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The process of formation of magnetic clusters in perpendicular recording media was investigated through micromagnetic simulation and compared with data for longitudinal recording media. The size of magnetic clusters, which is obtained from the autocorrelation length of the demagnetized state by a dc reverse field, was almost the same as that for written states. It increased with increasing intergrain exchange coupling and was independent of the process of cluster formation. Also, the thermal switching unit size (activation volume) was found to be smaller than the magnetic cluster size. It remained almost the same as the grain size until the exchange coupling reached a critical value, and then increased further. The transition noise, the main part of the medium noise, increased with increasing magnetic cluster size.