Observation of underlayer domain noise in perpendicular recording disks

The authors employed a disk test system to extract samples of media noise from dual layer perpendicular recording disks. The samples of the media noise show it to have a distinct high and low frequency structure. The high frequency structure is the noise arising from the micromagnetics of the CoCr recording layer. The low frequency structure does not arise from the recording layer but from the NiFe underlayer. Using an imaging technique, the underlayer noise is characterized by lines that span inwards from the outside diameter of the disk. The appearance and behavior of these lines shows them to be attributable to domain walls in the underlayer of the disk