Control of exchange coupling for overwritable magneto-optical media by sputter etching

It has been found that the interface wall energy density σw for exchange-coupled trilayer (ECTL) media, which is a key characteristic in obtaining overwritable magneto-optical media, is controlled by sputter etching the SiN underlayer. The sputter-etching effect of the SiN underlayer on film surface morphology, magnetic anisotropy, σw, and overwrite characteristics in ECTL media has been investigated. The media consist of a polycarbonate substrate, a SiN underlayer, TbFe memory layer, a NdFeCo exchange coupling control layer, a GdTbFeCo writing layer, and a SiN protective layer. By measuring saturation magnetization and switching field for the writing layer, σw between the memory layer and the writing layer was determined, according to the exchange-coupled double-layer theory. The effective perpendicular anisotropy of the NdFeCo layer decreased with sputter etching of the SiN layer, whereas the interface wall energy density changed from 0.6 to 1.7 erg/cm2 as 50 Å of SiN was sputter etched away. These effects are believed to be related to morphological changes observed in the SiN. For mark lengths greater than 2.3 μm, the primary effect of sputter etching the SiN underlayer was to increase the noise by 10 dB with respect to the non-sputter-etched case, whereas the carrier was the same.