FABRICATION AND CHARACTERIZATION OF TERBIUM SUBSTITUTED FERRITES

Abstract

This dissertation presents the effect of terbium substitution on the structural, magnetic, electrical and dielectric properties of ferrites of nominal compositions Ni1−xTbxFe2O4 and Mg1-xTbxFe2O4 (where x = 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18 and 0.20). The samples were initially sintered at 1000 °C and final sintering was performed at 1230 °C in order to prepare homogeneous ferrites. The said ferrites were characterized by X-ray Diffractometry, Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy, Vibrating Sample Magnetometery and Ferromagnetic Resonance. Phase analysis from XRD patterns reveals that the samples have a cubic spinel structure along with a few traces of second phase recognized as orthorhombic phase (TbFeO3) and this phase becomes more conspicuous as the terbium (Tb) is substituted in Ni1−xTbxFe2O4 and Mg1- xTbxFe2O4 ferrites. The lattice parameter changes non-linearly as a function of Tb content in both series that was attributed to the differences in ionic radii of the cations involved and the solubility limit of terbium ions. A gradual increase in the bulk density was observed with the increase of terbium concentration in both series. FTIR absorption spectra of Ni1−xTbxFe2O4 and Mg1- xTbxFe2O4 ferrites exhibited two main absorption bands in the wave number range 370 cm-1 to 1500 cm-1, thereby confirming the spinel structure. The particle size measured from XRD analysis found to decrease with the increase of Tb substitution. The morphology of the samples indicates that samples are crack free along with a few agglomerates. The inclusion of Tb caused the magnetization index to drop which is due to redistribution of cations on the tetrahedral and octahedral sites. The coercivity is observed to decrease and this trend is deviated at higher concentrations of terbium ions in Ni1−xTbxFe2O4 and Mg1-xTbxFe2O4 ferrites. Ferromagnetic resonance linewidths are ≤ 1000 Oe for most of the samples. The nominal composition Ni.94Tb.06Fe2O4 have minimum linewidth, ΔH = 593 Oe, which is minimum of the reported linewidths for spinel ferrites. Hence these ferrites have potential in high frequency applications. The relative initial permeability generally decreases for all the Tb-substituted samples and these are attributed to the decrease in magnetization. The magnetic loss factor is damped with respect to frequency and has very low values in the high frequency region. The electrical resistivity increases and it is attributed to the hinderence in the hopping mechanism caused by the presence of Tb-ions in these ferrites which enhance hopping length between the cations involved in the conduction mechanism. The study on resistivity as a function of temperature shows that all the samples obey semiconducting behaviour. The drift mobility drops while the activation energy increases in a similar manner as that of resistivity. The samples NiFe2O4 and MgFe2O4 have high dielectric constant as compared to the substituted samples. Hence, introduction of Tb-ions in Ni1−xTbxFe2O4 and Mg1-xTbxFe2O4 decrease the dielectric constant. All the substituted samples indicate small values of dielectric loss. These features make these ferrites suitable for various applications.