Reduction of intergranular exchange coupling for CoPt-B₂O₃ granular media by employing a RuCoCr-oxide buffer layer with oxide of various melting points

CoPt-oxide granular films have been widely used for perpendicular magnetic recording media. To further increase recording density of the media, enhancement of magnetocrystalline anisotropy $(K_{u})$ to overcome the thermal stability issue and reduction of media noise to increase signal-to-noise-ratio are essential. The requirements can be realized through the promotion of columnar growth of magnetic grains which are phase separated with oxide boundary material and intergranular exchange decoupling ¹), respectively. Generally, for the grain boundary materials of the granular media, oxides with amorphous phase which do not dissolve into CoPt metal have been utilized. The oxide is expected to segregate into the trench of Ru underlayer so that CoPt grains can grow heteroepitaxially on the bump of Ru underlayer. Many authors have proposed various kinds of oxide materials to be segregated into the grain boundaries, like SiO₂²), TiO₂³), Ta₂ $\mathrm {O}_{5}^{3)}$, Nb₂ $\mathrm {O}_{5}^{4)}$, WO₃⁴), MgO ⁵), Cr₂ $\mathrm {O}_{3}^{5)}$, and $\mathrm {Y}_{2} \mathrm {O}_{3}^{5)}$. Previously, we had found that a granular medium with high magnetocrystalline anisotropy $(K_{u})$ was successfully realized when B₂O₃ was applied for the grain boundary material ⁶⁾. However, the separation between magnetic grains at the initial growth region in the Ru trench by low melting point oxide such as B₂O₃ is not sufficient ⁷). Therefore, we have carried out an investigation on the deposition of a buffer layer (BL) with non-ferromagnetic metal and oxides of various melting points $(T_{m})$ on the Ru underlayer to grow the non-ferromagnetic metal and oxide on the Ru bump and trench, respectively, aiming to intergranular exchange decouple the magnetic grains.