Characterization of Calcinated Ni–Cu Ferrite Nanoparticles Synthesized by Citrate-gel Auto combustion Technique

Citrate-gel auto combustion technique was used to synthesize nickel-copper mixed ferrite nanoparticles NixCu1-xFe2O4 (x= 0.0, 0.4, 1.0) with different calcinating temperatures (200, 450, 650 and 850 °C). Structural, morphological, magnetic, and electrical properties were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR), vibrating sample magnetometer (VSM), and LCR meter in order to determine significant influences of Cu 2+ cations content in nickel ferrite. The XRD patterns showed that all compositions had cubic spinels, except CuFe2O4 samples. The system structure in Cu-ferrite was deformed at 650 °C from a cubic to a tetragonal system with a secondary phase CuO. Lattice constant was decreased with increasing Cu 2+ cations substitution in Ni-ferrites, while crystalline volume was increased. With the help of Debye-Scherrer`s equation using XRD data, we found that the crystallite size at 850 °C is lying in the range from 38.70  to 48.00 nm. The FTIR spectrum of samples under investigation showed two significant absorption bands, which refer to the formation of a single-phase cubic spinel. The magnetization test revealed a soft ferromagnetic behavior for all the compositions sintered at 850 °C. The saturation magnetization (Ms) was decreased with the substitution by Cu 2+ cations, while remnant magnetization (Mr) and coercivity (Hc) were increased in Cu-ferrite. The highest Ms value was 42.25 emu.g -1 for Ni-ferrite, while the highest Hc value was 517.16 Oe for Cu-ferrite. The electrical measurement of samples showed an increase in the real dielectric constant and AC conductivity at a frequency range of 20Hz3MHz with the addition of copper cations.


Introduction
In recent years, the ferrites family has gained a great importance among researchers, as these materials have been greatly desired in various electrical industries. They are electrically insulated and accompanied by high resistance, leading to their employment in changing magnetic fields because their loss of eddy currents is very slight. This feature made such materials more widely used in the applications of microwave and magnetic fluids [1], recording disks, transformer molds, gas sensors, and moisture sensors [2,3]. The general formula for spinel ferrites is MFe 2 O 4 , where M is a metallic divalent ion (e.g., Cu, Fe, Cd, Ni, Zn, etc.) [4]. Among the most famous ferrites are CuFe 2 O 4 and NiFe 2 O 4 , due to their inverse spinel structure. They have ferromagnetic behavior and wide device applications. The substitution of copper in nickel ferrite enhances the dielectric properties. Besides, it reduces the magnetic properties of nickel ferrite, which is useful in many device applications [5]. CuFe 2 O 4 and NiFe 2 O 4 were prepared by various methods, such as the ceramic method [6], electrodeposition method [7], hydrothermal reaction [8], oxidation precipitation [9], sol-gel method [10], and electrochemical method [11]. Among the best methods for preparing ferrite compounds is the self-combustion method for the gel, because it is easy to prepare, does not need high temperature, the interacting elements and the granular size of the powder nanoparticles can be controlled, and free from impurities [12]. The aim of the present work is to understand the role of Cu 2+ cations in controlling the dielectric and magnetic behavior of Ni-ferrite. Therefore, a series of Ni x Cu 1-x Fe 2 O 4 (x= 0.0,0.4,1.0) spinel ferrite samples were prepared and the consequent changes in several parameters, such as lattice constant, crystalline volume, density, saturation magnetization, dielectric permittivity, and dielectric loss tangent, were discussed in this paper. Furthermore, we investigated the effect of calcination temperature on the parameters above.

Experimental Part 2.1. Synthesis of Ni x Cu 1-x Fe 2 O 4 nanoparticles
The spinel samples of Ni were used as raw materials for preparing the compounds of Ni x Cu 1-x Fe 2 O 4 nanoparticles. A stoichiometric ratio of nickel nitrate (1.163 g) and ferric nitrate (3.232 g) was mixed in 50 ml deionized water. The molar ratio of nickel nitrate and ferric nitrate was 1:2. Citric acid was added as a fuel in the metal nitrate with a ratio of 3:1 under vigorous stirring conditions. The ferrite samples were successfully prepared by doping of Cu 2+ with different molar ratios (Ni x Cu 1-x Fe 2 O 4 , x= 0.0, 0.4, 1.0) to nickel ferrite. Copper nitrate amounts that were added to nickel nitrate solution were 1.182 g and 0.177 g for molar ratios of 0.0 and 0.4, respectively. The pH value of the solution mixture was maintained at 8 by ammonia solution. The mixing process was continued to ensure complete mixing of the solution and a good distribution of the components at room temperature. After that, the temperature was gradually raised and then fixed at 90°C with the stirring process continuing until the gel form was reached. After around 10 min the gel began to burn out to form a dry gel (Xerogel). The final burnt powders were then thermally calcinated at 200, 450, 650, and 850 °C for 3 h to obtain wellcrystallized Ni x Cu 1-x Fe 2 O 4 .

