Study of Holmium Doped Lithium Nano-Structured Ferrites: Synthesis, Characterization and Application

Abstract

In this work we have studied the influence of Ho-substitution on various properties of Li1.2Me0.4Fe2-xHoxO4 (Me: Co, Ni, Mn, Zn; x = 0.00-0.15) spinel ferrites synthesized by sol-gel auto-combustion method. The thermo-gravimetric analysis (TGA) of the as prepared samples was carried out to estimate the annealing temperature essential for complete decomposition of precipitates and spinel phase formation which was seen to be around 950 °C. It is seen that face centered cubic phase is obtained by this method of preparation. The overall average crystallite sizes as determined from X-ray diffraction (XRD) fall in the range of 29-17 nm, relatively smaller than those reveal from transmission electron microscopy (TEM). The densities are seen to increase, whereas the porosity is reduced by the substitution of Ho ions. X-ray photoelectron spectroscopy (XPS) has confirmed the presence of all metal ions in the required valence states. Fourier transform infrared (FTIR) spectra featured two intrinsic vibrational bands (υ1) and (υ2) in a wave number range 400-600 cm-1 due to the tetrahedral and octahedral stretching vibrations, respectively. We found that the Ho-substituted lithium ferrites show low dielectric losses for all samples. The effective contribution of grain-interior mechanism to dielectric behavior has confirmed by complex impedance study. The saturation magnetization is seen to decrease while coercivity is effectively increased with increasing the Ho-content proposing these ferrites for magnetic recording media. Ferromagnetic resonance (FMR) studies at X-band show the reduction in FMR line width with Ho substitution. However, the effects are to be related to the densification of the material since we find that the more dense samples actually lead to decreased microwave losses (narrow linewidth), which is prerequisite for their use in microwave applications. Electrical measurements revealed that the resistivity is higher in samples with smaller grains possessing a greater number of thin insulating grain boundaries. In high temperature resistivity measurements, a decrease in dc resistivity is observed with increasing the temperature. Most significantly we observed a change in slope around the Curie transition temperature (Tc) separating the ferromagnetic region from the paramagnetic with the activation energies being higher for paramagnetic region (Ep > Ef). The substitution of Ho3+ for Fe3+ at octahedral sites causes a decrease in Tc due to the damping of A-B exchange interactions. The drift mobility is seen to decrease with increase in resistivity and vice versa.