Effect of Copper Doping on the Structural, Electrical, and Magnetic Properties of Mg-Co Ferrites

2. Materials & Methods

2.1. Chemicals

High-purity analytical grade (A.R) chemicals of transition metal oxides were used for standard ceramic synthesis. Magnesium Oxide (MgO ≥99.99%), Ferric oxide (Fe₂O₃ ≥99.99%), Copper oxide (CuO ≥99.99%), and Cobalt (II) oxide (CoO ≥99.99%), were purchased from Himedia. Acetone (C₃H₆O), Potassium Bromide (KBr), Silver (Ag), and Poly Vinyl Alcohol (C₂H₄O)_x were purchased from Merck. No additional purification was required for all chemicals.

2.2. Synthesis of Cu²⁺ Substituted Mg-Co Ferrites

Cu²⁺ substituted Mg_0.6-xCu_xCo_0.4Fe₂O₄(x=0.0,0.1,0.2, and 0.3) were prepared using a solid-state reaction technique. Analytical grade oxides Fe₂O₃, MgO, CoO, and CuO, were considered in stoichiometric proportions and grounded for 6 hours in an agate motor and pestle to obtain in powder form. Subsequently, the powder is calcined for four hours at 900 °C. The calcined samples are combined with a few drops of PVA binder, compacted into a disk-shaped pellet, and sintered at 1200 °C for four hours at a rate of 5°C per min while in the air. To prepare the pellets for use as electrodes, their surfaces are carefully polished, cleaned in acetone, and then covered in silver paste. Figure 1 depicts the schematic of the synthesis method of Cu-doped magnesium-cobalt ferrites. Additional characterization was done using the produced samples.

2.2.1. Structural Characterizations

The identification of phase formation of synthesized magnesium ferrite powder samples was done on a PANalytical XPERT-PRO diffractometer fitted along with Cu Kα radiation (λ=1.54060Å) at the scan rate of 2° per min in the 2θ range of 10°-80°. The observations were noted at 40 mA and 45 kV. The created samples' XRD patterns were evaluated with standard reference data cards from the “International Center of Diffraction Data” (ICDD) or the “Joint Committee on Powder Diffraction Standards” (JCPDS).

The structural and chemical variations are confirmed by the FTIR spectrometer (IR Prestige21 Shimadzu). The synthesized ferrite powdered samples were mixed with solid KBr (Potassium Bromide), grounded, and then pressed in a standard hydraulic press to form a spherical shape pellet for measurements. The spectrum of every synthesized sample was collected in the range between 200-4000cm^-1 at room temperature.

2.2.2. Morphological and Elemental Characterization

FE-SEM: “Field Emission Scanning Electron Microscope (Carl Zeiss, EVO MA 15, Oxford Instruments, Inca Penta FETx3.JPG equipment”) operating at 30 kV was applied to assess the microstructure and morphology of the produced magnesium ferrite samples. The National Institutes of Health, USA, developed the public domain Java-based image processing application ImageJ version 1.53e, which was used to quantify the grain size from the electron micrographs [14].

2.2.3. Magnetic Measurements

The VSM (Lakeshore 735) was applied to measure the magnetization behaviour of the synthesized ferrite samples. Finely powdered samples of synthesized ferrites (approximately 125 mg each) were carefully mounted on the Vibrating Sample Magnetometer (Lakeshore 735) holder. To ensure correct alignment within the magnetic field of the VSM, the samples were isolated from other metallic components to prevent any interference. An external magnetic field at a magnetic field strength of 1Tesla was used, and the magnetization of the vibrating samples was recorded, producing a hysteresis loop. Every measurement was conducted at room temperature to maintain stability, as temperature fluctuations could alter magnetization behaviour. Following measurements, data analysis was performed to identify key magnetic properties: coercivity, remanence, and saturation magnetization. Rigorous calibration of the VSM was maintained throughout, with thorough cleaning post-use to prevent contamination.

