AbstractThe grain size reduction mechanism of CoCrPt‐SiO2 perpendicular magnetic recording media by oxygen reactive sputtering of RuCr intermediate layer has been investigated as a function of oxygen content in the Ar sputtering gas. The media stack was Ta/Ru/RuCr/CoCrPt‐SiO2. It was found that, by the oxygen addition, Cr and Ru oxides were formed at grain boundaries of the RuCr layer and this was the reason of the intermediate layer grain size reduction, which induced the reduction of the recording layer grain size. C‐axis alignment of RuCr (0002) plane, however, was degraded as the oxygen gas ratio increased. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)