Ultra clean sputtering process (UC-process) was introduced in the fabrication of Co85.5Cr10.5Ta4/Cr thin film media. Magnetic properties, microstructure and their effect on media noise are discussed against the thickness of Cr underlayer. By applying the UC-process, coercive force Hc shows high values of about 2.3 kOe isotropically until 10 nm in Cr thickness. High Hc of about 1.5 kOe remains at even 2.5 nm. UC-process is found to enable the enhancement of the formation of Cr segregated grain boundary structure, which reduces intergranular exchange coupling even in the media with 2.5 nm in Cr thickness. The ratio of readback signal to media noise S/Nm gradually increases with decreasing Cr thickness, and shows the highest value at 2.5∼5 nm. In the media with sufficiently separated grains by segregated grain boundaries, the reduction of the grain size with decreasing Cr thickness is found to be most effective for the improvement of S/Nm.
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