INFLUENCE OF Mn SUBSTITUTION ON THE STRUCTURAL, ELECTRICAL AND MAGNETIC PROPERTIES OF Ni-Cu-Cd BULK FERRITES SINTERED FROM NANOCRYSTALLINE POWDER

Abstract

Maintaining nanoscale properties in a high-density bulk form of ferrite prepared from powdered nanoparticles is quite desirable in many high frequency applications. Various Ni0.5-xMnxCu0.2Cd0.3Fe2O4 (NMCCFO, x = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) dense bulk ferrites were consolidated from nano-crystalline powders by the sol-gel auto-combustion technique. Commercially available different nitrate salts of the ingredients were mixed thoroughly in stoichiometric amount and were calcined at 700°C for 5 h. Pellet and toroid shaped samples prepared from each composition were sintered at 1200°C for 5 h. X-ray diffraction (XRD) was used to carry out the structural analyses. The XRD data confirm that all compositions are single phase spinel structure. The lattice constant increase with increasing Mn content which is a clear indication of Mn incorporation in spinel structure. The theoretical density and the bulk density decrease but the porosity increases with increasing Mn content. The Rietveld refinement method confirms the goodness of fit with refined XRD data of NMCCFO for different Mn content. Rietveld technique is also adopted to determine the cation distribution between tetrahedral and octahedral sites and shows that maximum migration of Fe ions from A to B-sites occurs for x = 0.2 of Mn content. The Maximum Entropy Map analysis reveals the variation of the electron density with increasing Mn content and the presence of strong covalent bonding. Field emission scanning electron microscopy (FE-SEM) is used to carry out the surface morphology analyses. The average grain sizes increases from 1 μm to 4 μm for all compositions except x = 0.5 of Mn content. Energy dispersive X-ray (EDX) findings confirm the absence of traceable impurities and presence of Ni, Mn, Cu, Cd, Fe, and O in the samples. The dielectric measurements as a function of frequency and compositions are carried out at room temperature in the frequency range 100 Hz to 100 MHz. The dielectric constant (ε / ) and the dielectric loss tangent (tan δ) remains high at low frequency but becomes independent of frequency at higher frequencies for all the compositions of NMCCFO. This phenomenon may be explained by the Maxwell–Wagner model. The ac conductivity (σac) is derived from the dielectric measurements and it increases with increasing of frequency for all the compositions of NMCCFO. Frequency dependence of real (M/ ) and imaginary (M//) parts of the electric modulus and real (Z / ) and imaginary (Z //) parts of the complex impedance for different composition are measured at room temperature. The Cole-Cole plots (M/ vs M//) of electric modulus exhibit a tendency of formation of a single semicircular arc for all compositions indicates the existence of single-phase nature of the materials as well as the improvement in conductivity. Also, the Cole-Cole plots (Z/ vs Z//) of complex impedance exhibit a tendency of formation of semicircles end in the high frequency region. It explains the dominancy of the grain boundary. The vibrating sample magnetometer (VSM) was used for magnetization measurement at room temperature. From the hysteresis loop, the saturation magnetisation (Ms), remanent magnetisation (Mr), coercivity (Hc), the ratio (R) of Mr and Ms, anisotropy constant (K1), and magnetisation magnetic moment (μB) are calculated. All the compositions show the nature of soft ferrite due to the small amount of remanence and coercivity. Theoretical law of approach to saturation (LAS) shows that the values for both the saturation magnetization (Ms) and anisotropy constant (K1) are lesser than the experimental value. Therefore, the unique combination of electric and magnetic properties like low dielectric loss tangent, high ac conductivity and soft ferrite like behavior make the NMCCFO materials suitable for manufacturing high frequency devices like Multilayer Ferrite Chips Inductor (MLFCI), phase shifters, switches, etc.