Time dependence in perpendicular media with a soft underlayer

In this paper we describe measurements of magnetic viscosity or time dependence in magnetic thin films suitable for use as perpendicular recording media. Generally, such effects cannot be measured using conventional magnetometry techniques due to the presence of a thin (0.1 /spl mu/m) soft underlayer (SUL) in the media necessary to focus the head field. To achieve our results we have developed an ultrastable MOKE magnetometer, the construction of which is described. This has enabled us to measure nominally identical films with and without the presence of the SUL. We find that the presence of the SUL narrows the energy barrier distribution in the perpendicular film increasing the nucleation field (H/sub n/), reducing the coercivity (H/sub c/) and results in an increase in the squareness of the loop. This in turn results in an increase in the magnitude of the viscosity in the region of the H/sub c/ but that the range of fields over which the viscosity occurs is reduced.