Barium ferrite has wide applications in electronic devices, and its preparation process optimization plays an important role in the improvement of microwave electromagnetic and absorption properties. In this paper, M-type barium ferrite was synthesized by the citrate sol–gel method. During sintering, the precursor gel first experienced free water evaporation and impurity volatilization, and then went through two endothermic reactions, eventually generating M-type barium ferrite. As the sintering temperature increased from 800 to 1400 °C, the products transformed from the particles exhibited a long rod shape without an obvious M-type barium ferrite phase to the hexagonal block particles mainly composed of M-type barium ferrite. Meanwhile, with increasing temperature, the particles and grain size became larger, the size uniformity was first strengthened and then weakened, the magnetic properties were first enhanced and then weakened, and the dielectric loss capacity continued to weaken, which were related to the changes of structure and microstructure, e.g., densification, magnetic domain state, and specific surface area. The products were equipped with the main loss mechanisms of dipole turning polarization, space charge polarization, interfacial polarization, natural resonance, eddy current loss, and multiple reflections/scattering. Besides they possessed great impedance performance and obvious interference cancelation, further improving the absorption ability. After optimization, the barium ferrite sintered at 1200 °C exhibited a maximum microwave absorption bandwidth of 6.16 GHz with a matching thickness of 2.34 mm, and it achieved the minimum reflection loss of − 38.97 dB at 39.93 GHz when the matching thickness is 3.10 mm. It is indicated that the nanoparticles sintered at the appropriate temperature are the potential candidates as wide-band electromagnetic wave absorbers in the Ka-band.
