Role of Mn 2+ ion in the optimization of the structural and dielectric properties of Co–Zn ferrite

Mn-substituted Co–Zn ferrite nanomaterials with the general form Co 0.8−x Mn x Zn 0.2 Fe 2 O 4 (x = 0.0, 0.1, 0.2, and 0.3) were prepared using the coprecipitation method. Based on X-ray diffraction, it can be confirmed that all samples have a single-phase cubic structure with an average crystallite size ranging from 23.46 to 32.66 nm. In addition, the lattice parameter increased from 8.32 to 8.37 Å. In this study, the theoretical density was calculated, and the Fourier-transform infrared spectra of the prepared samples were investigated. Further, the cation distribution of the system was estimated. The proposed cation distribution was confirmed by calculating the theoretical lattice parameter and comparing it with the experimental lattice parameter. We conducted transmission electron microscopy to confirm the obtained particle size. At room temperature, the electrical measurements of the prepared samples were performed using the LCR circuit as a function of frequency up to 5 MHz. In this study, we obtained enhanced dielectric properties by substituting Mn ions in the Co–Zn ferrite. As the Mn concentration increases, the AC resistivity of the samples increases. Consequently, the dielectric loss decreases, and its minimum value can be observed at x = 0.2, making the Co–Mn–Zn ferrite suitable for applications in the microwave frequency range. The results show that all dielectric properties exhibit a normal behavior with frequency. We obtained an improvement in the dielectric properties of the prepared samples, making them suitable for use in high-frequency applications due to the substitution by Mn ions.


Introduction
Spinel ferrites are chemical, magnetic materials with various applications in electric, magnetic, electronics, and microwave devices, catalysts, and transformer cores owing to their high electrical resistivity at room temperature, low electrical loss, high dielectric constant, chemical stability, and low cost [1,2]. The physical properties of these ferrites depend on their chemical composition, preparation method, particle size, and microstructure [2,3]. Among the spinel ferrites, Co-Zn ferrites have attracted considerable attention owing to their chemical stability, mechanical hardness, and good coactivity [2] as well as their wide range of applications in electronic devices, such as transformer cores, electric motors, and generators [4], and because of their chemical stability.
The structural and magnetic properties of Co-Zn ferrite doped with Cu ions using a sol-gel combustion method have been studied [3]. This study demonstrates that all samples show a cubic spinel structure, and the lattice parameter decreases as the Cu concentration increases. The average crystallite sizes range from 51 to 100 nm. The cation distribution reveals that Cu occupies tetrahedral (A) and octahedral (B) sites with a different ratio, and Zn and Co ions occupy A and B sites, respectively. The grain size and density of samples increase as the Cu concentration increases. These samples can be helpful in magnetic recording applications [3].
Co-Zn ferrite substituted with Si ions using the double sintering ceramic method was studied [4]. It has been reported that the samples have cubic spinel structures, and the lattice parameter decreases as the Si concentration increases. It has also been reported that Si 4+ ions occupy A sites, Co 2+ ions occupy B sites, and Zn 2+ ions occupy A sites. The dc resistivity (ρ dc ) increases as the Si concentration increases and then decreases [4]. The influence of magnesium on the structural properties of Co 0.6−x Mg x Zn 0.4 Fe 2 O 4 using the combustion method has been studied [5]. X-ray diffraction (XRD) patterns confirm the formation of spinel ferrite nanoparticles with an average crystallite size of 30 nm. With increasing x content, the lattice constant first decreased because of the replacement of Co ions. Then, at x = 0.6, the lattice constant showed a slight decrease, and values of a exp were lower than those of a XRD . Wu prepared Li x Co 0.5 Zn 0.5−x Fe 2 O 4 by calcining precursor oxalates in the air method, which has a cubic structure when calcined at 900 °C. It has also been reported that the lattice parameters decrease with increasing Li content, the crystallinity increases with increasing calcination temperature, and the saturation magnetization value increases with increasing calcination temperature and Li content [6]. Kumar [7] reported the synthesis of In-doped Co-Zn nano ferrites (Co 0.9 Zn 0.1 In x Fe 2−x O 4 ) using the solution combustion method and investigated its structure, Fouriertransform infrared (FTIR) spectroscopy, and electrical properties. It is observed in a single phase with crystallite size ranging from 23.8 to 20.2 nm, and the lattice constant increases from 8.385 to 8.426 Å. FTIR confirmed no deviation in the structure of Co-Zn spinel ferrites with the addition of In 3+ ion. The resistivity decreased as the temperature increased, and an enhancement in resistivity was observed with an increasing In 3+ concentration. Meanwhile, a normal dispersion curve was observed for the variation of the dielectric constant as a function of frequency, showing a decrease in the dielectric constant with the increasing frequency.
Co-Zn ferrite has many applications such as highdensity magnetic recording media, gas sensing, magnetic drug delivery, ferrofluids, magnetic resonance imaging, biomedical application, MRI contrast agent, drug delivery, DNA hybridization, dyes removal, and cell separation [8,9]. Co-Zn ferrites are one of the most important soft ferrites due to their magnetic characteristics such as high magnetic permeability, high coercivity and low cost but it has relatively low resistivity. So, in the present work we tried to obtain an improvement in the electrical properties of Co-Zn ferrite using the substitution by a new ion such as Mn 2+ ion to improve its resistivity and consequently reduce its energy loss to be used for promising applications. Also in the present work we throw a light on the critical concentration of Mn 2+ ion to obtain more applicable samples.

