Medium noise of perpendicular double layer media is investigated in conjunction with transition irregularity. The noise is typically proportional to the square root of linear density, but it saturates at a particular density. This nonlinear noise behavior is explained by clusters consisting of several grains becoming minimum magnetic units. A simulation with a media model using Voronoi cells is carried out based on this resu it also gives good agreement with experimental results. Measured partial erasure quantitatively corresponds to the noise saturation. Clustering and partial erasure both suggest transition percolation; however, no supra-linear noise increase is usually observed. Noise characteristics in terms of percolation density suggest grain-clustering in perpendicular media.
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