Graphitic carbon-coated Ni metal core nanoparticles are synthesized using controlled thermal treatment of Ni-hydroxide in an aniline-formaldehyde copolymer matrix under a N2 atmosphere. The structural and microstructural studies substantiate the formation of a graphitic shell on the metallic nickel core. The 850 °C heat-treated core-shell structured material is ferromagnetic in nature with high saturation magnetization. The sample with the highest value of magnetic moment is impregnated in a rubber matrix to prepare composite samples with core-shell nanomaterials of different weight fractions. The sample with 70 wt% Ni/graphitic carbon core-shell powder in the elastomeric rubber matrix showed the optimal microwave absorption at a fairly low thickness of ∼1 mm. This is attributed to the optimized combination of the effective impedance matching and interfacial polarization losses. Further increase in the Ni/graphitic carbon filler material resulted in reduced microwave absorption due to the greater mismatch of impedance as compared to the desired free space impedance of ∼377 Ω. Thus, the optimized composite material is of great potential for microwave absorption in the Ku band. © 2018 The Royal Society of Chemistry.