Materials characterization
The X-ray diffraction patterns of the samples were identified using X-ray Diffractometer (XRD-6000 from Shimadzu, Japan) using Cu Kα radiation (λ = 1.5406 Å), with (2θ) = 10°-80° in steps of 5 deg/s. Spectra of the powders were recorded using FTIR spectrometer (FTIR 8400S from Shimadzu, Japan) in the wave number range of 400-3500 cm -1 to confirm the spinel structure of the samples. The surface morphology was analyzed through the use of FE-SEM (QUANTA 400, USA, 0.5-30 kV) and TEM (CM120 Philips, Netherlands). The magnetic properties of CuNi ferrites were investigated using VSM (Cryogenic Limited PPMS, UK) under the applied field of ±1 (T). LCR meter (GW INSTEK LCR-8105G, Taiwan) was used to measure dielectric properties at a frequency range of 20Hz-3MHz. respectively. For all the samples prepared and calcinated at different temperatures, it was noticed that when the temperature was increased, the peak of the reflections became sharper. This indicates an improvement in the crystallinity when the calcinating temperature increases. Additionally, it is noticed that as the calcinating temperature increases, the width of the central maxima decreases. This is due to the grain size increase with the increase in the calcinating temperature. The lattice constant (a) was calculated using equations (1) and (2) for cubic and tetragonal structures, respectively [13,14]: √ where d is the interplanar spacing, h, k, and l are Miller indices, and a and c are lattice constants. The crystallite size (D) was calculated using the high intensity peak for all the compositions using Scherrer's formula [15]:

Results and discussion 3.1. XRD and FTIR Analyses
where k is the shape factor (0.9), λ is the wavelength (1.5406 Å, CuKα), β is the full width at half maximum (FWHM), and θ is the diffraction angle. X-ray density (ρ x-ray ) and surface area (S) of the ferrite samples were calculated by using the following formulas [16,17]: where M is the molecular weight of the sample, N is the Avogadro's number, and a is the lattice constant. The values of the lattice constant, crystallite size, unit cell volume, X-ray density, and surface area of the calcinated samples are summarized in Table-1.  From Table-1, we can observe that x-ray density increases, while surface area decreases, with increasing calcinating temperature. Figure- (413), respectively. The XRD shows that the crystal structure was deformed at 650ºC from the cubic into a tetragonal system, with a secondary phase that belongs to CuO at 38.7 (JCPDS Card no 48-1548). Figure- (533), respectively. It can be noted clearly that the crystalline volume increases with the increase in doping concentration of Cu, which may be due to a larger ionic radius of Cu 2+ (0.73 Å) compared with that of Ni 2+ (0.69 Å) [18][19][20]. On the other hand, the crystalline volume increases with increasing calcinating temperature, which is due to crystal growth. Values of the lattice parameter a for NiFe 2 O 4 and Cu 0.6 Ni 0.4 Fe 2 O 4 samples were found to be equal to 8.310 and 8.371Å, respectively. The higher a value for Cu 0.6 Ni 0.4 Fe 2 O 4 compared with that of NiFe 2 O 4 is also due to the larger ionic radius of Cu 2+ than that of Ni 2+ . On the other hand, the lattice constant decreases with increasing calcinating temperature.  Figure-1 (b, d, and f) shows the FTIR spectra of Ni x Cu 1-x Fe 2 O 4 (x= 0.0, 0.4 and 1.0) samples at calcinating temperatures of 200, 450, 650, and 850 °C for 3 h. The gels are mostly amorphous (noncrystalline) in nature but can be crystallized by heat treatment [21]. Therefore, when the temperature was raised to 200 ºC, the phase beginnings of all samples were shown respectively. On the other hand, as the temperature was increased, we observed a decrease in the intensity of the bands, indicating the disappearance of water molecules and organic groups from the sample. In the case of samples calcinated at 850 ºC, we noticed the appearance of two strong absorption bands. One bond was in the high-frequency range (ν 1 ) due to the frequencies of the Fe-O bonds of the tetrahedral (A site) between 557 and 586 cm -1 . The other bond was in the low-frequency range (ν 2 ), which is attributed to the frequencies of the M-O bonds of the octahedral (B-site) between 418 and 430) cm -1 [22][23][24][25].

