Influence of manganese substitution on microstructural and electro-magnetic properties of nanocrystalline Li-Ni-Zn ferrites

Abstract

Mn substituted Li-Ni-Zn ferrite nanoparticles with nominal compositions of Li0.15Ni0.30-xMnxZn0.40Fe2.15O4 (where x=0.00, 0.03, 0.06, 0.09, 0.12, and 0.15) were synthesized by the chemical sol-gel auto-combustion technique. Disk- and toroid-shaped samples prepared from each composition have been sintered at the temperature of 1373, 1423, 1473, and 1523 K for 4~5 h. The internal structure was characterized by X-ray diffraction (XRD), which has confirmed the formation of spinel structure for each composition consist of major spinel cubic phase with the minor impurity phases (Fe2O3 and MnO). The crystallite size of the as-grown powder varied from 46 to 52 nm estimated by using Debye-Scherrer formula. The lattice constant (ao) slightly increased with the increase of Mn content up to x=0.12, beyond that a slight decrease in ao was observed. Both theoretical density (ρth) and bulk density (ρB) decreased in a similar fashion with the increase of Mn content except x=0.15, in present ferrite samples for a fixed sintering temperature (Ts=1523 K). An opposite trend was found for the porosity of the samples. The density of various samples increased and as expected porosity decreased with Ts. The mean ionic radius of the variant ions, as well as hopping lengths of the samples sintered at optimum Ts, were also observed. To reveal the microstructure of the ferrites, the surface micrographs and the elemental analyses of the samples were taken using Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray Spectroscopy (EDS), respectively. The average grain size (Dgrain) was found to increase with increasing Mn2+ substitution up to x=0.12, beyond that Dgrain decreased, which was estimated by linear intercept technique from the FESEM micrograph. The real part (μi′) and imaginary part (μi′′) of initial permeability, relative quality factor (RQF), and magnetic loss factor (tanδM) varied with Mn content for a fixed Ts. It was also observed that μi′ of Li0.15Ni0.18Mn0.12Zn0.40Fe2.15O4 increased with Ts. The maximum value of μi′ (=334) and RQF (=14067) were observed for Li0.15Ni0.18Mn0.12Zn0.40Fe2.15O4 sintered at 1523 K, which are 7% and 16% greater compared to the parent composition, respectively. As a result, the minimum tanδM was observed in this composition. The weakening of exchange interaction confirmed by increasing the ao and decreasing the Néel temperature (TN) as the increase of Mn content up to x=0.12. The real part of dielectric constant (ε′), dielectric loss tangent (tanδE), complex impedance, ac conductivity (σac) as well as ac resistivity (ρac) were studied in the frequency range 100 Hz-100 MHz to achieve the information about the dielectric features of the studied ferrites.