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Nanocomposite CoχPt100100-χ :C thin films with χ = 57 and χ = 50 were prepared by magnetron sputtering. Structures and activation volumes were studied systematically in order to better understand the magnetic behavior.We found that activation volumes of these two samples were almost the same. However, physical grain sizes of these two samples were different. Magnetic grains in the sample with equiatomic Co and Pt composition were almost independently switched and magnetic grains in the sample with higher Co content were exchange coupled and switched together. The reduction of the physical grain size in the sample does not always cause the reduction of the exchange coupling in the sample. Our results indicate that the Co concentration is an additional factor to be used for controlling the properties of nanocomposite CoPt:C films.