2.2.4. DC Electrical Resistivity

The DC electrical resistivities of the synthesized magnetic ferrite samples were determined for conductivity investigation by using a two-probe method and determined between the temperatures of 303K and 873 K. Two electrodes were used to hold the pellet-shaped sample. Good ohmic contact was ensured by applying the silver paste to both surfaces of the pellet. The current and voltage measurements were carried out during rising temperatures. A chrome-aluminum thermocouple was applied to test the temperature of sample.

3. Results and Discussion

3.1. Identification and Interpretation of Crystal Structure by XRD Analysis

The phase purity, crystal structure, and structural parameters of the synthesized samples were verified by examining their XRD patterns. Figure 2 indicates the XRD patterns of the synthesized samples. XRD patterns with the corresponding (hkl) planes; (111), (220), (311), (400), (422), (511), and (440) were observed for all the samples. The XRD patterns were well matched with the standard diffraction patterns of cubic spinel structure with the space group Fd3m for every sample (JCPDS card no. 73-1720 and 22-1086) [15]. All of the reflections are more comprehensive than ceramic samples. The presence of broader peaks suggests that the size of the synthesized ferrite particles is small. No impurity peaks were observed. The XRD result confirms the synthesis of Cu²⁺ions substituted magnesium ferrites with phase purity and small crystallite size [16].

The lattice constants of the samples were computed with the equation below (eq. (1)) and the lattice constant (a) values for “the synthesized ferrite samples are reported in Table 1 below.

$$\mathrm{a}=\mathrm{d}\times \sqrt{\left({\mathrm{h}}^2+{\mathrm{k}}^2+{\mathrm{l}}^2\right)}$$

(1)

where d indicates inter-planar spacing and (h k l) signifies the Miller indices of a cubic crystal plane.

The value of lattice constant values in the samples show irregular changes with different levels of substitution x, in the compound. Specifically, the lattice constant grows when x, changes from 0.0 to 0.1, suggesting that the unit cell might be expanding. However, it decreases at x=0.2 hinting at a decrease in the space between atoms. Then, it grows again at x=0.3. These changes at different substitution levels indicate a modification in the distance and interactions between atoms in the crystal, which can significantly affect the material’s properties.

The introduction of Cu²⁺ ions has a greater ionic radius (0.72Å) as compared to Mg²⁺ ions (0.65 Å) [17], which could be causing these changes in the lattice constant. This might also be related to the incidence of Fe²⁺ ions having a greater radius (0.78 Å) than Fe³⁺ ions (0.64 Å). The differences in ionic sizes, along with changes in the shape and internal stress in the lattice, are likely causing the lattice constant to increase.

The average crystallite sizes (D₍₃₁₁₎) of the synthesized Cu-doped Magnesium-Cobalt ferrite samples were determined with the Debye-Scherer formula (eq. 2):

$$\mathrm{D}={\displaystyle \frac{\mathrm{K}\mathsf{\lambda }}{\mathsf{\beta }\cos \mathsf{\theta }}}$$

(2)

Here D indicates average crystallite size, β presents the Full width at half maximum of the most substantial reflection in radians, θ signifies the angle that represents the position of the peak, λ denotes the wavelength of x-ray radiation which is equivalent to λ=1.5406 Å, and K denotes the shape factor of average crystallite, and its value is ~ 0.9.

The values of crystallite sizes for the synthesized ferrite samples are revealed in Table 1, which are in good agreement with earlier research [18]. The average crystallite sizes have been determined in the ranges between 57.29 to 48.57 nm, and the trend of this change is consistent with variations in the lattice constant as shown in Table 1. The “plot of lattice constant (a) along with crystallite size with Co content (x) is revealed in Figure 3. The crystallite size has a random value due to the randomness in the sintering condition and cation distribution” [19].

Average crystallite size is found in the ranges between 57.29 to 48.57 nm calculated using the Debye-Scherrer equation.

