The ultra-clean sputtering process (UC-process), which has been developed by the present authors, is an indispensable infrastructure for the precise control of the microstructure of metallic thin films. Developing new thin film fabrication techniques under the UC-process, the authors succeeded to control the grain size of the magnetic thin films for ultra-high-density magnetic storage devices. In the present paper, we demonstrate the enlarged in-plane grain diameter in Co/Cu mutilayers and the sharpened grain size distribution in magnetic recording media, by using novel buffer layers and ultra-thin seed layers, respectively. The guiding principles for the material design of such buffer and seed layers are also presented.