SEM and TEM Analysis
The morphology of the nanoparticles was examined through FE-SEM/EDX and TEM. Figure -shows SEM micrographs, the respective energy-dispersive X-ray spectroscopy (EDX), and the particle diameter distribution of NiFe 2 O 4 , CuFe 2 O 4 , and Ni 0.4 Cu 0.6 Fe 2 O 4 nanoparticles calcinated at 850 °C. As can be seen from Figure-2 (a), pristine NiFe 2 O 4 nanoparticles exhibit polyhedron and spheric morphology, some being accumulated to form larger particles. This agglomeration reduces the surface area of NiFe 2 O 4 nanoparticles. Figure-2 (b) reveals the diameter histogram for NiFe 2 O 4 nanoparticles, which displays a homogenous particle size distribution around 35.5nm range. Figure-2 (d) shows the SEM micrograph of CuFe 2 O 4 nanoparticle, revealing a polyhedron shape with few agglomerations and negligible porosity. Figure-2 (e) shows a broad histogram of particle diameter distribution of CuFe 2 O 4 , with average size around 77.7nm. Figure- Figure Table-2. The Ms value of NiFe 2 O 4 nanoparticles was 42.25 emu.g -1 , which was high as compared to the value in other samples. While the Ms value of CuFe 2 O 4 nanoparticles was the lowest, being around 32 emu.g -1 . This might be due to the presence of CuO as an impurity, which was in fact detected in XRD analysis. Additionally, this could be due to the smaller magnetic moment of Cu 2+ ion (1.37μ B ). However, CuFe 2 O 4 still has a high saturation magnetization. The experimental magnetic moment per formula unit (μ B ) in Bohr magneton was calculated using the following relation [26]:

VSM Analysis
As related to remnant magnetization, we noticed that the highest value was recorded for CuFe 2 O 4 , followed by NiFe 2 O 4 , and Cu 0.6 Ni 0.4 Fe 2 O 4 , respectively. Coercivity value was increased with the substitution of Cu 2+ ions into Ni-ferrite and the highest value was recorded in CuFe 2 O 4 due to the potential for a cationic disturbance on the surface [27]. In general, NiFe 2 O 4 has an inverse spinel structure with Ni 2+ ions in octahedral sites, and Fe 3+ ions are equally distributed between tetrahedral and octahedral sites [28]. CuFe 2 O 4 has a partial inverse spinel structure with 85% of Cu 2+ are occupying octahedral sites, while the other 15% are occupying tetrahedral sites. However, Fe 3+ ions are equally distributed in the tetrahedral and octahedral sites [29]. This means that the Ni 2+ ions have a strong preference for octahedral sites, while Cu 2+ ions have a similar preference for octahedral and tetrahedral sites. When Cu 2+ is substituted by Ni 2+ in Ni x Cu 1-x Fe 2 O 4 nanoparticles, the cation distribution will change and thus changes the magnetic properties subsequently. Table-2 shows that ferrite samples have various coercivity values. This may be attributed to several reasons, such as composition, surface conditions, and magnetic anisotropy.
The squareness ratio (Sq=Mr/Ms) was increased with increasing the content of Cu. Therefore, the maximum Sq value was found to be 0.46, indicating the enhanced contribution from the cubic anisotropy for CuFe 2 O 4 . Hence, a large coercivity can be expected for CuFe 2 O 4 . The values of anisotropy constant (K) were evaluated using the following relation [30,31]: Waheed et al. Iraqi Journal of Science, 2021, Vol. 62, No. 3, pp: 878-888 778 The anisotropy constant was observed to increase with increasing Cu 2+ content.

