Effect of Al3+ Substitution On Magnetic and DC Electrical Resistivity Properties of NiZnCu Nanoferrites

: Al substituted Ni 0.4 Zn 0.35 Cu 0.25 Fe 2-x Al x O 4 (x = 0.00, 0.05, 0.10, 0.15, 0.20) samples is synthesized using the sol-gel auto-combustion process. X-ray diffraction shows its cubic spinel structure. The lattice constant decreases as the Al 3+ content increases. The sizes of the crystallites are also decreasing in the range of 32.15 nm to 22.89 nm. The wavenumbers of tetrahedral and octahedral sites sighted in the FT-IR spectra are similar to that of the precursor. The increment in the Al 3+ content increases the DC conductivity. The electrical resistivity decrease with an increase in the temperature, i.e., it has a negative temperature coefficient with resistance similar to semiconductors. VSM results show their isotropic nature forming single domain ferrimagnetic particles. The resultant material is widely significant, as indicated by its result. Al 3+ substituted NiZnCu ferrite, XRD, FT-IR, Magnetic, Electrical properties.


Introduction
These spinel nanomaterials or ferrites with cations in their tetrahedral and octahedral sites are used as soft magnetic materials in everyday life [1][2][3]. Changing their internal with doping can make them applicable in more advanced devices. Most of the spinel ferrite materials have M 2+ Fe2 3+ O4 2the type of structure where M is a Transitional Metal Cation. M is on the tetrahedral site, and Fe on the octahedral site of the spinel. This soft magnetic ferrite's high resistivity and low power loss made them be used in multilayered chip inductors, microwave absorption materials, information storage systems, transformer core, computer circuitry, and electronic communication. Among them, Ni-Zn ferrite is highly stable at a higher frequency [4,5].

Ni-Cu-Zn ferrites are used in Multilayer Chip Inductors (MLCI) and Surface
Mounting Devices (SMDs). As a result, they are utilized in cellular phones, video cameras, notebook computers, personal wireless communication systems, etc. their properties can be changed by varying the concentration of Cu 2+ and found to be more applicable in inductive devices like transformers. The ferrites prepared at low temperatures are used in multilayer power inductors and transformers [6][7][8].
The purpose of this study is to a sol-gel auto-combustion method for the synthesis of the samples Ni0.5Zn0.5−xCuxFe2O4 (0.0 ≤ x ≤ 0.4) nanoparticles and investigates the influence of chemical composition, DC electrical resistivity, and magnetic properties. Ni-Cu-Zn ferrites have been studied for different electrical and magnetic applications [9]. Albeit high dielectric constants, i.e., electrical properties, have been reported for these ferrites, the correspondingly high dielectric loss, especially at high operating frequencies, hampers their application in reality [10].
A spinel system's magnetic parameters collectively respond to the synthesis method, cation types, and occupancy in different sites. The distribution of cations depends on the types of bonding and the ionic radii of the cations. A. Dzunuzovic et al. [11] fabricated Znsubstituted Ni-ferrites and studied their structural and morphological properties. The results show that Zn-substituted Ni-ferrites are of cubic spinel structure and the grain size of samples increases with Zn composition. Charalampos Stergiou [12] reported dielectric and magnetic properties of rare-earth (Y and La) substituted Ni-Co-Zn spinel ferrites. The dielectric constant is increased with the rare-earth substitution due to the improvement of the dielectric orientation polarization. D.R.S. Gangaswamy et al. [13] fabricated Co-substituted Ni-Zn-Mg ferrite and investigated the magnetic properties. It is found that saturation magnetization is decreased, and the soft magnetic nature increases with Co substitution. S.J. Haralkar et al. [14] prepared Cr-substituted Mg-Zn ferrite. They studied the cation distribution. It is observed that the fraction of Fe ions in octahedral sites decreases, and the fraction of Cr ions in octahedral sites increases with the substitution of Cr. Yu Gao et al. [15] synthesized Lisubstituted Ni-Zn ferrite and investigated elastic, structural, and magnetic properties. They observed that the characteristic band of ferrite shifts towards a lower frequency with Li [16].
Different ion substituted ferrites have been reported in the literature; very few are given above, but Ni-Zn-Cu-Al is still not fabricated and studied.
In this research, we have reported that the synthesis of Ni-Zn-Cu ferrites by the solgel auto combustion method by doping a portion of nonmagnetic Al 3+ content and studying the concentration of contents on the structure dc electrical resistivity and magnetic properties.  Few drops of polyvinyl alcohol were mixed with the powder for shaping them into disc-like pallets after pressing them in a die under the hydraulic press of 5 tons. The pallets were then made as electrodes by sintering them in 800 o C in a muffle furnace and polishing their flat sides with gold. Their conductivity was checked by the two-probe DC resistivity method.

