Control of exchange coupling for overwritable MO media by sputtering conditions (abstract)

Abstract

The sputtering condition effect on interface wall energy σw in overwritable MO media has been investigated. The media consist of a polycarbonate substrate, a SiN underlayer, a TbFe or TbFeCo memory layer, a NdFeCo exchange coupling control layer, a GdTbFeCo complementary layer, and a SiN protective layer. Underlayer SiN sputter etching effect on σw and Ar gas pressure effect on σw for memory layer deposition were examined. The sputter etching rate was fixed to 13 Å/min. By measuring saturation magnetization and switching field for the complementary layer in the M-H loop, σw between the memory layer and the complementary layer was calculated according to the exchange-coupled double-layer theory.1 With increasing sputtering etching time, σw changes from 0.6 to 1.7 erg/cm2 at about 5-min sputter etching. Correspondingly, not only does memory layer coercive force Hc change from 6.5 to 1.8 kOe, but also coercive squareness S* is improved. FE-SEM observation reveals that the structureless uniform memory layer is formed when the SiN underlayer surface is treated by sputter etching. With decreasing Ar gas pressure, S* is improved and film becomes structureless. Based on these facts, the authors conclude that sputtering condition control to form magnetic films without fine microstructures is effective for reducing interface wall energy σw. A 45-dB overwrite C/N ratio has been obtained at 11.3 m/s linear velocity and 7.4-MHz carrier frequency by controlling these sputtering conditions, as well as by adjusting film compositions and thickness.