the XRD patterns belong to the tetragonal structure of spinel ferrite and confirm the lack of formation of any secondary phase [13]. Fig. 3 shows that the peaks are shifted to higher angles with an increase in cobalt concentration in the Mn–Zn ferrite. The peak position was found to shift towards higher diffraction angles due to the smaller ionic radius of cobalt (0.938 Å) compared to Mn and Zn radius.

Fig. 2

Fig. 3

XRD analysis can also be used to evaluate peak broadening with crystallite size, and lattice strain due to dislocations [14]. The crystallite sizes of the Co_xMn_yZn_yFe₂O₄ samples found using Debye-Scherrer formula (equation 1) were 35 nm, 37 nm and 28 nm at x = 0.1, 0.5 and x = 0.9, respectively:

where D is the crystallite size, k is the shape factor (k = 0.9), λ is the wavelength of CuKα radiation and β_hkl is the instrument corrected integral breadth of the reflection (in radians) located at 2θ, and θ is the angle of reflection (in degrees).

The values of lattice parameters of Co_xMn_yZn_yFe₂O₄ nanopowders were calculated and listed in Table 1.

The values were compared with the standard values of Mn–Zn ferrite. The ratio of c/a had the value greater than unity, attributed to a tetragonal structure. Fig. 4. shows the graphical representation of lattice parameter values with respect to enhancement in cobalt content in the Mn–Zn ferrite. The lattice parameter values were found to increase with increasing cobalt content up to x = 0.5. Upon further increase in the cobalt content, the value decreased due to the density of cobalt in the unit cell. This behavior was confirmed by the volume of unit cell and strain calculation.

Fig. 4

The scanning electron micrographs of Co_xMn_yZn_yFe₂O₄ nanopowders are shown in Fig. 5. It implies that the materials have a well-defined microstructure. The micrographs indicate homogeneous distribution of polycrystalline grains throughout the material. This also indicates that the CMZF nanopowders possess relatively narrow size distribution.

Fig. 5

3.1.1

W-H analysis

Since the breadth of the Bragg peak is the combination of both instrumental and sample dependent effects, it is necessary to collect a diffraction pattern from the line broadening of a standard material such as silicon to determine the instrumental broadening [15].

The instrumental corrected broadening β_hkl corresponding to the diffraction peak of Co_xMn_yZn_yFe₂O₄ was estimated from the relation:

βhkl=[(βhkl)measured2−βinstrumental2]1/2$${\beta _{hkl}} = {\left[ {\left( {{\beta _{hkl}}} \right)_{measured}^2 - \beta _{instrumental}^2} \right]^{1/2}}$$(2)

Williamson and Hall (W-H) proposed a method of deconvoluting size and strain from the mathematical expression given by:

βℏklcosθ=[kλD]+4εssinθ$${\beta _{\hbar kl}}\cos \theta = \left[ {\frac{{k\lambda }}{D}} \right] + 4{\varepsilon _s}\sin \theta $$(3)

where k is the shape factor, λ is the X-ray wavelength, θ is the Bragg angle, D is the effective crystallite size, ∊_s is the strain and β_hkl is the full width at half maximum of the corresponding (h k l) plane. A plot drawn between 4 sinθ along the X-axis and β_hkl cosθ along the Y-axis is shown in Fig. 6. From the data of linear fit, the value of strain was calculated from the slope of the line. W-H analysis (equation 3) is a simplified integral-breadth method, where size-induced and strain-induced broadening is deconvoluted by considering the peak width as a function of 2θ [16]. The strain ∊ was evaluated from the slope of the linear fit and the values were found to be 0.00153 for x = 0.1, −0.0007048 for x = 0.5 and −0.0008518 for x = 0.9 respectively. The strain results show that the strain changes from tensile to compressive nature with the increase in the cobalt content in the Mn–Zn ferrite.

Fig. 6

The dielectric properties of Co_xMn_yZn_yFe₂O₄ with x = 0.1, 0.5, 0.9 and y = 0.45, 0.25, 0.05 were studied using HIOKI 3532-50 LCR HITESTER. Dielectric loss, dielectric constant were computed according to Smit and Wijn [17] as a function of frequency. Fig. 7 and Fig. 8 show that the dielectric constant as well as dielectric loss decrease with increasing frequency, exhibiting normal ferrimagnetic behavior for all the compositions. It can be seen that the dielectric constant decreases with increasing frequency in the lower frequency region. As the frequency increases to higher values, the dielectric constant remains almost constant. This is a typical behavior of ferrites and it can be explained with the help of Maxwell-Wagner model [18]. According to this model, ferrites consist of two layers, first of which contains well conducting grains (ferrous ions), and the second one is composed of poorly conducting grain boundaries. These grain boundaries are more active at lower frequencies; hence, the hopping frequency of electrons between Fe³⁺ and Fe²⁺ ions is less at lower frequencies.

Fig. 7

Fig. 8

The dielectric properties of ferrites cause localized accumulation of charge under the influence of electric field. The polarization in ferrites is similar to the conduction of electrons in Fe²⁺ and Fe³⁺ . At lower frequencies the conduction is favorable and, as a result, the dielectric constant is high but it continuously decreases with increasing frequency because the electron exchange between Fe²⁺ and Fe³⁺ cannot follow the higher AC field frequency. Largeteau et al. [19] showed that permittivity values are composition dependent and for ferrites containing ferrous ions in excess, the relative real permittivity usually attains values as high as 10⁵ to 10⁶ for the frequency ranging from 10² Hz to 10⁵ Hz. The observed permittivity values ∊’ are of the order of 10⁴ in the frequency range of 10² Hz to 10³ Hz. The low dielectric values make these ferrites useful in higher frequency applications [20].

