Impact of V substitution on the physical properties of Ni-Zn-Co ferrites: structural, magnetic, dielectric and electrical properties

We have investigated the Vanadium- (V) substituted Ni-Zn-Co ferrites where the samples were prepared using solid-state reaction technique. The impact of V5+ substitution on the structural, magnetic, dielectric and electrical properties of Ni-Zn-Co ferrites has been studied. XRD analysis confirmed the formation of a single-phase cubic spinel structure. The lattice constants have been calculated both theoretically and experimentally along with other structural parameters such as bulk density, X-ray density and porosity. The FESEM images are taken to study the surface morphology. FTIR measurement is also performed which confirms spinel structure formation. The saturation magnetization (Ms), coercive field (Hc) and Bohr magneton (B) were calculated from the obtained M-H loops. The temperature dependent permeability is studied to obtain the Curie temperature. Frequency and composition dependence of permeability was also analyzed. Dielectric behavior and ac resistivity are also subjected to investigate the frequency dependency. An inverse relationship was observed between the composition dependence of dielectric constant and ac resistivity. The obtained results such as the electrical resistivity, dielectric constants and magnetic properties suggest the appropriateness of the studied ferrites in microwave device applications.


Introduction
Ferrites, influential ceramic materials are composed by combining, firing and blending a huge portion of Fe2O3 (iron (III) oxide) with a small amount of at least one metallic element, for example, manganese, zinc, nickel, barium, cobalt, etc. [1]. Ferrites mainly used in three sectors of electronics: power applications, low-level applications and Electro-Magnetic Interference (EMI) suppression. Therefore, a constant challenge is being posted to improve their characteristics for ever-increasing demands in home communication appliance, computer, electrical and other technical fields [2]. The interests in this oxide emerge from their versatile applicability from relatively high to microwave frequency region. Among various ferrites, soft ferrites being highly resistive at high frequencies are used extensively in electronic applications such as transformer cores, inductors, antenna rod, microwave devices, computer memory chip, magnetic recording media, etc. after their first commercial introduction [3]. Mostly these electrical and magnetic properties depend on the method of preparation, preparative parameters, preparative conditions, particle size, nature of dopants etc. [4]. Hence, many researchers bestowed their time and efforts on various ferrites with many dopants to enhance its electric and magnetic characteristics.
The important electronic properties make the spinel Ni-Zn-Co ferrites prominent [5]. A good combination of its magnetic and electric properties as well as it low-cost aspect makes them potential candidates for application in high-frequency purpose [6]. These ferrites have high saturation magnetization, low eddy current loss, high resistivity, high permeability, high Curie temperature [7][8][9][10]. It is technologically sound due to its prospective use in targeted microwave devices, sensors, catalysis, magnetic recording applications and drug delivery systems [11][12][13][14][15].
Owing to the mentioned interest many researchers have paid their attention to the Ni-Zn-Co ferrites. Mallapur et al. have investigated the structural and electrical properties of the spinel ferrite system of Ni-Co-Zn [16]. Electric properties of nanocrystalline Ni-Co-Zn ferrites have been reported by Ghodake et al [17]. Spectral studies such as room temperature Mossbauer, Xray and infrared IR spectra of Ni-Co-Zn ferrites have been carried out by Amer et al. [18].
The dependency of the physical properties of ferrites on the distribution of cation over tetrahedral (A) and octahedral (B) sites open the way of changing their properties by introducing different ions into these two sites [32][33][34]. Therefore, alteration of physical properties by the incorporation of a small amount of V into Ni-Zn-Co is also expected. Reports on the addition or substitution of vanadium into different ferrites systems are available [35,36]. Korkmaz [41]. The decrease of saturation magnetization owing to the presence of V with the low melting temperature which causes the formation of the liquid phase, accelerating the grain growth process at low sintering temperature [42,43]. Based on the available report, a considerable alteration of properties of Ni-Zn-Co ferrites is expected through V substitution. Therefore, the objective of the current study is to investigate the influence of V substitution on the electrical and magnetic properties of Ni-Zn-Co ferrites.

EXPERIMENTAL DETAILS
Conventional double sintering method was used to prepare V 5+ substituted Ni-Zn-Co ferrite [Ni0.7Zn0.2Co0.1Fe2-xVxO4 (0 ≤ x ≤ 0.12) ] . The following operation are used to prepare the desired samples the details of which can be found elsewhere [31]. However, the sample preparation procedure is shown in Fig. 1.    The experimental lattice constant (a) for all the samples are calculated using the formula [45]:

