Co–Ni alloy nanoparticles, a potential candidate for microwave absorption material, were successfully synthesized by tuning the reduction timing of Co and Ni ions by introducing oleylamine as a complexing agent and 1-heptanol as a reducing solvent. The formation mechanism elucidated using time-resolved sampling and in situ X-ray absorption spectroscopy (XAS) and ultraviolet-visible (UV-vis) spectrophotometry measurements suggested that the delay in the reduction of Co ions via complexation with oleylamine facilitated the co-reduction of Co with Ni ions and led to the formation of Co–Ni alloys. The successful synthesis of Co–Ni alloys experimentally confirmed the differences in magnetic properties between alloy and core–shell structured Co50Ni50 particles. Further, the syntheses of Co–Ni alloys with different compositions were also possible using the above technique. In addition, the microwave absorption properties were measured using the free-space method utilizing a vector network analyzer of Co50Ni50polyethylene composite with different sheet thicknesses. A reflection loss (RL) value of −25.7 dB at 13.6 GHz for the alloy structure was more significant than the core–shell counterpart. The above values are high compared to results reported in the past. The validity of the measurements was confirmed by utilizing the parameter retrieval method to extract permittivity and permeability from the scattering parameter (S) and recalculation of the RL as a function of frequency.

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