SiO(2) was doped into Ru underlayer to reduce the grain size of CoPt-SiO(2) perpendicular media. The effects of SiO(2) volume fraction and sputtering deposition pressure of Ru-SiO(2) underlayer on the microstructure and magnetic properties of CoPt-SiO(2) media were studied. Increasing SiO(2) volume fraction in Ru-SiO(2) layer decreased the grain sizes of Ru and CoPt. Adding 5% SiO(2) to Ru-SiO(2) layer increased the coercivity and enhanced the exchange decoupling and thermal stability of CoPt-SiO(2) layer. A further increase in SiO(2) volume fraction caused the deterioration of magnetic properties of CoPt-SiO(2) layer. Deposition of Ru-SiO(2) layer at 1.3 Pa resulted in a smaller activation volume and higher thermal stability of the CoPt-SiO(2) media than that deposited at 0.4 Pa. (C) 2010 Elsevier B.V. All rights reserved.