B₂O₃: Grain boundary material for high-K_u CoPt-oxide granular media with low degree of intergranular exchange coupling

The magnetic properties and structures of Co₈₀Pt₂₀-30 vol% oxide (ZrO₂, Cr₂O₃, Y₂O₃, Al₂O₃, MnO, TiO₂, WO₂, SiO₂, Mn₃O₄, WO₃, Co₃O₄, MoO₃, and B₂O₃) granular media deposited at room temperature were investigated. As a result, the following were found. 1) By employing oxides with low melting point temperatures as the granular media, magnetic grains with high saturation magnetization (M_s^grain) and perpendicular magnetic anisotropy (K_u^grain) are obtained; the increases in M_s^grain and K_u^grain are due to the promotion of the columnar growth of magnetic grains and phase separation or two-phase precipitation between magnetic grains and oxides. 2) The increase in the K_u^grain of the granular media followed by the decrease in the melting point of oxides is due to the reduction in the amount of stacking faults. 3) Among these granular media, the CoPt-B₂O₃ granular medium has the highest coercivity (H_c) and ratio of H_c to the magnetic anisotropy field (H_c=H_k^grain) of 8.0 kOe and 0.4, with M_s^grain and K_u^grain of 1115 emu/cm³ and 1.1 × 10⁷ erg/cm³, respectively. 4) The CoPt-B₂O₃ granular medium has well-isolated and columnar growth magnetic grains with an average grain size of 6.5 nm.