Sample preparation
Using the co-precipitation method, a series of Co 0.8−x Mn x Zn 0.2 Fe 2 O 4 nanoferrites (x = 0.0-0.3 with a step 0.1) was prepared, as shown in Fig. 1. The mixture was stirred with a magnetic stirrer until the reactant was dissolved completely. During the stirring, 30 g sodium hydroxide into 500 ml was added dropwise to the salt solution as a precipitating agent. As a result, a brown precipitate appeared quickly as shown in Fig. 1. When pH of the solution reached 12 under vigorous stirring, the mixed solution was heated to 80 °C to transform into a black solution and subsequently maintained for 45 min to obtain good results for the reaction process and then the sufficient precipitation phase was observed. The chemical reactions in this process and specific amounts of each composition in mole, are given in the following equations [10]: Then, air is passed over the metal hydroxide to oxidize it and to produce Co 0.8−x Mn x Zn 0.2 Fe 2 O 4 .
The Co 2+ , Zn 2+ , Mn 2+ ratio has been controlled such that the M 2+ /Fe 3+ mole ratio equal 1/2. The prepared magnetic nanoparticles were washed with distilled water several times until they were free from sodium and sulfate ions. The product was dried in fresh air to remove water contents. The dried product was ground well in a cleaned mortar and then pestled into a cylindrical-shaped disk using a hydraulic press by applying uniform pressure.

Sample characterization
Structural characterization was studied using an X-ray diffractometer (D8 ADVANCE) with Cu-Kα (λ = 1.5406 Å) radiation. The crystal structure, lattice parameter, crystallite size, and theoretical density were calculated from the XRD pattern. The FTIR absorption spectra of the samples were recorded using the FTIR spectrometer in the wave number range 200-1000 cm −1 with potassium bromide (KBr) as the solvent. We conducted transmission electron microscopy (TEM) to analyze the sample's topography and morphological features of the nanoparticles. The AC conductivity (σ ac ), dielectric constant (ε′), and dielectric loss angle (tan δ) of the prepared samples were determined at room temperature (300 K) as a function of frequency ranging from 50 Hz to 5 MHz using an LCR Bridge (HIOKI) Model 3531 Z HI tester.

