Mn–Zn ferrites with high Fe2O3 content obtained by the nitrate–citrate sol–gel auto-combustion method

In this paper, powders and sintered cores of Mn–Zn ferrites with Fe₂O₃ contents ranging from 61.8 to 76.9 mol.% were investigated. The high iron oxide powders obtained by the nitrate–citrate sol–gel auto-combustion method contained 96 wt.% ferrite phase without additional heat treatment. After air quenching at 1300°C, the ferrite cores obtained from these powders have a two-phase structure. Single-phase ferrites with Fe₂O₃ contents ranging from 61.8 to 76.9 mol.% were obtained by sintering in an atmosphere with a residual air pressure of 20 Pa at 1280°C. As the Fe₂O₃ content increases from 61.8 to 76.9 mol.%, initial magnetic permeability decreases from 1100 to 150, coercivity (H_c) increases from 70 to 240 A/m, but high temperature stability of maximum flux density (B_m) is achieved—in the range of 25°C–100°C, the decrease of B_m is not more than 3%. Sintering at 1300°C resulted in a decrease of initial magnetic permeability and B_m values due to active evaporation of zinc.