Structural properties
where, h, k and l are the Miller indices of the crystal planes. The calculated values are given in Table 1. The obtained value of aexp for x = 0.00 is 8.362 Å and the reported value is 8.3719 Å [26]. Our obtained value is lower than the reported value, might be due to different synthesis condition. The change of lattice constant (a) with V content is shown in Fig.2(c). At first aexp decreases with the increase of V contents and then increases for x = 0.12. The trend in the variation lattice constant is observed to have a good agreement with earlier report [46] of Vsubstituted Ni-Zn ferrites. The decrease of the lattice constant is owing to the differences of ionic radii (Shannon 6-coordination numbers) of V 5+ ions (0.59Å) and Fe 3+ ions (0.645Å) [47,48].
The lattice constant of studied samples Ni0.7Zn0.2Co0.1Fe2-xVxO4 (0 ≤ x ≤ 0.12) has also been calculated to compare with the experimental one. The lattice constant can be calculated theoretically using the following equation [2]: where, R0, rA, rB are the ionic radii of oxygen (1.32 Å) [49], A-and B-sites, respectively. The rA and rB can be calculated by the following equations: The calculated X-ray density, bulk density and porosity for the studied samples Ni0.7Zn0.2Co0.1Fe2-xVxO4 (0 ≤ x ≤ 0.12) were represented in Table 1. The equations used can be found elsewhere [31]. A decreasing trend of X-ray density and the bulk density with increasing V-content till x = 0.12 is noted while an inverse relation is followed by the porosity. This trend can be explained on the basis of atomic mass of V 5+ (50.94 amu) and F 3+ (55.84 amu). Here, V with lower atomic mass is substituted for Fe ions with comparatively higher atomic mass.
Whereas, for x = 0.12 the increase in density was observed due to the increase of lattice constant. Moreover, the b is found to be lower than the x, because the pores are considered in the calculations of bulk density which are absent in the x-ray density calculations [52].  Table 1] by linear intercepting method and a decreasing trend with V contents is also observed except x = 0.12 which is similar to the variation of density and porosity with V contents. The changes in the average grains size, density and porosity can be understood from the FESEM images. The change in average grains size is also related to the difference in between ionic radii of V 5+ (0.59 Å) ions and Fe 3+ ions (0.645 Å).

FTIR analysis
FTIR Spectroscopy is an important technique to study the completion of the solid-state reaction and inspect the presence of deformation in the spinel ferrites due to substitutions of ions [53,54].
In case of ferrites, there are two characteristic peaks in the FTIR spectra. The first peak at low     Study of temperature dependent permeability: Curie temperature

Magnetic properties
The magnetic properties such as saturation magnetization, permeability etc. are very sensitive to temperature. One of the characteristic parameters, Curie temperature (Tc) can be obtained from the temperature dependent permeability. The permeability remains almost constant up to certain temperature and after increasing slightly by exhibiting a peak, known as Hopkinson's peak, it finally drops sharply to be zero. The temperature at which the permeability becomes zero is known as Curie temperature (Tc), the temperature at which completely disorderliness of atomic moments took place and the ferrimagnetic materials converted to paramagnetic. The temperature dependent initial permeability was measured and shown for different composition in Fig. 6(a).
The mentioned features are also observed for our obtained data and the calculated Curie temperature is shown in Fig. 6 (b).  Fig. 7 (a) shows the real part of permeability (µ') as a function of frequency. The toroid shaped samples were prepared for this characterization. The value of remains almost constant up to ~ 30 MHz, a noticeable increase is noted at which the became maximum and then fall sharply to certain low values. The steady value is important for many applications such as in transformer as a broadband pulse and in video recording system as read-write heads (wide band) [61].

Frequency dependence of real permeability and Relative Quality Factor
In different circumstances, a significant peak is exhibited by [figure is not shown] at the frequency where sharp decline of is taken place. This phenomenon is termed as ferrimagnetic resonance [62] and the prepared compositions are found to follow the Snoek's limit [63].  where Ms and D are also found to decrease with V contents up to x = 0.10 and then increases for The relative quality factor (RQF) measures the performance of a material for use in filter. Fig. 7 (c) shows the usual behavior of RQF versus frequency for different V contents in which a prominent peak is observed for each composition [31,65,66]. The exhibition of peak is followed by very low value both in the low frequency and in the high frequency regions. The declination of RQF corresponds to the frequency (~30 MHz) wherein the (loss component) started to increase sharply. Moreover, the value of RQF is observed to increase owing to V substitution with the maximum for x = 0.12. The values of RQF are given in Table 3.

