Investigation of frequency dependent permeability of LixNi0.2Mg0.8-2xFe2+xO4 ferrites

Abstract

Polycrystalline LixNi0.2Mg0.8-2xFe2+xO4 ferrites with x= 0.00, 0.10, 0.20, 0.30 and 0.40 have been synthesized by solid state reaction technique. Pellet and toroid shaped samples are prepared from the ferrite powder and sintered at 1050, 1100, 1150 and 1200 ˚C in air for 6 hours. Structure and surface morphology are studied by X-ray diffraction (XRD) and a high resolution optical microscope, respectively. The magnetic properties of these ferrites are characterized with high frequency (300 kHz-110 MHz) complex initial permeability. DC magnetization of all samples is measured by vibrating sample magnetometer method. The influence of microstructure, various cation distribution and sintering temperature on the complex permeability of these samples are discussed. X-ray diffraction pattern show the formation of spinel crystal structure. Lattice parameters are calculated using the Nelson Riley function. There is a compression of unit cell dimension depending on Li substitutions in these compositions. This result is explained with the help of ionic radii of substituted cations. The microstructural study shows that both sintering temperatures and cations substitutions have great influence on the average grain size. As the sintering temperatures increase, the bulk density increases (depending on compositions), and hence the porosity decreases for all ferrites. The grain size, density, and lattice parameter of LixNi0.2Mg0.8-2xFe2+xO4 are found to have highest value. The initial permeability value increases with increasing average grain size of the samples. It is also observed that the real part of permeability increases with sintering temperatures for all ferrites because high sintering temperature helps develop uniform grain growth. The real part of the initial permeability remains fairly constant in the frequency range up to some critical frequency which is called resonance frequency. The high natural resonance frequency (77.69 MHz) is found for Li0.1Ni0.2Mg0.6Fe2.1O4. The relative quality factor (Q) is found to be maximum for Li0.3Ni0.2Mg0.2Fe2.3O4. The DC magnetizations as a function of applied magnetic field are measured at room temperature (300K). From this result a possible cation distribution