3.2. Fourier Transformed Infrared (FTIR) Spectroscopy

The infrared spectrum of Cu²⁺ substituted Mg_0.6-xCu_xCo_0.4Fe₂O₄ with x=0.0,0.1,0.2, & 0.3 samples are revealed in Figure 4. Two absorption bands, ν₁ & ν_2, were observed which lie between 600cm^-1 and 400 cm^-1. These bands correspond to the lower and higher frequencies of the octahedral and tetrahedral metal-oxygen vibration respectively [20]. These bands were found in the wave number range as listed in Table 2. With increasing Cu²⁺ ion concentration, the value of ν₁ is found within the range between 572.88 cm^-1 to 582.52 cm^-1 and values of ν₂ in the range from 401.56- 407.96 cm^-1. The cation distribution is the cause of the variation in the band positions. These band positions are consistent with earlier spinel ferrite studies [21].

Table 2. Wave number value of Cu doped Mg_0.6-xCu_xCo_0.4Fe₂O₄ (x = 0, 0.1, 0.2, and 0.3).

Composition (x)	x = 0.0	x = 0.1	x = 0.2	x = 0.3
Tetrahedral ʋ1 (cm-1)	572.88	574.81	579.63	582.52
Octahedral ʋ2 (cm-1)	410.24	407.96	406.03	401.56

Table 2. Wave number value of Cu doped Mg_0.6-xCu_xCo_0.4Fe₂O₄ (x = 0, 0.1, 0.2, and 0.3).

Composition (x)	x = 0.0	x = 0.1	x = 0.2	x = 0.3
Tetrahedral ʋ1 (cm-1)	572.88	574.81	579.63	582.52
Octahedral ʋ2 (cm-1)	410.24	407.96	406.03	401.56

3.3. Assessment of Morphology Using SEM (Scanning Electron Microscopy)

SEM micrographs of the Cu²⁺ substituted Mg_0.6-xCu_xCo_0.4Fe₂O₄ with x = 0.0,0.1,0.2, and 0.3 are shown in Figure 5. The micrograph indicates the growth of regular cubic crystals with the mean grain size determined by using Image J software, ranging from 1 to 1.5 µm, depending on the substitution of Cu²⁺. Copper has a major effect on the microstructure of produced ferrite samples due to facilitated liquid phase sintering. Furthermore, the force driving grain boundary movement and the force resisting it due to pores can be considered competing factors throughout the grain growth process [22]. Grain boundaries cover pores as a result of the force created by the thermal energy during sintering, which decreases the volume of the pores and raises the material's density. If each grain is subjected to a homogeneous driving force, the grain sizes will distribute uniformly. The current investigation's non-homogeneous driving force on the grains is what results in the non-uniformity of grain size [23]. The observed microstructures of synthesized materials give them suitable electromagnetic properties such as magnetization and permeability.

Synthesized ferrites show the growth of regular cubic crystals with average grain size ranges from 1-1.5 µm, depending on the substitution of Cu²⁺

3.4. Magnetic Measurements

The magnetic measurements were done with VSM, to determine the values of saturation magnetization (Ms) and coercivity (Hc) of the Cu²⁺ substituted Mg_0.6-xCu_xCo_0.4Fe₂O₄ (x = 0.0, 0.1, 0.2, and 0.3) at a magnetic field strength of 1T at room temperature. Figure 6 displays a plot of saturation magnetization plotted against the applied magnetic field for” various contents of produced ferrite samples. The figure demonstrates that magnetization rises as the applied magnetic field increases.

The M_s and H_c of synthesized ferrite samples were attained from the hysteresis loops as revealed in Figure 6 and their values are presented in Table 3 below. It was found that the value of M_s first increased and then reduced with the increasing dopant (Cu²⁺) concentration. The coercivity was found to depend on Cu²⁺ concentration and ranges from 157 Oe to 256 Oe. This result is comparable with prior studies indicating that magnetic characteristics are improved as a result of cation substitution. In the present research, Hoyos-Sifuentes et al. noted the synthesis of magnesium ferrite material (MgFe₂O₄) and a M_S value of 17 emu/g [24]. Balavijayalakshmiet al in their work synthesized the cobalt-substituted magnesium ferrites and reported an increase in Ms, M_r (Remanent Magnetization), and H_c as the content of cobalt substitution rises, attributed to the modification in the content of cation distribution [25]. Thus, the excellent “magnetic behavior of the synthesized Cu-doped Mg-Co ferrites can be utilized for high-frequency magnetic storage device applications.