Dielectric behavior
A very common dielectric behavior of ferrites is that the εʹ and εʹʹ decrease with frequency but exhibit three different responses in three ranges of frequencies: (i) a sharp decrease with low frequency in ԑʹ and ԑʹʹ (ԑʹʹ is not shown), (ii) a slow decrease within mid frequency range in ԑʹ and ԑʹʹ and (iii) finally become almost frequency independent at high frequencies [31,65,67,68].
Comparatively slower decrease in ԑʹ and ԑʹʹ is observed in the mid-frequencies region where the orientational polarization is mainly responsible for dielectric constant. The dielectric constant becomes constant at the very high frequency region where the contribution comes from the atomic and electronic polarization [69]. The frequency dependence of dielectric constant (ԑʹ) and dielectric loss factor (tanδ = ε"/ʹ) of Ni0.7Zn0.2Co0.1Fe2-xVxO4 (0 ≤ x ≤ 0.12) ferrites are shown in Fig. 8 (a and b). The dielectric properties can be explained by considering the studied ferrites as composed of two layers: grains of high conductivity which are separated by grain boundary of high resistivity [70][71][72]. The hopping mechanism is responsible of dielectric mechanism [73,74].
The charges produced through electron hopping between Fe 3+ and Fe 2+ are responsible for electrical conduction in ferrites. The motion of these charges are supposed to be hindered at the grain boundaries owing to their activity in the low frequency region and accumulated at the interface causing space charge polarization. Although, the grains are active at the high frequency region but electron hopping cannot follow the frequency of applied external electric that causes a reduction of the charges produced by hopping between Fe 3+ and Fe 2+ [75]. Therefore, the decrease of ԑʹ and ԑʹʹ with increasing frequency is expected. From Fig. 8 (a), it is obvious that the dielectric constant is decreased due to V substitution which is explained later. Fig. 8 (b) demonstrates the unusual behavior of dielectric loss (tanδ) as a function frequency. The curves tanδ vs frequency exhibit a maximum at a particular frequency but different with V concentration variation. This peak is observed to shift to the high frequency side owing to the increasing V substitution. The similar abnormal behavior (of tanδ) has also been reported for V substituted Ni-Zn-Cu ferrites [41], Sn substituted Ni-Zn ferrites [65] and Y substituted Mg-Zn [68]. This type of maximum in tanδ usually occurred when the jumping frequency of electron hopping becomes approximately equal to that of the externally applied ac electric field [76]

Frequency dependence of AC resistivity
The frequency dependent ac resistivity (ρac) is shown in Fig. 9(a) in the frequency range 1 kHz -120 MHz. Again, the normal behavior in the frequency dependence of ac resistivity is observed for each sample. The resistivity is observed to decrease with increasing frequency and then becomes invariant at high frequency [65]. Fig. 9 (a) also demonstrates that the ac resistivity is noted to increase with V contents as shown in Fig. 9 (b). It is seen from Fig. 8 (a) and 9 that the dependence of dielectric constant and ac resistivity on V contents follows the opposite trend that is normal trends in ferrites. The dependence of dielectric constant or ac resistivity on V contents can be explained by assuming two mechanisms. Firstly: The V is substituted for Fe 3+ ions at the B-sites that lead to the decrease of electronic hopping between Fe 3+ and Fe 2+ occurs at B-sites due to reduced Fe 3+ ions. Thus, the decrement of electronic conductivity attributed from the electronic hopping mechanism is expected. Such type of reducing electronic conductivity is also reported by means of Fe 3+ ions replacement with ions that have a tendency to occupy the B-sites [65]. Secondly: The decrement of grain sizes with V contents also contributed to the enhanced resistivity. The FESEM micrographs shows that the average grains size is observed to be decreased [ Table 1] with V contents that leads to the increased in number of grain boundaries, the more grain boundaries results more insulating barriers in the way of charge carriers [69].
Consequently, the decrease of grain size also revealed the reduction of the conductive area. [77] Hence, the electrical resistivity is expected to be increased owing to V substitutions as shown in Fig. 9 (b).

Conclusion
Ni0.7Zn0.2Co0.1Fe2-xVxO4 (0 ≤ x ≤ 0.12) ferrites have been synthesized by conventional ceramic technique. The decrease of lattice constant with V content is observed. A good correlation is observed among lattice constant, density and porosity for different V contents. Average grains size is found to decrease with V contents. The formation of spinel cubic ferrites is confirmed from the obtained values of vibrational frequency υ1 (in the range: 574 to 590 cm -1 ) and υ2 (in the range: 364 to 382 cm -1 ). The soft ferromagnetic nature is observed from hysteresis loops with low values of coercive field. The lowering of Ms (from 71.6 emu/g to 18 -28 emu/g) is due to non-magnetic V substitution. The Curie temperature obtained from temperature dependent permeability is observed to decrease owing to V substitution by means of decreasing the strength of exchange coupling constant. The study of the frequency dependent permeability exhibits a good correlation among the permeability, average grain size and saturation magnetization. The dielectric constant is found to decrease in V-substituted Ni-Zn-Co ferrites where the ac resistivity varies inversely with V contents. It is also expected that the results obtained in this paper will encourage the materials scientist to investigate the effect of V substitution on other ferrites system.