Fabrication and microwave absorbing properties of Ni–Zn spinel ferrites

Ni–Zn spinel ferrite and Cu-doped, Co-doped Ni–Zn spinel ferrites were prepared by a conventional ceramic processing method. Microwave absorption, complex permittivity and permeability of the (Ni0.5Zn0.5)Fe2O4, (Ni0.4Cu0.2Zn0.4)Fe2O4 and (Ni0.4Co0.2Zn0.4)Fe2O4 spinel ferrites within the frequency range of 0.5–18 GHz were investigated. The reflection loss calculation results show that the prepared Ni–Zn spinel ferrite and Cu-doped, Co-doped Ni–Zn spinel ferrites are good electromagnetic wave absorbers in the microwave range. The single layer (Ni0.4Co0.2Zn0.4)Fe2O4 spinel ferrite absorber with a thickness of 3 mm achieved a reflection loss below −10 dB (90% absorption) at 3.9–11.5 GHz, and the minimum value was −17.01 dB at 6.1 GHz. Compared with the performances of the single layer spinel ferrite absorbers, not only the matching frequencies of the double layer spinel ferrite absorbers were shifted to higher values, but also the microwave absorbing frequency bands were expanded. When the first layer and second layer are (Ni0.5Zn0.5)Fe2O4 and (Ni0.4Co0.2Zn0.4)Fe2O4 spinel ferrites, respectively, the laminated double layer spinel ferrite absorber with a thickness of 3 mm achieved a reflection loss below −10 dB at 3.3–12.7 GHz, and the minimum value was −49.1 dB at 11.8 GHz.