Fig. 9 shows the variation of dielectric constant with an increase in cobalt concentration at different frequencies (1 kHz, 10 kHz and 100 kHz). The decrease of dielectric constant with cobalt substitution can be explained on the basis of the mechanism of polarization process in ferrites, which is similar to that in the conduction process.

Fig. 9

The whole polarization in ferrites is mainly contributed by the space charge polarization which is governed by the number of space charge carriers. The conductivity in materials and the hopping exchange of the charges between two localized states is governed by density of the localized states and the displacement of the charges with respect to the external field. The addition of cobalt ions reduces the iron ions in B-sites, which is mainly responsible for both space charge polarization and hopping exchange between the localized states. Therefore, the increase in cobalt content causes a decrease in polarization which is accomplished by a decrease of dielectric constant, ∊’, of the composition. Koops et al. [21] suggested that the effect of grain boundaries is predominant at lower frequencies. The thinner grain boundary, the higher the dielectric constant. The decrease in dielectric constant promotes the quality factor at high frequencies. The numerical values for dielectric constants at different frequencies are given in Table 2.

The variation of dielectric loss tangent with cobalt concentration is shown in Fig. 10. It is observed that at frequencies 1 kHz and 10 kHz the dielectric loss increases with an increase in cobalt content and then decreases. But at a frequency of 100 kHz the dielectric loss is found to be gradually decreasing.

Fig. 10

The AC conductivity of the material is frequency dependent. The AC conductivity of the CMZF samples was estimated from the dielectric parameters.

Fig. 11 shows the dependence of AC conductivity on cobalt concentration and frequency of the applied AC field. It suggests that AC conductivity is strongly frequency dependent. It is evident from the graph that AC conductivity increases with increasing frequency. The conduction mechanism in ferrites can be explained on the basis of hopping of charge carriers between Fe²⁺–Fe³⁺ ions in octahedral sites [22]. The AC conductivity of a system depends on the dielectric properties and capacitance of the material. The variation of AC conductivity of CMZF indicates the conductivity dispersion throughout the range of frequency under investigation. This behavior may be attributed to the presence of space charge in the material [23]. The doping of CMZF with different concentrations of cobalt results in the modification of the conductivity spectrum. Further, it is observed that as cobalt content increases the AC conductivity decreases.

Fig. 11

Impedance spectroscopy is an important method to study the electrical properties of ferrites as the impedance of grains can be separated from the other impedance sources, such as impedance of electrodes and grain boundaries. One of the most important factors which influences the impedance properties of ferrites is the micro structural effect. The impedance measurement gives us information about the resistive (real part) and reactive (imaginary part) components of a material. The impedance behavior can be described by Debye’s formula for a serial-parallel resistor-capacitor (RC) circuit with elements that correspond to the dielectric behavior of different components [24]. The complex impedance response commonly exhibits semicircular forms in the measured Cole-Cole plot as shown in Fig. 12.

Fig. 12

It is evident that a single semicircle is seen for all the compositions, however, the semicircle appears to be smaller for the composition Co_0.5Mn_0.25Zn_0.25Fe₂O₄. Decentralization of the semicircles has been observed in compositions of Mn ferrite [25]. The grain-boundary resistance is normally higher than the grain interior and the probing interface resistance is higher than that of the boundary. The larger radius of the Cole-Cole plot in frequency space corresponds to contribution from a constituent of lower resistance [26]. Therefore, the semicircle can be attributed to the behavior of grain interior.

The FT-IR spectra analysis was carried out with an FT-IR spectrometer, having the resolution of spectral range covering 4000 cm⁻¹ to 400 cm⁻¹, range commonly used in a first determination step. The sample pellets were prepared using 100 mg of dry KBr and 2 mg of the obtained Co_xMn_yZn_yFe₂O₄ nanopowder. The FT-IR spectra of the Co_xMn_yZn_yFe₂O₄ materials are shown in Fig. 13. The patterns confirm the presence of cobalt ferrite and hydroxides. The peaks at ∼3414 cm⁻¹ have been assigned to asymmetric and symmetric OH stretching vibrations of lattice water. The adsorbed water is featured by the bands at 1690 cm⁻¹ assigned to the δ H–O–H bonding mode. The bands at 2924 cm⁻¹ are assigned to C–O stretching vibrations. Two bands are observed around of 585 cm⁻¹ and around 450 cm⁻¹ for all Co_xMn_yZn_yFe₂O₄ samples. These two vibrations bands correspond to the intrinsic lattice vibrations of octahedral and tetrahedral coordination compounds in the spinel structure, respectively [27]. The different frequency between the characteristic vibrations may be attributed to the long band length of oxygen-metal ions in the octahedral sites and shorter band length of oxygen-metal ions in the tetrahedral sites. The difference in the absorption position in the octahedral and tetrahedral omplexes of Co_xMn_yZn_yFe₂O₄ crystal is due to the difference in the distance between Fe³⁺–O²⁻ in the octahedral and tetrahedral sites [28]. The XRD results and FT-IR results confirm the presence of impurity free nanocrystalline Co_xMn_yZn_yFe₂O₄.

Fig. 13