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Investigation of magnetic properties and nanostructure for full granular media with CoPt−Gd2O3 granular cap layers (CLs) of various Gd2O3 content were conducted. Media with low surface roughness of 0.19 nm is maintained even though Gd2O3 content is reduced from 30 to 5 vol%, which is lower than that of media with CoPtCrB continuous CL. The result is supported by surface morphology observed by transmission electron microscopy. According to cross section element analysis, homogeneous nanostructure in the thickness direction with one- text on-one columnar growth of magnetic grain is observed. Reduction of Gd2O3 content increases indirect intergranular exchange coupling through CL which results in media with higher nucleation field and lower saturation field than that of media with CoPtCrB continuous CL. Furthermore, at a CL thickness of 6 nm, when Gd2O3 content is reduced from 30 to 15 vol%, switching field distribution is reduced from 0.11 to 0.05. These results suggest that the reduction of Gd2O3 content of the CoPt−Gd2O3 granular CL is effective to reduce the switching field distribution of the full granular media to be lower than that of media with CoPtCrB continuous CL while retaining the extremely smooth media surface.

Perpendicular 90 kOe torquemetry analysis of magnetic anisotropy ( $K_{\mathrm {u\bot }}^{\mathrm {exp}}$ ) for L10-typed FePt granular media with various grain boundary materials (GBMs) was conducted. According to the analysis for granular films with a constant GBM volume of 30 vol% deposited at a high substrate temperature, $K_{\mathrm {u\bot }}^{\mathrm {exp}}$ was not dependent on the saturated torque coefficient of fourfold components ( $L_{4\theta }^{\mathrm {sat}}$ ) but was dependent on that of twofold components ( $L_{2\theta }^{\mathrm {sat}}$ ) originating from magnetic grains with $c$ -axes normal to the film plane. Furthermore, $L_{4\theta }^{\mathrm {sat}}$ mainly originated from FePt grains with $c$ -axes parallel to the film plane. $K_{\mathrm {u\bot }}^{\mathrm {exp}}$ also had a strong correlation with grain diameter ( $GD$ ) and degree of order ( $S_{\mathrm {in}}$ ). On the other hand, $K_{\mathrm {u\bot }}^{\mathrm {exp}}$ had no significant correlation with the melting point ( $T_{m}$ ) of the GBM, which indicates that the dominant factor affecting $K_{\mathrm {u\bot }}^{\mathrm {exp}}$ was not phase separation. Based on these evaluations, an increase of $GD$ caused an increase of $S_{\mathrm {in}}$ , regardless of the GBMs, which resulted in enhancement of $K_{\mathrm {u\bot }}^{\mathrm {exp}}$ for the FePt granular films.

Magnetic properties and nanostructure for FePt-30vol%TiN deposited while flowing N2 gas are investigated. When N2 gas addition of 3%–5% are introduced, compared with that of without N2 gas, reduction in the lattice constant ratio and the unit cell volume, the increase in the degree of order of the granular film are observed. These indicate the segregation of Ti into grain boundaries as the result of the N2 gas addition through nitridization of Ti dissolved in the FePt grains which lead to the enhancement of saturation magnetization, perpendicular magnetic anisotropy, and magnetic anisotropy field of the granular film. Therefore, magnetic decoupling in the FePt-TiN similar to the FePt-BN granular films is promoted by the segregation of nitrides formed by nitridization of metal element solidly soluble in FePt magnetic grains due to nitrogen deficiency.

Magnetic properties and nanostructure for FePt-30vol%C (2 nm) / FePt-30vol% GBM (0–8 nm) stacked media was investigated. Following effects of the employment of FePt-C nucleation layer (NL) were confirmed: 1) Msavevaries linearly against melting point (T m) with value (500–600 emu/cm3) smaller than that of media without NL (500–750 emu/cm3) due to volume average of Msave for FePt-C and FePt-GBM granular layer, 2) There is no strong correlation between grain diameter (GD) and degree of ordering (Sinfilm) of the stacked media with T m of the GBM, 3) Ku┴ film dependence on Sinfilm for media with NL shows similar behavior with that of without NL, 4) for media with the same total thickness, the smaller I 110 inplane for media with NL than that of without NL reveals that disorder portion in the media with NL increases, 5) the improvement of linearity in I 110 inplane for media with NL against total thickness suggests that the homogeneity of the media with is improved, 6) reduction of I 100 linplane from XRD in-plane profile, reflected in l4θsat from the torque curve which suggests the reduction of the ratio of FePt grains with their c-axes parallel to the film plane. Further effort would be required to completely exclude the in-plane component.

