Effects of heat treatment on structure and magnetic properties of Fe/(NiZn)Fe2O4 soft magnetic composite powders prepared using a co-precipitation method

Abstract Fe powder was coated with precipitate via a co-precipitation method using chlorate as the raw material. Then, the coated powder was heat treated under different conditions. The effects of the heat treatment atmosphere and temperature on the morphology, phase formation, crystallinity, grain size and lattice constant of the coated layer of powders were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermogravimetric–differential scanning calorimetry (TG–DSC). The magnetic properties of the composite powder were determined using a Quantum Design vibrating sample magnetometer (QD-VSM). The results show that a fine, uniform coating layer of particles formed on the surface of the Fe powder after co-precipitation. Through heat treatment, the coating generated spinel Ni–Zn ferrite. The coated layer of argon-treated powders had a more uniform and regular morphology, purer phase and better crystallinity than air-treated powders. The saturation magnetisation first increased and subsequently decreased as the heating temperature increased. Similarly, the coercivity first decreased and subsequently increased. The sample annealed in air at 300 °C had the highest saturation magnetisation ( M s  = 186.3 emu/g) and the lowest coercivity ( H c  = 23.6 Oe), whereas the sample annealed in argon at 450 °C had the greatest saturation magnetisation ( M s  = 196.4 emu/g) and relatively lower coercivity ( H c  = 17.5 Oe). The powders annealed in argon atmosphere had better M s and lower H c than those annealed in air, mainly because coated powders heated in air produce a large amount of non-magnetic Fe 2 O 3 .