Extremely flat surface full granular media with cap layer consisting of high-Ku CoPt grains and ferromagnetic oxide grain boundaries

Investigation of magnetic properties and nanostructure of stacked media with a CoPt-B2O3 granular recording layer (GRL) and granular cap layers (CL) with CoPt and a typical ferromagnetic oxide of rare earth (RE) oxide (RE oxide: Gd2O3, Nd2O3, Sm2O3, and CeO2) are reported. CoPt-RE oxide granular layer shows higher saturation magnetization and magnetic anisotropy than that of CoPtCrB continuous layer. In stacked media with CoPt-Gd2O3 granular CL (9 nm), nucleation field increases from 0.7 to 0.8 kOe and saturation field decreases from 21.5 to 12.5 kOe with comparable values to those of CoPtCrB continuous CL. This indicates that CoPt-RE oxide granular CL is effective to control intergranular exchange coupling. Reduction of coercivity distribution ratio (∆Hc/Hc) from 0.15 to 0.09 which is comparable to that of CoPtCrB continuous CL when CoPt-Gd2O3 granular CL thickness is changed from 4 to 9 nm is observed. This means CoPt-Gd2O3 granular CL is effective to reduce switching field distribution. Furthermore, significant reduction of surface roughness (Ra) for media with CoPt-Gd2O3 granular CL (9 nm) from 0.32 to 0.25 nm is observed. This reveals that CoPt-RE oxide granular CL has a great impact to induce stacked media with flat surface. Based on the cross-section observation by TEM, the media with CoPt-Gd2O3 granular CL shows continuous structure of grain boundaries from the GRL up to the CL and which results in one-on-one growth of magnetic grain with homogeneous columnar structure. Focusing on the surface morphology, media with CoPt-Gd2O3 granular CL shows smoother surface than that of media with CoPtCrB continuous CL which supports the Ra evaluation result. These results indicate that the employment of granular CoPt-RE oxide for CL is not only to reduce switching field distribution but also to flatten surface of the stacked media.