Abstract: The nanocrystalline Ni_0.8Zn_0.2Fe₂O₄ was synthesized by a sol-gel auto-combustion method using citric acid as the chelating agents. The analysis methods of FT-IR, XRD, DSC-TG,
TEM, wave-guide were used to character the gel and the products. The phenomena of melt, combustion and forming crystal for gel were investigated in the processes of thermo-decomposition.
The results show that metal ions and carboxyl are coordinated by the oriented manner. The temperature of decomposition for the gel is 220℃,
and the nanocrystalline is formed after the combustion of the gel. The mixture, which consists of Ni_0.8Zn_0.2Fe₂O₄ nanocrystalline and
paraffin, has both dielectric loss and magnetic loss in the frequency range of 8～12GHz. The reflectivity of spinel nanocrystalline increases and
the reflectivity absorption peaks shift from high frequency to low frequency with the increase of thickness.

Key words: sol-gel auto-combustion, nanocrystalline, Ni_0.8Zn_0.2Fe₂O₄, microwave absorption