XRD Studies
The XRD plots of different Ni0.4Zn0.35Cu0.25Fe2-xAlxO4 (x = 0, 0.05, 0.1, 0.15, 0.2) samples are shown in Figure 1. The structure of the sample is found to be a cubic spinel structure according to the JCPDS card No.48-0489. The lattice constant 'a' is determined with the following relation [17].

= √ + +
where dhkl is more interplane spacing for given hkl planes and is calculated by Bragg's law, the highest intensity (311) peak indicates the crystallite's appropriate orientation to measure its degree crystalline nature to find the average crystallite size of all samples. Debye-Scherer's formula gives the average size of the crystallite size [18].
where D311, λ, β and θ are volume-averaged crystallite size, the wavelength of X-ray (1.5406 Å), full width at half maximum of (311) peak and diffraction angle respectively.    Table   1. The crystallite size first decreases from 32.15 nm to 22.89 nm (for x = 0.0 to 0.2). In contrast, the lattice parameter decreases with increases in the Al 3+ ions concentration. This is due to the greater ionic radius of Fe 3+ ions (0.67 Å) [19] as compared to Al 3+ (0.51 Å) [20], thereby expanding the unit cell or decreasing the lattice constant [21], as shown in figure 3.
The obtained value of the lattice parameter of the base sample is 8.343 Å to 8.323 Å. It is well-matched with the value reported in the literature [22]. When the smaller Al3+ ions replaced the larger Fe 3+ ions, the unit cell shrank while preserving the overall cubic symmetry. The lattice compression may also be due to the partial oxidation of Fe 3+ , Al 3+ . The variation of the lattice constant is more significant in the smaller size of the nanoparticle.
The diffraction peak width (β) is inversely proportional to the crystallite size from Scherer's formula. The increase in the lattice parameter expands the volume of the unit cell accordingly. Sintering decreases the lattice defects and involved strain but facilitates the crystals' coalition increasing in particle size.
The grains sizes are nearly equal [23]. The microstructure images like grain size, pores, inclusions, grain boundaries, particle size, homogeneity, defects, etc., can be obtained with the Electron microscope's help. The smaller grain size with low porosity controls excessive spin-wave production, which is essential for microwave devices. Similarly, the large grain size supports the mobility of the domain wall, resulting in high permeability with low coercive value. Also, the eddy current losses are checked by the grain boundaries acting as current barriers. to 405.35 cm -1 are seen as two depressions in figure 6, and values listed in Table 2 are the two respective characteristics bands of each spinel ferrite. The bond length between Fe 3+ and O 2is varied for the sample composition, resulting in the deviation in the peak position of v1and v2 towards the high-frequency region [24].

Magnetic properties
Ferrites have antimagnetic moments with unequal magnitudes. As a result, they have a large value of spontaneous magnetization. The exchange integral, depending on interatomic distance, is negative for ferrite. This indirect exchange interaction through oxygen ions limits the easy flow of electrons. So, ferrites have high resistivity [25].