A systematic investigation on magnetic properties and microstructure of Co80Pt20-B2O3 mixed with a high-melting-point oxide of various contents was conducted. It was found that the dual oxides granular media with moderate magnetic properties could be realized by changing the oxide fraction, which could not be realized by only utilizing a single oxide. The reduction of magnetic grain size could be obtained by employing a second oxide with high melting point due to the suppression of lateral magnetic grain growth by high-melting-point oxide at the initial growth region. The control of volume fraction of the high melting point was very essential to obtain small magnetic grains with columnar growth. To obtain a granular medium with small average grain size and narrow distribution, the application of an underlayer with smaller grain size would be needed. A significant reduction of grain size from 5.6 to 4.7 nm in a typical dual oxides granular medium could be observed when 30 vol% B2O3 was partially replaced by 15 vol% Cr2O3.

Investigation of magnetic properties and microstructure for the Co80Pt20–30vol% B2O3 granular medium to clarify the effect of utilizing buffer layer (BL) on intergranular exchange coupling was conducted. It was found that the employment of a Ru50Co25Cr25–30vol% TiO2 BL with thickness of around 2 nm will avoid physical contact between magnetic grains in the initial growth region, which is effective to intergranular exchange decouple the magnetic grains. On the other hand, for deposition of BL thicker than 2 nm, underlayer will be flatten that brings the possibility of physical contact of magnetic grains and forms a granular film with strong intergranular exchange coupling.

Investigation of the effect of the melting point ( $T_{m})$ of various single oxides on the columnar growth and generation of the nucleation site for CoPt granular media deposited at room temperature were conducted. When oxide with $T_{m}$ from 1857 °C to 2410 °C is added to the granular media, thickness of the heteroepitaxial growth of the magnetic grains on the Ru underlayer (columnar structure) decreases from 13 to 5.5 nm. The employment of oxide with $T_{m}$ of the range from 450 °C to 2410 °C to the granular media will lead to the reduction of grain size from 5.6 to 3.0 nm, due to the suppression of the lateral grain growth by the high $T_{m}$ oxide.

The dependence of the degree of order and perpendicular magnetic anisotropy (Ku⊥) on magnetic layer thickness (dmag) for FePt granular films with various grain boundary materials (GBMs) were investigated. According to the analysis for granular films with a constant GBM volume of 30 vol%, the order, disorder, and non-epitaxial portions co-existed in the FePt granular films. From the separate quantitative evaluation for these portions, the following was found; 1) the degree of order and magnitudes of Ku⊥ for disorder and order portions against dmag were constant, while the film average evaluation resulted in the dependence of the degree of order and Ku⊥ on dmag and the positive correlation between the grain diameter and the degree of order, 2) for the order portion, Ku⊥ had linear correlation with the degree of order, 3) the magnitude of Ku⊥ and the degree of order for the order portion depended on GBM.

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 SiO 2 2), TiO 2 3), Ta 2 $\mathrm {O}_{5}^{3)}$, Nb 2 $\mathrm {O}_{5}^{4)}$, WO 3 4), MgO 5), Cr 2 $\mathrm {O}_{3}^{5)}$, 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 B 2 O 3 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 B 2 O 3 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. In this paper, we will discuss about the effect of utilizing BL with oxide of various $T_{m}$ on the intergranular exchange decoupling of CoPt$- \mathrm {B}_{2} \mathrm {O}_{3}$ granular media in relation with magnetic properties.

CoPt-oxide granular media have been widely used for the perpendicular recording. To further extend the recording density, the increase of thermal stability through the enhancement of uniaxial magnetocrystalline anisotropy of the magnetic grains $(K_{\text{u}}^{\text{grain}})$ while maintaining low intergranular exchange coupling between grains is essential. It can be achieved by realizing the phase separation between grains and the grain boundaries material. Many authors have proposed various oxides for the grain boundaries material. However, for some oxides, $K_{\text{u}}^{\text{grain}}$ will be degraded to much lower than CoPt alloy. In this paper, we discuss the effect of utilizing B 2 O 3 and the guideline of choosing the oxides from the aspect of magnetic properties and microstructure.

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 ( ) and perpendicular magnetic anisotropy ( ) are obtained; the increases in and 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 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 ( ) of 8.0 kOe and 0.4, with and 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.