Effect of sintering temperature on the structural, dielectric and magnetic properties of Ni_0.4Zn_0.2Mn_0.4Fe₂O₄ potential for radar absorbing

Ni_0.4Zn_0.2Mn_0.4Fe₂O₄ nanopowders were prepared by sol-gel auto-combustion method, densified at different temperatures 400–700 °C/4 h using conventional sintering method. The grain sizes of all the samples vary between 18 nm and 30 nm. The hysteresis loops show high saturation magnetization and low coercivity, indicating magnetically soft behaviour of the material. The real and imaginary parts of permittivity is almost constant upto 1 GHz and increases with further increase of frequency. The permeability is ruled by Snoek's law, the values of μ′ increases with increase of temperature and the resonance frequency increases with an increase of temperature. The reflection coefficient is however increasing with sintering temperature and the maximum loss is observed in the range of 100 MHz–1 GHz. Sample sintered at 700 °C has shown maximum reflection loss and this loss occurs due to absorption, destructive interference and multiple internal reflections in the sample. Quality factor is constant upto 380 MHz and increases with frequency for all the samples sintered at different temperatures. The T_C for all the samples is above ~230 °C. The room temperature EPR spectra confirm the oxidation state of Fe³⁺. The g-factor is in the range of ~2.