Activation volumes in CoPtCr-SiO/sub 2/ perpendicular recording media

The activation volume (V/sub act/) is the minimum unit of magnetization reversal and the value of the associated diameter (D/sub act/) indicates the potential recording density of media. In this study, we measured D/sub act/ for CoPtCr-SiO/sub 2/ media by two methods: the Sharrock's equation, and the second is based on the measurement of the fluctuation field from time dependence data determined using a magneto-optical Kerr effect (MOKE). We discuss the values of D/sub act/ obtained from the two measurements for CoPtCr-SiO/sub 2/ media with and without soft under layers (SUL).