Synthesis and magnetic properties of hydrothermal magnesium–zinc spinel ferrite powders

Mg–Zn ferrite powders with the nominal composition Mg_0.5Zn_0.5Fe₂O₄ were synthesized via hydrothermal method, and their synthesis, magnetic properties and microstructures were studied. It was found that the pH value affected the amount of impurity Fe₂O₃ and the purity of ferrites greatly. It was also found excess Zn content (5 at.%) in starting materials was not helpful to synthesize pure spinel ferrite, while the prolonged reaction time was harmful for the formation of pure spinel structure. The specimens presented small coercivity lower than 10 Oe, which showed a typical magnetically soft behavior. With the increase of pH value, the saturation magnetization of specimens with excess Zn ions (5 at.%) kept increasing from 23.90 to 41.82 emu/g due to the decreasing amount of impurity Fe₂O₃. The study of microstructures showed that the large particles in powders were the aggregates of small nanoscale crystallites. The analysis of actual Zn and Mg content in synthesized ferrites confirmed that the best experimental conditions for synthesis of pure spinel Mg–Zn ferrite are the hydrothermal temperature is 200 °C, the reaction time is 8 h, the pH value is 12 and the excess amount of Zn(NO)₃ in starting materials is 5 at.%.