DC Electrical Resistivity and Magnetic Properties of Co Substituted NiCuZn Nano Ferrite

Co substituted Ni 0.3-x Co x Cu 25 Zn 0.45 Fe 2 O 4 (x = 0, 0.05, 0.1, 0.15 and 0.2) samples is synthesized using the sol-gel auto-combustion process. X-ray diffraction shows its cubic spinel structure. The lattice constant decreases as the Co content increases. The sizes of the crystallites are in the range of 20.18 – 26.24 nm. The wavenumbers of tetrahedral and octahedral sites sighted in the FTIR spectra are similar to that of the precursor. The increment on the Co content increases the DC conductivity. The electrical resistivity decrease with increase in the temperature, i.e. it has a negative temperature coefficient with resistance similar to semiconductors. The remnant ratios R obtained from VSM show their isotropic nature forming single domain ferrimagnetic particles. The resultant material is widely significant, as indicated by its result.

found to be more applicable in inductive devices like transformers. [4][5][6][7]. The ferrites prepared at low temperature are used in multilayer power inductors and transformers.

EXPERIMENTAL TECHNIQUES
Co Substituted NiCuZn nanoparticles are prepared by the sol-gel auto-  Figure 1. The structure of the sample is found to be cubic spinel structure according to the JCPDS card No.48-0489. The lattice constant 'a' is determined with the following relation [8].
where dhkl is interplaner spacing for given hkl planes and is calculated by Bragg's law.
The plot of intensity against the diffracting angle (2 ) for different concentration of Co, x = 0, 0.05, 0.1, 0.15 and 0.2 in Ni0.3-xCoxCu25Zn0.45Fe2O4 ferrite nanoparticles (NPs) are shown in Figure 2. The highest intensity (311) peak indicates the crystallites appropriate orientation to measure its degree of crystalline nature to find the average crystallite size of all samples [9]. Debye-Scherer's formula gives the average size of the crystallite size [10].
where, D311, λ, β and θ are volume-averaged crystallite size, the wavelength of X-ray (1.5406Å), full width at half maximum of (311) peak and diffraction angle respectively.
The crystallite size, lattice parameters and cell volume of the composition  and gain are usually credited to the surface, forming a common boundary structure between surfaces with a more volume fraction. The variation of lattice constant is more significant in a smaller size of the nanoparticle. From Scherer's formula, the diffraction peak width (β) is inversely proportional to the crystallite size. 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.

DC Electrical Resistivity
The  nanoparticles ferrite samples are shown in Figure 9. Verwey and de Boer hopping mechanism helps to interpret the resistivity variation for the Cu doped Ni-Zn 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 also possible in Ni 2+  Ni 3+ and Co 3+  Co 2+ existing together in a system [23].

Figure 9: Activation energies of the Co substituted NiCuZn ferrite
About the above calculation as in figure 8, the activation energies are found to be in the order of 0.42 to 0.51 eV which is for the Fe 2+  Fe 3+ electron hopping mechanism. It indicates that the major conduction mechanism is 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 that the energy required could be slightly more than 0.42 eV. The temperature-dependent resistivity and associated activation energies indicate the compositional dependence of resistivity [24].
The structure of the ferrite shown by the X-ray diffraction is single phase cubic spinel.
The increasing-decreasing pattern of crystallite size is due to the lattice strain produced in the process of synthesis. In comparison, the lattice parameter decreases with the Co 2+ ions concentration. This is due to the larger ionic radius of Co 2+ (0.73 Å) as compared Ni 2+ (0.74 Å), thereby expanding the unit cell or increasing lattice constant. Sintering decreases the lattice defects and involved strain but facilitates the crystals' coalition increasing in particle size. FESEM reveals microstructural growth along with heat action. The FTIR spectrum exhibits a prominent attribute of ferrite microstructure along with major impact of the mixture of ingredients. The magnetic measurements show that magnetization reduces and coercivity enhances as the composition x = 0.1. DC resistivity is decreasing with an increase in Co content due to its highly conducting property. The electrical resistivity decrease with increase in the temperature, i.e. it has a negative temperature coefficient with resistance similar to semiconductors. The temperature-dependent resistivity and associated activation energies indicate the compositional dependence of resistivity.