Electromagnetic wave absorbing properties of Ni-Zn ferrite powder–epoxy composites in GHz range

•The EM wave absorbing properties of Ni-Zn ferrite–epoxy composites in the GHz range were studied.•The EM absorbing properties can be modulated by the grain size and the composition of the ferrites.•Ni-Zn ferrite–resin composites are promising GHz range EM wave absorber. Spinel ferrites NixZn1-xFe2O4 (x = 0.4, 0.5, 0.6) with grain sizes ranging from tens of nanometers to several hundred nanometers were prepared using the sol–gel and post-annealing processes. X-ray diffraction (XRD) analysis confirmed that a crystalline single spinel phase could be formed after annealing the sol–gel processed powders at ≥400 °C. Reflection loss (RL) maps as functions of frequency (100 MHz ≤ f ≤ 15 GHz) and sample thickness were plotted for the Ni-Zn ferrite–epoxy (10 wt%) composites. The RL patterns were systematically modulated by the Ni-Zn ferrite grain size and Ni composition x. The electromagnetic wave absorbing area of RL