Magnetic moment of soft underlayers for perpendicular media

Summary form only given. The micromagnetic method is utilized to explicitly verify the notion that it is not possible to saturate a soft magnetic underlayer (SUL) when the pole tip and the SUL have the same saturation magnetization. Simple magnetostatic arguments reveal that, to lowest order, saturation of the SUL would require that the moment of the pole tip (M/sub s,tip/) be twice the moment of the SUL (M/sub s,SUL/). The magnetic field under the writer pole will certainly not exceed 2/spl pi/M/sub s,tip/. In response to the write field, a demagnetization field (4/spl pi/M/sub z,tip/, z along the perpendicular direction) develops in the SUL such that the torques generated by the write field and the demagnetization fields are equal and opposite. Hence, saturation of the SUL can be safely excluded unless M/sub s,tip/spl sim/2/ M/sub s,SUL/. This fundamental observation relaxes restrictions on the choice for materials for a low SUL.