Figure 6: Hysteresis curve of Al-doped NiZnCu nanoferrites
The magnetic properties like saturation permeability, coercivity, susceptibility, Curie temperature, etc., of ferrites, depend on the concentration of metal ions on both the octahedral and tetrahedral sites. The hysteresis loop shapes, resistivity, ac conductivity, and dielectric constant depend on the ferrite structure. So, these properties are more sensitive to the system.
These properties can be changed by adding external magnetic or non-magnetic metal ions.
The hysteresis curves of our respective samples are as shown in figure 6. Figure 7 shows the values of coercivity (Hc), saturation magnetization (Ms), etc., that are important for their magnetic properties. The values of Ms and Hc are listed in Table 3 [26].   ions to the Ni-Zn-Cu mixture, they exchange few magnetic ions Fe 3+ and Ni 2+ in B-site increases AB interaction that interrupts the antiparallel spin B site resulting from the increase in total magnetization [27]. A similar phenomenon occurs on the A site. According to Weiss's

Molecular field theory, the A-B and B-A interaction dominates the A-A and B-B interaction
resulting in the hysteresis loop [28][29][30]. substituted Ni-Zn-Cu is shown in figure 8. A graph between log ρ and 1000/T is a straight line. It shows that the resistivity decreases with an increase in temperature showing semiconducting behaviour. The slope of the line gives the activation energies of the ferrite samples. The dc resistivity data are used in Arrhenius to find the thermally activated charge carriers' activation energy [31]. The Arrhenius relation is,

DC Electrical Resistivity
where ρ is the dc electrical resistivity at temperature T, ρo is the pre-exponential factor, ∆E is the activation energy, K is the Boltzmann constant, and T is the absolute temperature. The calculated values of activation energies of the synthesized ferrite nanoparticles samples are shown in Figure 9. Verwey and de Boer hopping mechanism helps to interpret the resistivity variation for the Al-doped Ni-Zn-Cu ferrite nanoparticles. Electron hopping occurs between ions of the same element located at different valance states and the two sites. During sintering of the ferrites, the divalent and trivalent iron ions can be produced and exist in octahedral sites that help in electrical conduction through Fe 2+  Fe 3+ hopping mechanism. If the ferrite's sintering temperature is higher, more Fe 2+ ions are produced, thereby accelerating the hopping process. The hopping process is possible in Fe 2+  Fe 3+ and Al 3+  Al 2+ existing together in a system [32].

Figure 9: Activation energies of the Al-doped NiZnCu nanoferrites
About the above calculation, as in figure 8, the activation energies are found to be in the order of 0.48 to 0.60 eV which is for the Fe 2+  Fe 3+ electron hopping mechanism. It indicates that the major conduction mechanism is the Fe 2+  Fe 3+ process. Besides, the conduction processes such as Fe 2+ + Zn 3+  Fe 3+ + Zn 2+ ions require relatively more energy for electron hopping, so the energy required could be slightly more than 0.48 eV. The temperature-dependent resistivity and associated activation energies indicate the compositional dependence of resistivity [33].

CONCLUSIONS
Sol-gel auto-combustion method is utilized to fabricate nanocrystalline Ni0.4Zn0.35Cu0.25Fe2-xAlxO4(x = 0, 0.05, 0.1, 0.15, 0.2) ferrite NPs. The structure of the ferrite shown by the X-ray diffraction is single phase cubic spinel. The decreasing crystallite size values increasing the Al concentration. In comparison, the lattice parameter decreases with the increases in Al 3+ ions concentration. This is due to the greater ionic radius of Fe 3+ ions (0.67 Å) than Al 3+ (0.51 Å), thereby expanding the unit cell or decreasing the lattice constant.
FESEM reveals microstructural growth along with heat action. The FTIR spectrum exhibits a prominent attribute of ferrite microstructure and a major impact on ingredients' mixture. The magnetic measurements show that magnetization reduces and coercivity enhances. DC resistivity is decreasing with an increase in the content of copper due to its highly conducting property. The electrical resistivity decrease with an increase in the temperature, i.e., it has a negative temperature coefficient with resistance similar to semiconductors. The temperaturedependent resistivity and associated activation energies indicate the compositional dependence of resistivity.