To enhance the microwave absorption of Fe₃O₄ nanocrystals, ZnO nanoshells with a thickness of 2 nm were grown on Fe₃O₄ nanocrystals by heterogeneous nucleation. After being coated with ZnO nanoshells, the material possesses a far more improved ability for microwave absorption. A minimum reflection loss of −3.31 dB for Fe₃O₄ nanocrystals alone was improved to a minimum reflection loss of −22.69 dB and with an effective absorption band (R_L < −10 dB) covering a frequency range of 10.08–15.97 GHz. The reasons for the enhanced microwave absorption were studied by the use of X-ray absorption near-edge structures at O K-edge, electron spin resonance analysis and microwave electromagnetic parameters mapping. The results indicate that the decoration of the dielectric ZnO shell had varied the dielectric property as well as the oxidization environment and distribution of Fe ions on the surface of the Fe₃O₄. This well balances the permeability and the permittivity of the nanomaterials and decreases the difficulty of impedance matching the microwave absorber within the free space. This leads to the Fe₃O₄@ZnO nanohybrids possessing vastly improved microwave absorption as compared to Fe₃O₄ nanocrystals alone.

In this paper, both the leaf-like dendritic Co_xFe_1−x (x = 0.1, 0.3, 0.5 and 0.7) alloys and novel grain-like dendritic nanostructure Co_0.9Fe_0.1 alloy and ε-Co were synthesized by an electric field-induced and electrochemical reduction method under over-polarization current. The formation of these two dendritic structures is not only attributed to the electric-field strength and ion concentration gradient, but also related to their crystalline structure. The trunks and branches of leaf-like dendritic bcc Co_xFe_1−x grow along three of the six equivalent directions. However, for grain-like dendritic hcp ε-Co, their six branches grow along the six equivalent <101> directions, and the trunks are oriented preferably along [0002] direction. The electromagnetic absorption (EMA) performance of leaf-like structure Co_xFe_1−x alloys are obviously enhanced with Co content increase from −21.7 dB for Co_0.1Fe_0.9 to −59.1 dB for Co_0.5Fe_0.5. Meanwhile, the response frequency of maximum reflection loss (RL) shifts to the higher band. In the case of grain-like dendritic Co_0.9Fe_0.1 alloy, it has the widest frequency band from 5 to 18 GHz with RL < −20 dB due to the mixed phase composition. We ascribe the enhancement of dendritic Co_xFe_1−x alloys' EMA performance to the different elemental compositions, crystalline structures, surface anisotropy and crystal defects, which contribute to enhancing the effective complementarities between dielectric loss and magnetic loss.