Structural, morphological and magnetic properties variation of nickel-manganese ferrites with lithium substitution

Mixed ferrites with nominal chemical compositions Li_xNi_0.2Mn_0.8−2xFe_2+xO₄ ranging from x=0 to 0.4 in the steps of 0.1 have been prepared by the auto combustion technique. The X-ray diffraction patterns consist of major cubic spinel Li_xNi_0.2Mn_0.8−2xFe_2+xO₄ phase with minor impurity phases (Fe₂O₃ and MnO) and with Li substitution phase purity has increased, such that for x=0.4 pure phase spinel structure has been obtained. The lattice parameter has decreased with the increase in Li content obeying Vegard's law. Both the bulk density and theoretical density have decreased with Li content and with sintering temperature (T_s) up to 1300 °C ρ_B has increased and beyond that it has decreased. Morphological studies have performed by a high resolution optical microscope and observed that average grain size noticeably dependent on Li substitution. The initial permeability (μ_i′′) has found to decrease with Li substitution. The Curie temperature (T_C) has determined from the temperature dependent μ_i′′ and found to increases with Li content. From the room temperature magnetization measurement, it has observed that all samples are in ferrimagnetic state at room temperature. The number of Bohr magneton has been obtained from the observed saturation magnetization. Dielectric constant, dielectric loss tangent, ac conductivity and complex impedance are studied in the frequency range 20 Hz–10 MHz. Frequency dependence of dielectric constant in lower frequencies indicates a usual dielectric dispersion due to the Maxwell-Wagner type interfacial polarization. Dielectric loss tangent shows similar behavior like dielectric constant. The complex impedance analysis has been used to study the effect of grain and grain boundary on the electrical properties and with Li content both grain and grain boundary resistance show an increasing trend. The ac conductivity shows frequency independent behavior at the low frequency side and with increasing frequency the conductivity increases. - Highlights: • Li-Ni–Mn ferrites have been prepared by auto combustion technique. • Impurity phases decrease with increasing Li concentration. • Frequency stability of μ_i′′ increase with increasing Li content. • The T_C increases almost linearly with increasing Li substitution. • The dielectric constant shows usual dielectric dispersion at low frequency.