Physical and magnetic properties of nanocrystalline LixMn0.8-2x Ni0.2Fe2+xO4 ferrites

Abstract

Structural and magnetic properties of a series of nanocrystalline spinel type ferrites with the nominal chemical composition LixMn0.8-2xNi0.2Fe2+xO4, where x=0.00 to 0.40 with step 0.10 have been synthesized by combustion technique. Fluffy lose powders obtained with combustion technique are calcined at 750C for 5h. Disk- and toroid-shaped samples prepared from these powders are sintered at various temperatures (1150-1350C). Structural and morphological studies were performed by X-ray diffractometer and a high resolution optical microscope, respectively. The magnetic properties like complex permeability, loss factor, quality factor of these ferrites were measured by Wayne Kerr impedance analyzer (Model No. 6500B) with frequency (100Hz-120MHz). The temperature dependence of initial permeability is also studied to obtain Neel temperature, TN. The formation of spinel structure was confirmed by X-ray diffraction analysis. Lattice parameters are calculated using the Nelson Riley function. Lattice constants decreased with increase in Li content. It is observed that the density of LixNi0.2Mn0.8-2xFe2+xO4 ferrites increases with increasing sintering temperature up to 1300˚C, then decreases. On the other hand, porosity (P) of the samples decreases with increasing sintering temperature up to 1300˚C and an increasing trend is shown beyond it. Average grain sizes of the samples are determined from optical micrographs by linear intercept technique. The microstructural study shows that both sintering temperatures and cations substitutions have great influence on the average grain size.The real ( / i  ) and imaginary ( // i  ) permeability decrease with Li substitution. With the increase of sintering temperature the real part of initial permeability / i  increases up to 1300˚C and above 1300˚C a decreasing trend is observed. It is found that resonance frequency (fr) is shifted from lower value to higher value as sintering temperature increases for all samples. The maximum value of fr (66.1MHz) is observed for Li0.3Ni0.2Mn0.2Fe2.3O4 ferrite sintered at 1150˚C. Q-factor is found to decrease for particular contents above 1300˚C because at higher sintering temperature abnormal grain growth occurs, which creates trapped pores inside the grain. It has been found that the TN increases almost linearly from 4000C to 6800C with increasing Li content sintered at 13000C.