▎ 摘　　要

High entropy spinel ferrite (HEF) nanoparticles were synthesized by solution combustion method as high-performance microwave absorbers. The structural, microstructural, magnetic and microwave absorption properties of (MnNiCuZn)(1-x)CoxFe2O4 powders (x 1/4 0.05, 0.1, 0.2, and 0.3) were studied as a function of Co contents by modern characterization methods including X-ray diffractometry, Raman spectroscopy, electron microscopy, vibrating sample magnetometry, and vector network analyzer. The as-combusted powders were single phase with the space group of Fd (3) over barm. The cation distributions showed that the fraction of Fe-3 thorn cations in (A) sites decreased with the substitution of Co cations. The (MnNiCuZn)(0.7)Co0.3Fe2O4 nanoparticles (similar to 30-70 nm) were dispersed on the graphene sheets. The coercivity increased from 94 to 225 Oe with the Co contents, while the saturation magnetization slightly changed in the range of 67-71 emu/g. The saturation magnetization and coercivity of the (MnNiCuZn)(0.7)Co0.3Fe2O4/graphene composite powders were lower than those of (MnNiCuZn)(0.7)Co0.3Fe2O4 powders due to their smaller particle size. The (MnNiCuZn)(0.7)Co0.3Fe2O4/paraffin composite sample at the mass fraction of 70 wt.% showed the maximum reflection loss of -27 dB at the matching thickness of 5.3 mm in Ku band. The interfacial relaxation and ferromagnetic resonance were determined as main dielectric and magnetic loss mechanisms, respectively. By compositing the (MnNiCuZn)(0.7)Co0.3Fe2O4 powders with graphene, the maximum reflection loss of -16 dB at the matching thickness of 4.5 mm in Ku band was obtained at the lower mass fraction of 30 wt.% because of the higher dielectric loss of graphene. (C) 2021 The Author(s). Published by Elsevier B.V.