An ultrafine barium ferrite powder of high coercivity from water-in-oil microemulsion

An ultrafine barium ferrite powder of high coercivity and high saturation magnetization has been successfully prepared from an inverse water-in-oil microemulsion consisting of cyclohexane, NP5/NP9, and an aqueous solution of mixed ferric nitrate and barium nitrate. An ammonium hydroxide solution was used as the precipitant to obtain the hydroxide precursor of particle size in the range of 10-30 nm from the microemulsion. The resulting precursor is characterized using techniques such as thermogravimetric analysis (TGA), differential thermal analysis (DTA) and a FT-IR analyzer. It was then calcined at various temperatures ranging from 600 to 950 degrees C in order to develop the designed barium ferrite phase. Phase development in the precursor at each calcination temperature was monitored using X-ray diffractometer and a Fe-57-Mossbauer spectrometer analyzer. The particle size and size distribution of the calcined powders were determined using light scattering technique, scanning electron microscope and a transmission electron microscope. A high purity barium ferrite was obtained when the precursor was calcined at 950 degrees C for 4 h. It exhibits an intrinsic coercivity (H-i(c)) of 5639 Oe and a saturation magnetization (M-s) of 69.7 emu/g, when characterized using a vibrating sample magnetometer (VSM). (C) 1998 Elsevier Science B.V. All rights reserved.