[en] To realize CoB₁₅/Pd multilayer perpendicular media with weak intergranular exchange coupling, we utilize PdSi₁₈ (at. %) alloy, which contains elements that have large difference of affinity for oxygen, as the underlayer. It is clarified that introducing oxygen into Ar atmosphere during deposition of the underlayer substantially reduces the intergranular exchange coupling of the multilayer. Microstructural analysis reveals that the introduction of oxygen has separated the PdSi₁₈ underlayer into two phases and enhances the formation of grain boundaries of the multilayer. It is suggested that the two-phase-separated Pd-SiO underlayer becomes a template for the growth of the multilayer and promotes the formation of grain boundaries in the multilayer, which reduces the exchange coupling through isolation of grains of the multilayer. In addition, independent of the oxygen pressure, coercivity and anisotropy energy maintain values of around 3.5 kOe and 2.3x10⁶ erg/cm³, respectively. It is concluded that the two-phase-separated Pd-SiO underlayer, fabricated by introducing oxygen during sputtering of PdSi₁₈ underlayer, is effective for reducing the magnetic exchange coupling without degrading the magnetic properties