Dielectric Properties
AC electrical properties, represented by the real part of dielectric constant (ε'), the imaginary part of dielectric constant (ε") and the loss factor (tanδ), in addition to the alternating conductivity (ζ ac ) of the samples of NiFe 2 O 4 , Ni 0.4 Cu 0.6 Fe 2 O 4 , and CuFe 2 O 4 , were studied as a function of frequency at room temperature in a frequency range of 20Hz-3MHz. The real and imaginary dielectric constant values were calculated by using the following equations [32] where C is the capacitance of the pellet, d is the thickness, A is the cross-sectional area, and ε˳ is the permittivity of free space (ε˳= 8.854×10 -12 F.m -1 ). Figure-5 (a and b) shows the variation of dielectric constant and imaginary part with frequency. It is seen that the real and imaginary dielectric constant values of the samples decrease with increasing frequency and then they remain constant at high values, which is the normal behavior of most ferrites. The high values of the real dielectric constant at low frequencies are due to the polarization of the space charge due to the large mass of its dipoles compared to other types of polarization (electronic, ionic, and directional). When the frequency is increased, these dipoles cannot rotate with the electric field and thus the dielectric constant decreases. This behavior of ferrite has been observed by many researchers [33][34][35]. Figure-4 also shows that the highest value of the real dielectric constant was recorded for CuFe 2 O 4 (24304) and the lowest value was for NiFe 2 O 4 (2323), whereas that for Ni 0.4 Cu 0.6 Fe 2 O 4 mediated the two values. Since ferrite has a semi-conductive nature, this is affected by doping with other atoms. The effect of doping on charge carriers is either reflected by an increase or a decrease in the real dielectric constant, depending on the type of atom. Therefore, the dielectric constant values differed as a result of the nickel doping process.
The loss factor is a measure of energy dissipation within the dielectric medium and is considered as

(a (b )
part of the primary loss in the ferrite. We notice from Figure-5 (c) that the maximum values of the loss factor and the loss tangent appeared at low frequencies, while they were decreased with the increase in frequency, until they became constant and stable. Similar results were observed in another research of ferrite systems [36]. This result of the distribution of values in the curve of the imaginary dielectric constant and loss factor is attributed to the Maxwell-Wagner theory of interpolarity [37]. According to this theory, the dielectric constant of ferritic materials consists of two layers, namely the grains and grain boundaries, where the grains are considered regions with low electrical resistance, due to the ideal crystalline properties, while grain boundaries are regions with high resistance. According to Maxwell-Wagner polarization theory, the polarization between surfaces within the grain boundaries occurs due to the accumulation of charge in these high-resistance boundaries. This polarization occurs in all heterogeneous crystalline materials (which are more than one phase). Figure-5 (d) shows the alternating electrical conductivity, as this study provides important information about this process and the effect of deformation on it. In general, the conduction process occurs in the ferrite materials as a result of the transfer of electrons between Fe +2 and Fe +3 located in octagonal locations [38]. The values of AC conductivity (ζ ac ) were calculated using the following equation [32,39]: (ζ ac )= ω ε o ε′ tanδ (10) where ω is the angular frequency. Figure-5 (d) shows that AC conductivity decreases with the addition of nickel atoms, as electronic transitions between Fe +2 and Fe +3 decrease in the octagonal locations [39]. On the other hand, grain boundaries in the low conduction operate in all samples at frequencies of 10 3 -10 4 H,z where the transmission of electrons between Fe +2 and Fe +3 decreases relatively, while electronic mobility increases in the larger frequencies (10 4 Hz) as the grain boundaries become more active, which enhances conductivity.

Conclusions
The Ni x Cu 1-x Fe 2 O 4 (x= 0.0, 0.4, and 1.0) nanoparticles were synthesized successfully via the citrate-gel auto combustion method. These Ni-Cu ferrite nanoparticles show polyhedron and spherical morphology as revealed via SEM and TEM results. XRD analysis exhibited deformation in system structure, from cubic to tetragonal system, when substituting Cu 2+ cations in nickel ferrite completely at 850 ºC. The Cu content has a significant effect on dielectric and magnetic properties, where the  (c (d substitution of copper in nickel ferrite decreases the saturation magnetization and improves dielectric properties of nickel ferrite. Thus, we obtained superb ferrite materials that have mixed properties from Ni-ferrite and Cu-ferrite, which make them good materials for electronic applications, including sensors, EM devices operating at high frequencies, and magnetic fluids.