X-ray analysis
different Mn-content. It showed that all prepared samples were in a single-phase structure with a cubic spinel ferrite structure, where XRD peaks were indexed according to the cubic spinel ferrite structure of the samples with their Miller indices (220), (311), (400), (422), (511), and (440) planes using the XRD reference card (JCPDC card no.  for CoFe 2 O 4 [3,9,11]. Well-defined sharp diffraction lines reflecting good crystallization with no secondary phases were observed in the samples.
where λ is the X-ray wavelength, β is the full width half maximum, and θ is the corresponding diffraction angle. It is worth noting that the average size of the prepared samples decreased from 23.46 to 20.33 nm at x = 0.2 and then increased to 32.66 nm at x = 0.3.

High-resolution transmission electron microscope
The morphological studies for the prepared sample of Co 0.6 Mn 0.2 Zn 0.2 Fe 2 O 4 were investigated through TEM analysis. Figure 3 shows the TEM image of the prepared sample (x = 0.2). It can be seen from the TEM image that the sample is uniform in the morphology and particle size distribution. The average particle size was calculated via TEM analysis and was found to be 23.26 nm, which is well-matched with the average crystallite size obtained from XRD analysis. The average particle size decreased with increasing Mn 2+ content, showing that doping decreased the grain size [11,12]. The general decrease in the particle size with increasing Mn 2+ content can be explained by the electronic configuration of Co 2+ (3d 7 ) and its further propensity to interact with ligands and oxyanions, as compared to Mn 2+ (3d 5 ) with a complete electronic configuration [5]. Additionally, this decrease is due to the flexibility of substitution into the available sites during particle growth, limiting the nucleation process and size [7].
TEM images also demonstrated a spherical shape of the prepared sample, and the agglomerations observed may be related to the interactions of magnetic dipoles arising within the ferrite nanoparticle [14,15].
The lattice parameters of the prepared samples were calculated using X-ray data using the following equation [14,16,17], and the results are shown in Table 1.
The experimental lattice parameter of the prepared samples calculated using Eq. (2) shows an increasing trend of the lattice parameter with an increase in doping level till x = 0.2 and a decrease at x = 0.3. The lattice parameter increased from 8.3256 to 8.377 Å with the Mn 2+ concentration. This occurred because the radius of Co 2+ ions (0.78 Å) was smaller than that of the Mn 2+ ions (0.80 Å) [5,18,19]. Thus, the substitution by larger ions resulted in the lattice expansion, leading to an increase in the lattice constant [16,17]. When the Co ions were replaced with manganese at x = 0.3, the lattice parameters decreased to 8.377 Å [5]. This decrease occurs because some Mn ions cannot enter lattice sites and stress on the grains. Consequently, the lattice constant decreased, and a similar behavior was observed [17].
The theoretical density (D th ) was calculated using the following equation [18,20]:  where M is the molecular weight of the ferrite, N A is the Avogadro's number, a is the experimental lattice parameter, and Z is the number of molecules per unit cell, i.e., 8 (in spinel lattice, each primitive unit cell contains eight molecules). Figure 4 shows the variation in the theoretical density with Mn 2+ content. Table 1 presents the theoretical density values calculated using Eq. (3). Figure 4 shows that the D th of the prepared samples decreases with increasing Mn 2+ concentration for x ≤ 0.2 and then increases at x = 0.3. The theoretical density depends on the lattice constant and molecular weight of the sample, where theoretical density is inversely proportional to the lattice constant [7,20]. The decrease in the theoretical density can be attributed to the increase in the lattice parameter, which increases the lattice volume, leading to a decrease in the theoretical density [3,16,20]. However, the theoretical density increased at x = 0.3. This increase is attributed to the decrease in lattice parameter because some Mn 2+ ions cannot enter lattice sites and stress on the grains, thereby increasing the theoretical density. Another factor that increases the theoretical density at x = 0.3 is that the molecular weight of Co 2+ ions (58.93 u) is greater than that of the Mn 2+ ions (54.93 u) [7].

