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Cobalt/polypyrrole(Co/PPy)nanocomposites were prepared via an in situ oxidation polymerization of pyrrole in an aqueous dispersion of Co nanoparticles(NPs).The Co/PPy nanocomposites showed good electromagnetic properties because of the coexistence of magnetic loss and dielectric loss to electromagnetic waves.The electromagnetic wave absorbing bandwidth(reflection loss ＜-10 dB)for Co/PPy(30 wt％in a paraffin matrix)was located at 11.7-16.47 GHz with a thickness of 2 mm,and with a maximum reflection loss(around-33 dB)at 13.6 GHz.More interestingly,the electromagnetic wave absorbing properties of the nanocomposites can be easily controlled by tuning the ratio of the two components in the composites.This improved electromagnetic wave absorption may be attributed to the excellent electromagnetic match at the corresponding resonance peaks for dielectric and magnetic loss.These magnetic nanoparticles/conducting polymer nanocomposites are great potential candidates for use as electromagnetic wave absorbents due to their excellent properties such as wide absorbing frequency,strong absorption,good compatibility,low density and controllable absorbing properties.