In this study, we used detonation to synthesise carbon-encapsulated Fe-Ni alloy nanoparticles using safety-composite explosive precursors doped with Fe (NO3)(3)center dot 9H(2)O and Ni(NO3)(2)center dot 6H(2)O. We characterised the morphology, components and structure of the synthesised carbon-encapsulated alloy nanoparticles by X-ray diffraction, Raman spectroscopy and a transmission electron microscope equipped with an energy dispersive X-ray spectroscope (EDS). The results revealed the carbon-encapsulated Fe-Ni nanoparticles to have a core-shell structure. The grains ranged in size from 40 nm to 60 nm and were uniformly distributed. We determined the encapsulated metal core to be mainly composed of different proportions of Fe and Ni and the outer shell to be composed of graphite and amorphous carbon. In addition, we observed onion carbon formed by the graphitisation of diamond clusters in the vicinity of the nanoparticles. We determined the electromagnetic characteristics of the Fe-Ni alloy nanoparticle composites using an Agilent microwave network analyser in a band range of 2-18 GHz. The experimental results for a coating thickness of 2 mm are as follows: In nanoparticles in which the atomic ratio of iron and nickel is 1: 4, the reflection loss R(dB) of the absorption layer has a double absorption peak, with peak values of -14.6 dB (9.7 GHz) and - 7.7 dB (14.3 GHz) and the absorption band of -10 dB ranges from 8.5 GHz to 11.8 GHz. In nanoparticles in which the atomic ratio of iron and nickel is 1: 1, the reflection loss R(dB) reaches 30 dB at 12.88 GHz and the absorption band of -10 dB ranges from 9.7 to 14.4 GHz, which indicates a wide absorption band and an excellent absorbing property.