Fourier-transform infrared analysis
The FTIR spectroscopy of Co 0.8−x Mn x Zn 0.2 Fe 2 O 4 (x = 0.0 to 0.3 with a step size of 0.1) was recorded in the range of 200-1000 cm −1 . Figure 5 shows the FTIR spectra of the prepared samples, and the absorption band results are presented in Table 2. It shows two absorption bands, ν 1 and ν 2 , at ~ 600 and 400 cm −1 , respectively, confirming the formation of the spinel structure. The high-frequency band (ν 1 ) corresponds to bending vibrations at the A site, and the low-frequency band (ν 2 ) corresponds to stretching vibrations of Fe 3+ -O 2+ -Fe 3+ at the B site [1,8,11,16,17]. As presented in Table 2, with the increase in the substitution of Mn 2+ ions, band (ν 1 ) decreases, whereas band (ν 2 ) increases. Thus, Mn ions have a greater effect on A sites than B sites.
As Mn +2 concentration (x) increases the vibrational frequency ν 1 shifts slightly towards lower value. This shift is attributed to the increase in the bond length due to the increase of the lattice parameter [21,22]. The sample of x = 0.2 has a maximum lattice parameter and consequently has relatively large shift to lower frequency referring to other x values. For x > 0.2, the lattice parameters decreased because some Mn 2+ ions cannot enter lattice sites, so it has relatively smaller shift than x = 0. 2.

Cation distribution
Based on FTIR analysis, the assumed cation distribution of the prepared samples presented in Table 1 [11,20]. As presented in Table 1, the assumed cation distribution also showed that some Mn 2+ ions occupy B sites in Co-Zn ferrites with a ratio of 20% and 80% of Mn 2+ ions in A sites, as reported in [17]. The values of the lattice parameter, theoretical density, and crystallite size with Mn 2+ substitution are presented in Table 1. To confirm the proposed cation distribution, the mean ionic radii of A and B sites and theoretical lattice parameters were calculated and compared with the experimental lattice parameter (Table 1). Figure 6 shows the variation of the theoretical lattice and experimental parameters with x content. As shown in Fig. 6, the experimental lattice parameter follows the same trend as the theoretical lattice parameter, which shows the increasing trend of the theoretical lattice parameter with the increase in doping level calculated using the following equations (Table 1) [3,9,14]: where r A and r B are the ionic radii of A and B sites, respectively, R o is the radius of oxygen, and r(Zn 2+ ), r(Fe 3+ ), r(Mn 2+ ), and r(Co 2+ ) are the ionic radii of Zn 2+ , Fe 3+ , Mn 2+ , and Co 2+ , respectively.