Table 3. Magnetization and coercivity values for Mg_0.6-xCu_xCo_0.4Fe₂O_4.

Parameters	x = 0.0	x = 0.1	x = 0.2	x = 0.3”
Magnetization Ms [emu/g]	19.95	23.68	20.30	25.64
Coercivity Hc	157	266	256	193

Table 3. Magnetization and coercivity values for Mg_0.6-xCu_xCo_0.4Fe₂O_4.

Parameters	x = 0.0	x = 0.1	x = 0.2	x = 0.3”
Magnetization Ms [emu/g]	19.95	23.68	20.30	25.64
Coercivity Hc	157	266	256	193

The magnetic behavior of the Cu-substituted Mg-Co ferrites samples can be described by Neel's model. A ferrite's coercivity and saturation magnetization are determined by its density, microstructure, composition, and cation occupancy. As per “Neel's two-sublattice model, the variation between the magnetic moments of every sublattice determines the magnetic moment per formula unit. The total magnetization value can be obtained by their differences:

M= |M_B-M_A|

here M_A and M_B indicate the A and B sublattice’s magnetic moments per formula unit respectively” in μ_B (Bohr magneton).

From Table 3, the observed alteration in saturation magnetization, i.e.; first increase and then decrease with the rising dopant (Cu²⁺) content, indicates the change in the “cationic distribution in 2 interstitial sites; tetrahedral (A) and octahedral (B) [26]. With the substitution of Cu²⁺ on the B site, the drop in B-site magnetization produces a reduction in the ferrite's magnetization value (M_s), and this substitution of Cu²⁺ ions into the B-sites will promote the migration of Fe³⁺ ions into the A-site, which then increase magnetization of A-site.

The different exchange interactions, like A-B, A-A, & B-B based on the distribution of magnetic as well as non-magnetic ions at the A and B sites, can also be applied to describe the variations in M_s value. The A-A & B-B interactions are” known to be subordinate to the A-B interaction, which is the strongest. As the Cu level increased, the iron ions moved to the A site, exhibiting less A-B interaction with iron on the B site and thus the ferromagnetic behavior decreased with increasing Cu²⁺ concentration. Moreover, the coercivity first increased and then decreased due to the greater anisotropic constant of the Cu²⁺ substituted Mg-Co ferrites, which is completely connected to the samples’ bulk density. This result indicates that samples with high coercivity can be utilized for applications in microwave absorption systems, DC-DC converters, MLCIs, and switch-mode power supplies.

3.5. DC Electrical Resistivity Studies

The prepared pellets of the synthesized samples have been examined by two probe methods to determine the resistivity of Cu²⁺ substituted Mg_0.6-xCu_xCo_0.4Fe₂O₄ with x = 0.0, 0.1,0.2, and 0.3 samples in the range 300K - 873K. The sample temperature was determined with a chrome-alumni thermocouple. The resistivity of the sample is obtained by using equation (3) below:

ρ = R_bA/L

(3)

R_b signifies the “resistance of the sample, A indicates the sample’s surface area and is given by πr², r represents the sample pellet radius, and L denotes the sample’s thickness.

The plot of log ρ against 1/T is revealed in Figure 8. The” plot obeys the “Arrhenius relation (equation (4)), presenting the semiconducting behavior of synthesized samples [27].

$$\mathsf{\rho }={\mathsf{\rho }}_{\mathrm{o}}\exp (∆\mathrm{E}/\mathrm{k}\mathrm{T})$$

(4)

Where k indicates the Boltzmann constant, T presents the absolute temperature, $∆\mathrm{E}$ denotes the activation energy for conduction, ${\mathsf{\rho }}_{\mathrm{o}}$ presents the resistivity at 0K, and$\mathsf{\rho }$ shows the resistivity of the material at TK.

