Magnetic and microwave-absorbing properties of SrAl4Fe8O19 powders synthesized by coprecipitation and citric- combustion methods

Al-substituted M-type hexaferrite is a highly anisotropic ferromagnetic material. In the present study, the coprecipitation and the citric-combustion methods of synthesis for SrAl4Fe8O19 powders were explored and their microstructure, magnetic properties, and microwave absorptivity examined. X-ray diffraction (XRD), scanning electron microscopy (SEM), a vibrating sample magnetometer, and a vector network analyser were used to characterize the powders. The XRD analyses indicated that the pure SrAl4Fe8O19 powder was synthesized at 900°C and 1000°C for 3 h by coprecipitation, but only at 1000°C for the citric-combustion processes. The SEM analysis revealed that the coprecipitation process yielded a powder with a smaller particle size, near single-domain structure, uniform grain morphology, and smaller shape anisotropy than the citric-combustion process. The synthesis technique also significantly affected the magnetic properties and microwave-absorptivity. Conversely, calcining temperature and calcining time had less of an effect. The grain size was found to be a key factor affecting the property of the powder. The powders synthesized by coprecipitation method at calcining temperature of 900°C exhibited the largest magnetization, largest coercivity, and best microwave absorptivity.