Electrical properties
Dielectric constant (ε′), dielectric loss factor (ε′′), and AC conductivity (σ ac ) were calculated using the following equations [10,14,16]: where C is the capacitance of the pellet in Farad, d is the thickness of the pellet in meters, A is the cross-sectional area of the pellet, ε o is the permittivity of free space, which is equal to 8.85 × 10 −12 F m −1 , and f is the frequency in Hz.
The dielectric properties of ferrite materials depend on factors such as the fabrication method, chemical composition, cation distribution, grain size, and porosity [7,18]. Figure 7 shows the change in the dielectric constant as a function of frequency at room temperature for all prepared samples. As shown in Fig. 7, the dielectric constant for all the prepared samples is high at low frequencies, decreases with increasing frequency range, and finally reaches its minimum value at high frequencies, which is the normal behavior of the ferrite materials. Similar results were observed in [1, 7, 11-14, 16, 18, 19, 23]. This variation can be explained by space charge polarization, which is due to the inhomogeneous structure of the ferrite material discussed using the Maxwell Wanger model [25][26][27][28]. According to the Maxwell Wanger model, each dielectric material consists of grains with small resistivity and grain boundaries with small conductivity [9,11,15,16]. The electric polarization is produced in spinel ferrite because of the movement of an electron from Fe 2+ to Fe 3+ . The high value of the dielectric constant can be attributed to the simultaneous presence of different types of polarization contributions, including space charge, dipolar, ionic, and electronic [14]. The exchange of electrons between Fe 2+ and Fe 3+ results in the local displacement of electrons in the direction of the electric field, which determines the polarization and dielectric constant [25]. Consequently, the hoping of charge carriers follows the applied field [11][12][13]16]. By increasing the frequency, each dipole requires some time to align in the direction of the field; therefore, each dipole fails to align itself in the direction of the field, and the electron hopping cannot follow the electric field [1,[11][12][13]16]. The hopping of charge carriers lags behind the applied field, thereby decreasing the dielectric constant [7,11,14,16,27]. At high frequency range 10 5 to 10 6 the speed of decreasing ε′ is getting faster. This is attributed to at high frequencies the electric dipoles cannot follow the electric field variation because at high frequencies the alternating voltage half period becomes shorter, so the space charge polarization fails to settle itself. So, the space charge polarization does not contribute in the polarization at high frequencies and the total polarization decreases and consequently the dielectric constant begins to decrease faster at low frequencies [26,[29][30][31] Figure 8 shows the variation of AC conductivity (σ ac ) as a function of frequency for a series of Co 0.8−x Mn x Zn 0.2 Fe 2 O 4 with x = 0.0, 0.1, 0.2, and 0.3 at room temperature. As shown in Fig. 8, the AC conductivity of the prepared samples increases as the frequency increases, which is the normal behavior of ferrite. This behavior can be interpreted using the Maxwell-Wagner polarization that agrees with Koop's theory [7,12,14,18,28]. The grain boundaries of high resistivity are more effective at a lower frequency, decreasing the hopping of charges between Fe 2+ and Fe 3+ . Thus, the observed conductivity is lower at low frequency. Further, as the frequency increases, the grains of low resistivity are more effective than grain boundaries; hence, the hopping of charges between Fe 2+ and Fe 3+ on B sites increases, thereby increasing the observed conductivity [7,14,28]. Figure 9 shows the variation of the dielectric loss factor (ε′′) as a function of frequency. As shown in Fig. 9, the dielectric loss factor decreases with increasing frequency, which is a normal behavior of ferrite [7,14,[18][19][20]. The dielectric loss behavior is similar to the dielectric constant due to space charge polarization [28]. The reduction in the dielectric loss factor with the frequency is interpreted using the Maxwell-Wagner model [7,18]. Figure 9 also shows high values of dielectric loss at a lower frequency, which may be due to the contribution of different types of polarization [28]. Figure 10 shows the compositional dependence of ε′ and σ at different frequencies at room temperature of the prepared samples. Figure 11 shows the variation of the dielectric constant and AC conductivity with Mn concentration at 1 MHz. As shown in Figs. 10 and 11, the dielectric constant and Ac conductivity of the prepared samples have the same trend of variation, confirming that the mechanisms of dielectric properties and AC conductivity have the same origin, which agrees well with Eqs. (7), (8), and (9). It also shows that the dielectric constant and conductivity decrease with increasing Mn 2+ content, reaching a minimum value at x = 0.2 and subsequently increasing at x = 0.3. The decrease in dielectric constant with Mn 2+ content is due to the increase in resistivity with Mn 2+ [7], which is related to the density of the samples. Consequently, it increases the porosity due to the trapped pores in the samples [20] and increases the grain boundary between the small grains [17], reducing the hopping probabilities across the grain boundaries [7,16,25]. This impedes the movement of electrons between Fe 2+ and Fe 3+ and reduces the space charge polarization. The increase in the dielectric constant at x = 0.3 may be attributed to the decrease in the density of the sample at x = 0.3 because some Mn 2+ ions cannot enter the lattice sites and create external stress on the grains [16].