It is clear from the variation of electrical resistivity (logρ) with temperature (1/T) curves (at different Cu²⁺ concentrations) that each plot adheres to the Arrhenius relation. The resistivity of ferrite samples (at different Cu²⁺ concentrations) decreases as the temperature rises, showing that the samples are” semiconducting. The DC electrical resistivity was maximum with a value of ~ 2.4×10⁶ without copper substitution in magnesium-cobalt ferrite samples. With the copper substitution, the electrical resistivity was calculated and found to decline from 1.4 x 10⁶ Ω-cm (for x = 0.1) to 6.7 x 10⁵ Ω-cm (for x = 0.3). It shows that with the increase in Cu cation concentrations synthesized ferrite samples show a decrease in resistivity. The increased resistivity value is caused by the smaller particle size. Large numbers of grain boundaries present in smaller grains act as locations where electrons can be scattered, thus increasing the resistivity. Bharathi et al. in a study, reported the temperature-dependent DC electrical resistivity and the semiconducting behavior of synthesized Cu²⁺ substituted Mg-Co ferrite [28]. This result is in line with our study, which showed the decrease in electrical resistivity dependent on temperature and cation substitution and the semiconducting nature of synthesized copper-doped Mg-Co ferrites.

Figure 9 displays the activation energy plot of the samples against the content of Cu2+. The activation energy and sample resistivity decrease as the concentration content rises because of the hopping mechanism [29]. The activation energy values for Mg_0.6-xCu_xCo_0.4Fe₂O₄ (x = 0, 0.1, 0.2, and 0.3) are shown in Table 4.

On the adjacent octahedral sites (B-sites) of the ferrite structure, electrons jump (or hop) from Fe2+ to Fe3+, which causes electrical conduction in ferrites containing Cu2+ ions. To preserve the neutrality of the spinel lattice, oxygen vacancies are created during the high-temperature sintering of samples owing to the conversion of Fe3+ into Fe2+ ions. The result of this study is consistent with a previous report, where Parajuli et al. synthesized magnesium-substituted copper-cobalt ferrites and reported their electrical properties and activation energy based on the hopping mechanism [30].

4. Conclusions

This study successfully synthesized Cu2+ substituted Mg-Co ferrites (Mg0.6-xCuxCo0.4Fe2O4 with x values of 0.0, 0.1, 0.2 & 0.3) with solid-state reaction approach, showcasing the effective incorporation of copper into the ferrite structure. The resulting materials displayed a single-phase cubic spinel structure across every sample, as validated using XRD analysis. This structural integrity is crucial for applications requiring precise magnetic and electrical properties. Incorporating larger Cu2+ ions increased the lattice constant, highlighting the impact t of ionic substitution on the ferrite's crystal structure. SEM showed uniformly sized cubic crystals, with grain sizes ranging between 1-1.5µm, indicating a homogeneous synthesis process. Further structural confirmation came from FTIR, where the observed absorption bands aligned with the characteristic metal-oxygen vibrations in octahedral and tetrahedral sites, consistent with spinel ferrites. These Cu-doped ferrites' electrical resistivity and magnetic properties showed significant variation with the level of Cu2+ substitution. A decrease in electrical resistivity with increasing Cu2+ content suggests enhanced electrical conductivity, making these materials suitable for applications with desirable lower resistivity and semiconducting behavior. Specifically, the drop in activation energy for electrical conductivity with higher Cu2+ concentration improves the efficiency of high-frequency electronic devices. Magnetic measurements revealed increased saturation magnetization and coercivity with Cu2+ concentration, indicating the potential for tailored magnetic properties in device applications.

The relevance of these findings extends to the application of ferrites in SODAR systems. The synthesized materials, with their high DC electrical resistivity and adjustable coercivity, offer promising prospects for enhancing signal quality and noise reduction in such sophisticated technologies. In SODAR pre-amplifiers, where signal clarity and noise suppression are paramount, Cu-doped Mg-Co ferrites' tailored magnetic and electrical properties could significantly improve environmental monitoring accuracy and reliability. This study not only underscores the versatility of ferrites in high-frequency and noise-sensitive applications but also paves the way for future research into other doping elements to further expand the application scope of ferrite materials in advanced technological systems.