Magnetic metals are vital microwave-absorbing materials in alleviating electromagnetic (EM) pollution; however, in practical applications they are limited by their high density and relatively narrow absorption band. An effective strategy for addressing this issue is compounding magnetic metals with carbon materials. In this work, we fabricated the carbon/nitrogen-doped graphite (Co/NC) composites using a simple and low-cost method by directly calcining a dry mixture of melamine and Co powders in Ar gas. Part of Co/NC composite was calcined at 800 °C (Co/NC−800). The effects of variations in phase composition and morphology on the EM parameters and microwave-absorbing performance were investigated. Compared to pure Co, Co/NC−800 composite exhibits excellent microwave-absorbing performance such as strong absorption, better EM wave attenuation, and a lighter weight. Particularly, Co/NC−800 composite achieved the minimum reflection loss (RL) of −57.76 dB at 4.88 GHz and an effective absorption bandwidth (EABW, RL ≤ −10 dB) of 14.26 GHz (3.74–18.00 GHz) at a coating thickness d of 1.00–5.00 mm. Further, the maximum EABW of 5.50 GHz was attained at d = 1.86 mm. The optimal impedance matching, multiple scattering and reflections of EM waves on rough surfaces, and strong dielectric loss including conduction loss, interfacial and dipolar polarizations were noted as the main factors for the enhanced EM energy absorbance of Co/NC−800 composite. We believe our work may serve as a guideline for the fabrication of other light carbon-based magnetic metal composites as EM wave absorbers.