Compositional dependence of the dielectric properties
The decrease in AC conductivity with Mn 2+ content is attributed to the decrease in Co/Fe due to the replacement of Co 2+ by Mn 2+ in the B site [18], decreasing the Fe 2+ /Fe 3+ pairs in the B site, which is responsible for the conduction process [15]. This implies a decrease in n-type electronic transitions, which may be related to the partial substitution of Mn and the difference in the grain size. The highest conductivity value was found at x = 0.0, whereas the minimum value was found at x = 0.2. Here, the AC conductivity decreases with a decrease in grain size, and smaller grains denote a large number of insulating grain boundaries, which act as a barrier to electron hopping [7]. Generally, the conductivity and permittivity values depend on important factors, such as the grain size, grain boundaries, Fe 2+ content, structural homogeneity, porosity, and stoichiometry, which mainly depend on the composition and synthesis methods [7].  Figure 12 shows the variation of the dielectric loss tangent and AC resistivity with the concentration of the Mn 2+ ion for Co 0.8−x Mn x Zn 0.2 Fe 2 O 4 at 1 kHz. As shown in Fig. 12, the resistivity increases as the Mn 2+ content increases from x = 0.0 to 0.2, and the resistivity decreases at x = 0.3. This may be attributed to the density of the samples, and a similar behavior was observed in [20,26]. This variation in resistivity may be due to the increase in the lattice parameter (a) that affects the hopping probabilities between the ions [24,26], where the variation in lattice parameter (a) directly affects the hopping length [20]. This may be because the substitution expands the lattice, increases the bond length, and decreases the overlapping of orbitals, which decreases the hopping probability between the A and B sites [26]. Figure 12 also shows that the dielectric loss tangent decreases as the Mn 2+ content increases from x = 0.0 to reach its minimum value at x = 0.2 and then increases at x = 0.3. Therefore, the energy losses decrease with increasing Mn 2+ content [7], which is inversely proportional to the resistivity related to the eddy current loss [20]. The dielectric loss tangent also depends on different factors, such as Fe 2+ content, structural homogeneity, and stoichiometry, which mainly depend on the composition and synthesis methods [7,9,11]. The minimum value of the dielectric loss tangent at x = 0.2 may be due to the large resistivity value [25].
The obtained results showed that Co-Mn-Zn ferrites has low values of dielectric loss at high frequencies make the prepared samples suitable for high-frequency applications in electrical circuits to decrease dielectric losses. Additionally, the increase in the electrical resistivity due to Mn 2+ ion makes these prepared samples are suitable for highfrequency applications.

Conclusion
A series of Co 0.8−x Mn x Zn 0.2 Fe 2 O 4 (x = 0.0, 0.1, 0.2, and 0.3) spinel ferrites was successfully prepared using the coprecipitation method. Further, the structural and electrical properties of the prepared samples were investigated. The XRD patterns of the prepared samples confirmed the singlephase cubic spinel structure. An increase in lattice parameter was observed in the range of 8.3256-8.377 Å. The average grain size decreases at x > 0. 2 0.0, 0.1, 0.2, and 0.3). The dielectric properties and AC conductivity were investigated as a function of frequency at room temperature. The dielectric constant and loss factor decrease with increasing frequency and then reach a minimum value at high frequencies. The conductivity increases with increasing frequency. With the increasing Mn substitution, the dielectric constant, dielectric loss angle, and AC conductivity decrease, whereas the resistivity increases. The low values of dielectric loss at high frequencies make the prepared samples suitable for high-frequency applications in electrical circuits to decrease dielectric losses. Additionally, the increased electrical resistivity makes these prepared samples suitable for high-frequency applications to decrease the eddy currents. The behavior of the dielectric constant and AC conductivity of all samples is similar to the normal behavior of ferrite that follows the Maxwell-Wagner polarization process and electron hopping. The different changes in the structural and dielectric properties of Mn-substituted Co-Zn ferrite can be attributed to the rearrangement of cations at different sites.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB). No funding sources.

Availability of data and material
The data that support the plots of this paper and other findings within this study are available from the corresponding author upon reasonable request.

Conflict of interest None declared.
Ethical approval Not required.
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